JP2004049884A - Radiation therapy apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To perform radiation therapy efficiently while preventing damages to normal tissues. <P>SOLUTION: A radiation material 13 comprising a heavy metal such as platinum that emits electron-positron pairs when exposed to gamma ray 12 is arranged in the vicinity of an affected area of a tumor 22. Gamma ray 12 is then irradiated to the radiation material 13 from the outside of a patient 21. When gamma ray 12 hits the radiation material 13, electron-positron pairs are emitted from the radiation material 13. The electron-positron pairs develop into an electromagnetic shower and irradiates the tumor 22 near the radiation material 13 for radiation therapy. Since the intensity of the electron-positron pairs and the electromagnetic shower developed therefrom reduces after travelling a predetermined distance, influences on the normal tissues surrounding the tumor 22 are prevented. <P>COPYRIGHT: (C)2004,JPO

Description

【００８５】
スペクトルはエネルギーに逆比例するので、制動輻射では低エネルギーのガンマ線が特に多くなる。低エネルギー成分は物質との相互作用が大きいので、放射線治療に用いると、どうしても表面で吸収が大きく、皮膚組織への影響が大きい。
【００８６】
一方、制動輻射で放出されるガンマ線は電子線の入射方向と同じく前方に多く放射されるが、一定の角度による広がりがあり、それは以下の式で与えられる。
【００８７】
【数２】

【００８８】
ここで、ｍは電子の質量、ｐは電子線のエネルギーでθはガンマ線の放出される角度である。ある角度θ内に放出されるガンマ線の全角度に対する割合は、ここで電子のエネルギーがその質量ｍより十分大きい場合を想定しているのでｍ／ｐ≪１で、かつ分布は前方に集中しているのでｓｉｎθ〜θとできる。よって以下のようになる。
【００８９】
【数３】

【００９０】
全角度積分した量はπ・ｐ^２／ｍ^２なので、ガンマ線はｍ^２／ｐ^２で表される角度内に半分が集まることになる。典型的に３０ＭｅＶの電子であれば広がりは１／６０ｒａｄとなる。
【００９１】
電子線を１０ＭｅＶ程度に加速して放射材に照射し、制動輻射によってガンマ線を生成する方法は癌治療で一般的に使われている。加速器としては上述の通りライナック（線形加速器）が一般的である。これは、エネルギーは１０ＭｅＶ程度であれば十分な透過力が得られるのと、不要な中性子の発生が抑えられることによっている。
【００９２】
さて、放射材を用いる治療には１０〜５０ＭｅＶ程度のガンマ線が適している。また細くコリメートされたガンマ線を得るにはエネルギーの高い方が効率的である。このためにはエネルギーの高い電子線が必要になる。電子線のエネルギーが１０ＭｅＶを越え始めると中性子の発生が無視出来なくなる。よってエネルギーが高い方が良いが、現実的には２０〜３０ＭｅＶ程度が適当であろう。
【００９３】
ガンマ線を細く絞るので十分な強度を得る為に必要な条件を探す必要がある。電子に制動輻射を起こさせる放射材の厚さは０．３Ｌ_Ｒ程度が適当である。この厚さではガンマ線への転換効率が３０％程度で、かつ生成されたガンマ線が相互作用して失われる確率が１５％程度なので、電子全エネルギーの２５％程度がガンマ線に転換される。実際の物質としては金や白金またはタングステンなどで１ｍｍ程度の厚さのものを置くことになる。
【００９４】
体内に置かれた放射材の大きさに当たるように細いガンマ線ビームを取り出すためには出来るだけ短い距離にする必要がある。コリメータと体内を通る距離を考えると、制動輻射を起こさせる放射材から体内の放射材までの距離として４０ｃｍを考えれば十分であろう。体内の放射材の直径を１ｍｍとすると、角度の広がりは１．２ｍｒａｄである。この角度内に入ってくるガンマ線は、３０ＭｅＶの場合は（０．００１２×６０）^２＝５×１０^−３である。この結果、３０ＭｅＶの電子線の電流を１μアンペアとすると、２×１０^９／ｓｅｃのガンマ線が得られる。典型的には数秒から数１０秒で、必要な線量が得られる。これが１０ＭｅＶの場合は（０．００１２×２０）^２＝６×１０^−４で、エネルギーが下がると前方の細い放射材に照射できるガンマ線の量が相対的に減少することで、３０ＭｅＶの場合に比較すると１桁近く効率が落ちる。実用上は電流を増やしたり時間をかけたりすることでカバー出来るので、比較的簡単に手に入る加速器での治療が可能になる意味は大きいであろう。
【００９５】
この方法で放射材を用いる治療に最適なガンマ線を得るために以下の装置を用いることができる。制動輻射により発生するガンマ線を小さなビームサイズに絞るためにコリメータを設置する。ガンマ線は飛行する距離と共に広がっていくので、コリメータから照射部位までの距離を短くするとガンマ線を効率良く使える。そのために材質として放射長の短い物質が適している。白金や金が最適ではあるが、コストを考えると、タングステンを主成分にするヘビーメタルが最も現実的である。ヘビーメタルの放射長は３．７ｍｍ程度であるので数センチメートルでガンマ線に対して十分な遮蔽効果がある。
【００９６】
制動輻射で生成されるガンマ線は低エネルギー程多いが、これは生体の特に表面に不要な線量を与える。これを除去するためにガンマ線の通る道筋とコリメータの中の穴に炭素を詰める。図１２に、制動輻射で得られるガンマ線のエネルギースペクトルを示す。吸収材が何もない時は式（１）で示されるエネルギーに反比例するスペクトルが、炭素の厚さを増やすに従って低エネルギー領域から減少して行くのが見える。道筋に沿って３０ｇ／ｃｍ^２程度の厚みがあれば人体の表面に影響を与える１ＭｅＶ以下のガンマ線は１桁近く減少するので、ほぼ実害のないレベルに出来る。グラファイトは密度が２．２程度なので１５ｃｍ程度あればよい。工業用のダイヤモンドを使えば更に短く出来る。
【００９７】
コリメータの形状は図１３、図１４のようなものである。中心部はヘビーメタルで出来ている細いガンマ線を引き出すコリメータである。周りに洩れていくガンマ線を止める役割も果たす。その外側には中性子の減速材が置かれている。パラフィンやポリエチレンなど水素を多く含む物質が優れている。減速材の層の厚さは中性子のフラックスをどの程度減少させたいかで決めることができる。典型的には１０ｃｍ程度の層の厚さであるが、もともとの中性子の発生量が少ない時は小さくコンパクトに作ることが出来る。
【００９８】
減速材の周りはカドミウムの金属シートがまかれている。０．５〜１ｍｍ程度の厚さがあれば減速された中性子のほとんどを吸収できる。尚、中性子を吸収したカドミウムはガンマ線を放出するので鉛の遮蔽をその外側におく。この時生成されるガンマ線は比較的エネルギーが低いので鉛５ｃｍ程度で十分遮蔽できる。
【００９９】
図１３、図１４を用いて、ガンマ線照射部５０の構成例を説明する。
【０１００】
タングステン（不要ガンマ線吸収部）５４は円柱であり、その内部が同心円状にくり抜かれている。この内部に、左から、タングステン（不要ガンマ線吸収部）５５、白金板（ガンマ線放出部）５６、コリメータ片５７、コリメータ片５８、白金板（低エネルギーガンマ線吸収部）６１、コリメータ片５９、コリメータ片６０がはめ込まれている。コリメータ片５７ないし６０はタングステン製である。コリメータ片５７ないし６０によってコリメータが形成されており、ガンマ線放出部から放出されるガンマ線のうち、所定の放射角以下のもののみを通す穴を有している。なお、ここでは４個のコリメータ片を用いているが、これに限定されず、設定に応じて１個以上、例えば２個以上を使用可能である。
【０１０１】
白金板５６・６１は、直径がタングステン５４の内径と同一である。
【０１０２】
タングステン５７ないし６０は外形が互いに合同な円柱形状であり、底面の直径がタングステン５４の内径と同一である。タングステン５７ないし６０は、いずれも、軸を通る同心円形状に内部がくり抜かれて貫通した穴を有している。タングステン５７ないし６０のそれぞれについては穴の径は一定である。そして、この穴の径は、白金板５６に近いほど小さく、
コリメータ片５７＜コリメータ片５８＜コリメータ片５９＜コリメータ片６０となっている。
【０１０３】
タングステン５５の内径は、穴の直径の最も小さい（白金板５６に最も近い）コリメータ片５７の穴の直径より大きく設定できる。
【０１０４】
５５〜６１は、タングステン５４内に、タングステン５４の底面から底面までの間に隙間が無いように詰まっている。
【０１０５】
コリメータ片５７〜６０の穴には、グラファイト（低エネルギーガンマ線吸収部）６４が詰められている。
【０１０６】
タングステン５４、５５は、パラフィン（中性子減速部）５３で覆われている。これは、ガンマ線放出部からガンマ線とともに放出される中性子を減速するものである。
【０１０７】
パラフィン５３は、カドミウムシート（中性子吸収部）５２で覆われている。これは、ガンマ線放出部からガンマ線とともに放出される中性子を吸収するものである。
【０１０８】
カドミウムシート５２は、鉛（副次ガンマ線吸収部）５１で覆われている。これは、中性子吸収部から放出されるガンマ線を吸収するものである。
【０１０９】
鉛５１は円柱形状であり、ガンマ線照射部５０全体としても円柱形状である。
【０１１０】
なお、図１４では、鉛５１を、５１ａ、５１ｂ、５１ｃのように分かれた部材として描いているが、このように別々の部材としてもよいし、これら３つを合わせて鉛５１として一体化した部材としてもよい。カドミウムシート５２（５２ａ、５２ｂ、５２ｃ）、パラフィン５３（５３ａ、５３ｂ）についても同様である。
【０１１１】
白金板５６には、図示しない電子線の発生源（照射源）から電子線６２が当たる。電子線の発生源としては、すでに述べた通り、医療用のガンマ線発生装置として一般に使われているライナック（線形加速器）等を用いることが可能である。
【０１１２】
白金板５６に電子線６２が当たると、白金板５６からガンマ線が発生する。このガンマ線のうち、低エネルギーのものは、グラファイト６４や白金板６１で吸収される。また、ガンマ線のうち、コリメータの穴を通らないものは、コリメータ片５７〜６０で吸収される。そのため、グラファイト６４を抜けてガンマ線照射部５０を出射するガンマ線６３は、高エネルギー成分を多く含んだものとなる。
【０１１３】
コリメータは、内径の異なるコリメータ片５７〜６０を積み重ねることで出来ている。すなわち、この構成では、コリメータ径が、段階的・不連続に変化し、変化する位置ごとに別々の部材となっている。しかしながら、これ以外にも、例えば円錐形状の内径を持つ１個のタングステン製部材をコリメータとして用いてもよい。この場合はコリメータ径が連続的に変化することになる。
【０１１４】
制動輻射で生成されるガンマ線は低エネルギーほど多いが、これは、生体の特に表面に不要な線量を与える。これを除去するために、ガンマ線の通る道筋とコリメータの中の穴に吸収材（低エネルギーガンマ線吸収部）を詰める。コリメータの穴の長さを１０ｃｍとすると、典型的に、最初の５ｃｍにグラファイト（炭素）６４を詰め、次に、１ｍｍ厚の白金（または、金、タングステン）板６１を詰め、次の５ｃｍにグラファイト６４を詰めるという構成が可能である。すなわちこの例は、２個所のグラファイト６４の長さ（５ｃｍ）が互いに等しく、かつ、コリメータの穴の長さ（１０ｃｍ）の半分となっている例である。
【０１１５】
最初に電子線ビームを約１ｍｍ厚の白金（または、金、タングステン）板５６（ｃｏｎｖｅｒｔｅｒ、変換材）（ガンマ線放出部）に照射し、ガンマ線を発生させる。発生点はできるだけ小さいことが、コリメータの効率を向上させる。典型的には、数分の１ｍｍ程度以下が望ましい。ここで発生したガンマ線はコリメータで絞り込まれる。
【０１１６】
変換材の点においては、エネルギーの下がった電子が存在するが、ガンマ線放出部が重い金属であると、この電子から新たにガンマ線が発生する。これらの多くは、不要な低エネルギー成分であり、除去したい。そこで、ガンマ線が発生した場所の後ろに吸収材を置き、せっかく生成されたガンマ線の高エネルギー成分については減少を抑えながら、低エネルギー成分を吸収除去する。これには上記のようにグラファイト６４のような炭素を用いる。
【０１１７】
炭素中での電子のエネルギー損失は２ＭｅＶ／（ｇ／ｃｍ^２）程度なので、５ｃｍ（１１ｇ／ｃｍ^２）程度あれば電子はほぼ止まる。
【０１１８】
次に、上記のように、１ｍｍ厚程度の薄い白金板６１を置く。これによって低エネルギーのガンマ線を有効に落とせる。さらにその後ろに５ｃｍ程度の炭素で吸収材とする。重い金属はＫ吸収端（８０ｋｅＶ程度）以下のエネルギーでは吸収効率がかえって落ちる性質があるので、その部分を再び炭素で吸収させる。
【０１１９】
白金（または、金やタングステン）板６１は、低エネルギー成分を吸収する（厚いと、高エネルギー成分まで吸収する）ものであり、グラファイト６４は、主として低エネルギー成分を吸収するとともに、電子を、ガンマ線の生成なく減速させることができる。
【０１２０】
図１５、図１６に、それぞれ、炭素、白金について、ガンマ線との相互作用断面積（全吸収面積）を示す。ほぼ、吸収の確率に対応すると考えることができる。
【０１２１】
図１５では、ガンマ線との相互作用断面積は、炭素のガンマ線吸収断面積エネルギーとともにゆっくり減少している。
【０１２２】
図１６では、ガンマ線との相互作用断面積は、白金のガンマ線吸収断面積エネルギーとともに急激に減少している。また、エネルギーが数ＭｅＶより上になると増加を始める。また、８０ｋｅＶ程度あたりに断面積の減少する点がある。
【０１２３】
ガンマ線が厚さｔのある物質を通ったときどの程度に減少するかは、吸収係数σを用いて
Ｎ（ｔ）＝Ｎ_０・ｅｘｐ（−σｔ）
と表される。ただし、物質を通る前の個数をＮ_０、通った後の個数をＮ（ｔ）とし、自然対数の底ｅのｘ乗をｅｘｐ（ｘ）と表す。
【０１２４】
図１５、図１６には、縦軸にσ（単位：ｃｍ^２／ｇ）が表されている。物質の厚さは、長さの単位ではなく物質の密度まで考慮したｇ／ｃｍ^２で決めておくと便利である。
【０１２５】
白金（比重２１．４５）を例に考えると、図１６より
Ｅ＝０．１ＭｅＶのときσ＝６ｃｍ^２／ｇ
Ｅ＝１ＭｅＶのとき　　σ＝０．０６ｃｍ^２／ｇ
であることがわかる。ここで、１ｍｍ厚の白金は
ｔ＝２２．４５×０．１＝２．２４５ｇ／ｃｍ^２
の厚さとなる。よって
Ｎ（０．１ＭｅＶ）＝ｅｘｐ（−６×２．２４５）Ｎ_０＝１．４×１０^−６Ｎ_０
Ｎ（１ＭｅＶ）＝ｅｘｐ（−０．０６×２．２４５）Ｎ_０＝０．８７Ｎ_０
となって、０．１ＭｅＶのガンマ線はほぼ完全に落とせるが、一方で、１ＭｅＶのガンマ線はほとんど残ることになる。
【０１２６】
炭素１０ｃｍ（グラファイトの比重は２．２程度である）については、ｔ＝２２ｇ／ｃｍ^２で、図１５より
Ｅ＝０．１ＭｅＶのときσ＝０．１６ｃｍ^２／ｇ、Ｎ＝０．０３Ｎ_０
Ｅ＝１ＭｅＶのとき　　σ＝０．０６ｃｍ^２／ｇ、Ｎ＝０．２７Ｎ_０
Ｅ＝１０ＭｅＶのとき　σ＝０．０２ｃｍ^２／ｇ、Ｎ＝０．６４Ｎ_０
となり、エネルギー依存性は極端ではないが、高エネルギー側を残せることがわかる。
【０１２７】
【発明の効果】
以上のように、本発明に係る放射線治療装置は、ガンマ線が当たると電子陽電子対を放出して電磁シャワーを発生させて治療対象体中の患部に照射する電子陽電子対放出部と、治療対象体の外部からガンマ線を照射して上記電子陽電子対放出部に当てるガンマ線照射部とを備えた構成である。
【０１２８】
これにより、ガンマ線照射部から出たガンマ線が電子陽電子対放出部に当たり、電子陽電子対放出部から電子陽電子対が放出される。この電子陽電子対は、それが電磁シャワーに発展して、ガンマ線照射部の近傍の患部に照射されて患部を放射線治療することができる。
【０１２９】
それゆえ、正常組織の損傷を極力抑えながら腫瘍等の患部に多くの放射線を照射することができ、効率的に放射線治療を行うことができるという効果を奏する。
【０１３０】
また、本発明に係る放射線治療装置は、上記の構成に加えて、治療対象体中の電子陽電子対放出部の位置を検出する電子陽電子対放出部位置検出部と、治療対象体中でガンマ線が照射されている位置を検出するガンマ線照射位置検出部と、上記電子陽電子対放出部位置検出部により検出された治療対象体中の電子陽電子対放出部の位置と上記ガンマ線照射位置検出部により検出された治療対象体中でガンマ線が照射されている位置とがずれている旨の情報が入力されると、上記電子陽電子対放出部の位置を、ガンマ線が照射されている位置に戻す位置制御部と、電子陽電子対放出部の位置とガンマ線が照射されている位置とがずれている間、ガンマ線照射をオフにするように上記ガンマ線照射部を制御するガンマ線照射制御部とを備えた構成である。
【０１３１】
これにより、上記の構成による効果に加えて、無駄なくガンマ線を照射することができるという効果を奏する。
【０１３２】
また、本発明に係る放射線治療装置は、上記の構成に加えて、上記ガンマ線照射部が、電子線が当たると制動輻射でガンマ線を放出するガンマ線放出部と、上記ガンマ線放出部から放出されるガンマ線のうち、所定の放射角以下のもののみを通す穴を有するコリメータと、上記ガンマ線放出部とコリメータとの間に置かれ、上記ガンマ線放出部から放出されるガンマ線のうちでエネルギーが所定値以下のものを吸収する低エネルギーガンマ線吸収部とを備えた構成である。
【０１３３】
これにより、上記の構成による効果に加えて、ギガレベルの高価な加速器を使わずに、癌治療などですでに利用可能な既存の加速器を利用して安全かつ効率よく高エネルギーのガンマ線を得ることができ、患部を電磁シャワーで十分に治療することができるという効果を奏する。
【０１３４】
また、本発明に係る放射線治療装置は、上記の構成に加えて、上記ガンマ線放出部から放出されてコリメータの穴を通らないガンマ線を吸収する不要ガンマ線吸収部を備えた構成である。
【０１３５】
これにより、上記の構成による効果に加えて、不要なガンマ線を吸収することができるという効果を奏する。
【０１３６】
また、本発明に係る放射線治療装置は、上記の構成に加えて、上記ガンマ線放出部の周囲に、上記ガンマ線放出部からガンマ線とともに放出される中性子を吸収する中性子吸収部を備えた構成である。
【０１３７】
これにより、上記の構成による効果に加えて、不要な中性子を吸収することができるという効果を奏する。
【０１３８】
また、本発明に係る放射線治療装置は、上記の構成に加えて、上記ガンマ線放出部と上記中性子吸収部との間に、上記ガンマ線放出部からガンマ線とともに放出される中性子を減速する中性子減速部を備えた構成である。
【０１３９】
これにより、上記の構成による効果に加えて、不要な中性子を減速することができるという効果を奏する。
【０１４０】
また、本発明に係る放射線治療装置は、上記の構成に加えて、上記中性子吸収部が、中性子を吸収するとガンマ線を放出する性質を有しており、上記中性子吸収部の周囲に、中性子吸収部から放出されるガンマ線を吸収する副次ガンマ線吸収部を備えた構成である。
【０１４１】
これにより、上記の構成による効果に加えて、不要なガンマ線を吸収することができるという効果を奏する。
【０１４２】
また、本発明に係る放射線治療装置は、上記の構成に加えて、上記コリメータが、外形が合同な円柱形状物であって、軸を中心として貫通している円柱状のガンマ線経路となる穴の径が異なる複数のコリメータ片が、ガンマ線放出部から離れるに従って穴径が大きくなるような順序で同軸上に隙間無く並んで成る構成である。
【０１４３】
これにより、各コリメータ片の組み合わせを変えるだけで、ガンマ線の出射角度範囲を簡単に変えることができる。それゆえ、上記の構成による効果に加えて、より容易に、必要なガンマ線強度を得ることができるという効果を奏する。
【図面の簡単な説明】
【図１】本発明に係る放射線治療の概念を示す図である。
【図２】ガンマ線の入射位置と放射材の配置との関係を示す平面図である。
【図３】ガンマ線を放射材に照射したときの放射材の深さと放射線線量の蓄積量との関係を示すグラフである。
【図４】ガンマ線を放射材に照射したときの放射材の深さと放射線線量の蓄積量との関係を示すグラフである。
【図５】ガンマ線を放射材に照射したときの放射材の深さと放射線線量の蓄積量との関係を示すグラフである。
【図６】ガンマ線を放射材に照射したときの線量の最も多い深さにおける半径と放射線線量の蓄積量との関係を示すグラフである。
【図７】ガンマ線を放射材に照射したときの線量の最も多い深さにおける半径と放射線線量の蓄積量との関係を示すグラフである。
【図８】ガンマ線を放射材に照射したときの線量の最も多い深さにおける半径と放射線線量の蓄積量との関係を示すグラフである。
【図９】本発明に係る放射線治療の概念を示す図である。
【図１０】ガンマ線の入射位置と放射材の配置との関係を示す平面図である。
【図１１】本発明に係る放射線治療装置の一構成例を示す平面図である。
【図１２】ガンマ線のエネルギースペクトルを示すグラフである。
【図１３】ガンマ線照射部を示す断面図である。
【図１４】ガンマ線照射部を構成する各部材を示す斜視図である。
【図１５】炭素におけるガンマ線のエネルギーと、ガンマ線との相互作用断面積との関係を示すグラフである。
【図１６】白金におけるガンマ線のエネルギーと、ガンマ線との相互作用断面積との関係を示すグラフである。
【図１７】放射線治療の概念を示す図である。
【図１８】放射線治療におけるガンマ線による損傷の変化を示すグラフである。
【図１９】ガンマ線を用いた放射線治療の例を示す図である。
【符号の説明】
１１　　水
１２　　ガンマ線
１３　　放射材（電子陽電子対放出部）
１４　　電磁シャワー
２１　　患者（治療対象体）
２２　　腫瘍
３１　　高精度位置検出器（電子陽電子対放出部位置検出部）
３２　　照射位置確認装置（ガンマ線照射位置検出部）
３３　　高精度位置制御装置（位置制御部）
３６　　ガンマ線ビーム生成装置（ガンマ線照射部）
３７　　コリメータ（ガンマ線照射部）
３８　　コントローラー（ガンマ線照射制御部）
５０　　ガンマ線照射部
５１、５１ａ、５１ｂ、５１ｃ　　鉛（副次ガンマ線吸収部）
５２、５２ａ、５２ｂ、５２ｃ　　カドミウムシート（中性子吸収部）
５３、５３ａ、５３ｂ　　パラフィン（中性子減速部）
５４　　タングステン（不要ガンマ線吸収部）
５５　　タングステン（不要ガンマ線吸収部）
５６　　白金板（ガンマ線放出部）
５７、５８、５９、６０　　タングステン（コリメータ）
６１　　白金板（低エネルギーガンマ線吸収部）
６２　　電子線
６３　　ガンマ線
６４　　グラファイト（低エネルギーガンマ線吸収部）[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a radiotherapy device for tumors and the like using gamma rays.
[0002]
[Prior art]
X-rays, gamma rays, and the like are used for treatment of tumors (cancer) by radiation, and gamma rays are widely used.
[0003]
Radiation therapy for tumors utilizes the loss of energy as radiation passes through the body, causing biological damage (hereinafter simply referred to as damage) to tumor tissue. As shown in the upper part of FIG. 17, radiation 41 is externally applied to a tumor 22 in a patient 21 such as a human or an animal. FIG. 17 also shows the relationship between the position (depth) of the tumor 22 in the patient 21 and the degree of damage.
[0004]
Since the influence of the above-mentioned radiation is exerted not only on the tumor but also on normal tissues, the basis of radiotherapy is to maximize the dose at the tumor and minimize the dose at the normal tissue. That is, as shown in the lower part of FIG. 17, it is required that the damage is remarkable only at the depth where the tumor 22 is present, and that the damage is not very small at other depths.
[0005]
There are Patent Documents 1 and 2 as known techniques.
[0006]
[Patent Document 1]
Japanese Patent Application Laid-Open No. H11-153310 (published on June 8, 1999)
[0007]
[Patent Document 2]
Japanese Patent Application Laid-Open No. 2000-331799 (Published November 30, 2000)
[0008]
[Problems to be solved by the invention]
Gamma rays are readily available, but also damage normal tissues, as shown in FIG. In FIG. 18, the horizontal axis is the same depth as above, and the vertical axis is the degree of damage as above.
[0009]
For this reason, as shown in FIG. 19, a method (gamma knife) of irradiating gamma rays 12 from all sides to concentrate on the tumor 22 has been developed. However, attempting to sufficiently damage the tumor 22 also increases the dose integrated into normal tissue.
[0010]
Recently, as disclosed in Patent Documents 1 and 2, a method using a Bragg peak using a high-energy charged particle beam is expected in this regard. Bragg peak refers to the emission of maximum radiant energy just before radiation stops. However, charged particles such as heavy ions and protons require a high-energy accelerator and, in practice, require a dedicated beam line. Further, since the effective range is small, there is little merit in a large affected area.
[0011]
The present invention has been made in view of the above problems, and an object of the present invention is to provide a radiotherapy apparatus capable of performing treatment efficiently while suppressing damage to normal tissues.
[0012]
[Means for Solving the Problems]
In order to solve the above problems, the radiation therapy apparatus according to the present invention, an electron positron pair emission unit that emits an electron positron pair when a gamma ray hits, generates an electromagnetic shower, and irradiates the affected part in the treatment target body, A gamma ray irradiating unit for irradiating a gamma ray from the outside of the treatment target and hitting the electron-positron pair emitting unit.
[0013]
With the above configuration, gamma rays are irradiated from the outside of the treatment target (human body or the like) by the gamma ray irradiation unit, and the gamma rays are applied to the electron-positron pair emission unit. Then, an electron-positron pair is emitted from the electron-positron pair emission portion, and the electron-positron pair develops into an electromagnetic shower and irradiates an affected part (tumor or the like).
[0014]
Therefore, by disposing the electron-positron pair emitting portion in the vicinity of the affected part, that is, within a distance range in which the electron-positron pair can hit the affected part with sufficient strength to have a therapeutic effect (damage effect), The affected area can be radiation-treated in pairs. When the affected part is present inside the treatment target, the electron-positron pair emitting part may be embedded in the center of the affected part. On the other hand, the intensity of the electron-positron pair and the electromagnetic shower developed from the electron-positron pair decrease when they travel a predetermined distance. Therefore, the influence on the normal tissue around the affected part can be suppressed.
[0015]
Therefore, it is possible to irradiate an affected part such as a tumor with a large amount of radiation while minimizing damage to a normal tissue, and it is possible to efficiently perform radiation treatment.
[0016]
Further, in addition to the above configuration, the radiation therapy apparatus according to the present invention further includes an electron positron pair emission unit position detection unit that detects a position of the electron positron pair emission unit in the treatment target, and a gamma ray in the treatment target. A gamma ray irradiation position detecting unit that detects a position where irradiation is performed, a position of the electron positron pair emitting unit in the treatment target detected by the electron positron pair emitting unit position detecting unit, and a position detected by the gamma ray irradiation position detecting unit. When the information indicating that the position where the gamma ray is irradiated is shifted in the treatment target body is input, the position of the electron-positron pair emitting unit is returned to the position where the gamma ray is irradiated, and a position control unit is returned. A gamma-ray irradiation control unit that controls the gamma-ray irradiation unit so as to turn off the gamma-ray irradiation while the position of the electron-positron pair emitting unit and the position where the gamma ray is irradiated are shifted. It is characterized.
[0017]
According to the above configuration, the generation of the gamma ray is turned off while the position of the gamma ray irradiating unit in the treatment target is shifted from the position of the gamma ray irradiation in the treatment target.
[0018]
Therefore, in addition to the effect of the above configuration, gamma rays can be irradiated without waste.
[0019]
Further, in addition to the above configuration, the radiation therapy apparatus according to the present invention, wherein the gamma ray irradiating unit emits gamma rays by braking radiation when an electron beam hits, and a gamma ray emitted from the gamma ray emitting unit. Among them, a collimator having a hole through which only those having a predetermined emission angle or less, and placed between the gamma-ray emitting unit and the collimator, the energy of the gamma rays emitted from the gamma-ray emitting unit is less than or equal to a predetermined value. And a low-energy gamma-ray absorbing section for absorbing an object.
[0020]
According to the above configuration, when a high-energy electron beam is applied to the gamma-ray emission unit in order to obtain a high-energy gamma ray, a high-energy gamma ray is emitted over a wide range (large emission angle). High-energy gamma rays can be emitted only in a desired narrow range. As a result, it is possible to prevent an unrelated part of the patient from being irradiated with high-energy gamma rays without entering the electron-positron pair emission part. Even if the emission range is narrowed in this way, if gamma rays of low energy are mixed, the efficiency is poor, so that it is removed.
[0021]
Therefore, in addition to the effects of the above configuration, high-energy gamma rays can be obtained safely and efficiently using existing accelerators that are already available for cancer treatment, etc., without using expensive giga-level accelerators. In addition, the affected area can be sufficiently treated with an electromagnetic shower.
[0022]
Further, the radiotherapy apparatus according to the present invention is characterized in that, in addition to the above-described configuration, an unnecessary gamma ray absorbing section for absorbing gamma rays emitted from the gamma ray emitting section and not passing through the hole of the collimator is provided.
[0023]
With the above configuration, there is provided an unnecessary gamma ray absorbing section for absorbing gamma rays emitted from the gamma ray emitting section and not passing through the hole of the collimator.
[0024]
Therefore, unnecessary gamma rays can be absorbed in addition to the effects of the above configuration.
[0025]
Further, in addition to the above configuration, the radiation therapy apparatus according to the present invention is characterized in that a neutron absorbing section that absorbs neutrons emitted together with gamma rays from the gamma ray emitting section is provided around the gamma ray emitting section. I have.
[0026]
According to the above configuration, a neutron absorbing unit that absorbs neutrons emitted together with gamma rays from the gamma ray emitting unit is provided around the gamma ray emitting unit.
[0027]
Therefore, unnecessary neutrons can be absorbed in addition to the effects of the above configuration.
[0028]
In addition, the radiotherapy apparatus according to the present invention, in addition to the above configuration, between the gamma ray emitting section and the neutron absorbing section, a neutron moderating section that slows down neutrons emitted together with gamma rays from the gamma ray emitting section. It is characterized by having.
[0029]
According to the above configuration, a neutron moderator is provided between the gamma ray emitter and the neutron absorber to reduce neutrons emitted together with gamma rays from the gamma ray emitter.
[0030]
Therefore, unnecessary neutrons can be decelerated in addition to the effect of the above configuration.
[0031]
Further, in addition to the above configuration, the radiation therapy apparatus according to the present invention has a property that the neutron absorbing section emits gamma rays when absorbing neutrons, and the neutron absorbing section is provided around the neutron absorbing section. And a secondary gamma ray absorbing section that absorbs gamma rays emitted from the device.
[0032]
With the above configuration, the neutron absorbing section has a property of emitting gamma rays when absorbing neutrons, and around the neutron absorbing section, a secondary gamma ray absorbing section that absorbs gamma rays emitted from the neutron absorbing section. Is provided.
[0033]
Therefore, unnecessary gamma rays can be absorbed in addition to the effects of the above configuration.
[0034]
In addition, in the radiotherapy apparatus according to the present invention, in addition to the above-described configuration, the collimator is a cylindrical object having a congruent outer shape, and a hole having a cylindrical gamma ray path penetrating around an axis. It is characterized in that a plurality of collimator pieces having different diameters are arranged coaxially and without gaps in such an order that the hole diameter increases as the distance from the gamma ray emitting portion increases.
[0035]
With the above configuration, the diameter of the collimator changes stepwise and discontinuously, and a separate member is provided for each change position. As a result, by variously combining the collimator pieces, the inclination of the collimator can be easily changed. For example, the diameter of the collimator can be reduced by combining many holes having different hole diameters little by little. Conversely, if a combination of holes having greatly different hole diameters is combined, the hole diameter increases rapidly as it advances, so that the diameter of the collimator can be increased.
[0036]
Therefore, the emission angle range of gamma rays can be easily changed only by changing the combination of the collimator pieces. Therefore, in addition to the effect of the above configuration, the required gamma ray intensity can be obtained more easily.
[0037]
BEST MODE FOR CARRYING OUT THE INVENTION
One embodiment of the present invention will be described below with reference to FIGS.
[0038]
First, the basic concept in the present embodiment will be described.
[0039]
Gamma rays having a certain level of high energy have low interaction with water and living organisms such as humans and animals, and can pass through with little effect on water and living organisms. That is, energy loss is small in a living body. Here, the interaction refers to an action that results in destroying a tumor tissue in the case of a tumor, and more generally refers to an action that results in applying high energy to an object. In the present embodiment, attention is paid to this fact, and radiation therapy that is minimally invasive to a living body using gamma rays is effectively performed.
[0040]
The energy value of the gamma ray to be adopted depends, in part, on the ability of the energy value of the gamma ray to have its own ability to cause interactions and other equipment and devices to prevent interaction with the living body. It can be determined in consideration of the presence / absence, their effects, and the like. For example, those having a value of 5 MeV or more and 50 MeV or less have low interaction with water or a living body as described above, and are considered to pass through with little effect on water or a living body. In addition, those having 5 MeV or more mainly generate electron-positron pairs as a result of the interaction. Therefore, such a value can be adopted.
[0041]
The energy value of the gamma ray to be employed can be determined in consideration of the characteristics of a radiating material that emits an electron-positron pair, which will be described later.
[0042]
As shown in FIG. 1, a radiating material (electron-positron pair emitting unit) 13 is placed in water 11 corresponding to a living body such as a human body or an animal. Then, gamma rays 12 are irradiated from outside the water 11. By doing so, the radiating material 13 emits electron-positron pairs.
[0043]
That is, as shown in FIGS. 1 and 2, a thin collimated high-energy gamma ray 12 is formed by cutting a bottom surface of a columnar heavy metal radiating material 13 (formed in a line shape and cut along a plane perpendicular to the length direction). When irradiated on the cross section, the radiating material 13 generates an electron positron pair, and emits the electron positron pair and the electromagnetic shower 14 developed by the electron positron pair mainly from a surface (side surface) in the radial direction (vertical direction in the figure). The affected part is irradiated with the electron-positron pair and the electromagnetic shower 14 developed from the electron-positron pair, and the treatment is performed.
[0044]
The radiating material 13 emits electron-positron pairs when irradiated with gamma rays, which develops into an electromagnetic shower 14 (see FIG. 2). The electromagnetic shower is radiation containing gamma rays, electrons, and positrons emitted from an object (the radiating material 13 in this case). Radiation length L of radiating material_RIt is preferable that the length is shorter, because only the affected part can be irradiated with the electromagnetic shower, and is preferably 5 mm or less. The emission length represents a material-specific length required to convert a gamma ray into an electron-positron pair at a constant conversion rate. The emission length is inversely proportional to the square of the atomic number and the density. The radiation length of water, which is the main component of the human body, is 360 mm, and the radiation length of platinum is 3.05 mm. Since the electron-positron pair generation rate per unit length of platinum is almost 100 times that of water, electron-positron pairs can be generated intensively at the position where platinum is placed. In order to achieve a sufficient therapeutic effect, it is preferable that most of the gamma rays be converted into electron-positron pairs in the size of the affected area. Gamma rays 3L in the radiating material_RAfter passing a certain distance, it can be said that most gamma rays are converted into electron-positron pairs. Therefore, as the radiating material, the distance through which the gamma ray passes in the radiating material is 3L._RThe length is about the size of the radiating material, and the size of the radiating material is about the size of the affected part (when radiating material is placed in the affected part, the size is such that the entire radiating material can be embedded inside the affected part) Substances are preferred. As an example of the radiating material 13, heavy metal is advantageous in that the radiation length is short. Examples of heavy metals include platinum, gold, tungsten, lead, and mercury. Considering that it can be put into the human body, lead and mercury are not preferred, and platinum is preferred.
[0045]
Heavy metals are usually used as radiation shielding materials. In the present embodiment, the treatment is performed by taking out the attenuated radiation (electron positron pair) and irradiating the affected part with the attenuated radiation utilizing the property in reverse.
[0046]
As the radiating material 13, for example, a material having a pillar (needle) shape can be used. In this case, for example, a material having a diameter (vertical direction in FIG. 1) of 2 mm or less and a length (horizontal direction in FIG. 1) of 5 mm or more can be used. For example, when the radiating material 13 is platinum, a material having a diameter of 1 mm and a length of 10 mm can be used as shown in FIG.
[0047]
The radiating material 13 is irradiated with gamma rays 12. For example, the gamma rays 12 can be applied to a surface (bottom surface) perpendicular to the length direction of the columns (or needles) of the radiating material 13.
[0048]
When the radiating material 13 receives the irradiation of the gamma rays 12, in each part inside the radiating material 13 through which the gamma rays 12 pass, in the passing direction of the gamma rays 12, that is, in the case of the columnar (needle) -shaped radiating material 13, the length direction ( The electron-positron pair and the electromagnetic shower 14 developed from the surface (the side surface in the case of a column (needle) as shown in FIG. 2) along the horizontal direction in FIGS. Is released. Therefore, from the viewpoint of efficiently emitting such electron-positron pairs, the shape of the radiating material 13 is long in the passing direction of the gamma ray 12 as shown in FIG. In the case of the distance between the gamma ray 12 and the surface of the radiating material 13 in a direction perpendicular to the direction of the radiating material 13, that is, a column (needle) -shaped radiating material 13, it is preferable that the thickness direction (vertical direction in FIGS. 1 and 2) is short.
[0049]
As shown in FIG. 10, the length direction of the pillar-shaped (needle) -shaped radiating material 13 is perpendicular to the incident direction of the gamma rays 12, and a surface along the length direction of the radiating material 13 (the vertical direction in FIG. 10), When the gamma ray 12 is irradiated on the side surface of the columnar (needle) -shaped radiating material 13, the ratio (conversion rate) of conversion of the gamma ray 12 into an electron-positron pair when passing through the radiating material 13 extremely decreases, Further, even if an electron-positron pair is generated from each gamma-ray passing portion of the radiating material 13, the distance of the radiating material 13 passing through the radiating material 13 until reaching the affected part such as a tumor is reduced. Since the length is long, attenuation becomes remarkable, and a sufficient amount of electron-positron pairs may not reach the affected part. Therefore, when such a column (needle) -shaped radiating material 13 is used, it is preferable to irradiate the gamma ray 12 to the bottom surface.
[0050]
The distance (depth) from the surface of the water 11 corresponding to the skin of the human body, where the gamma rays 12 are incident, to the center of the radiating material 13 corresponding to the position of the tumor part is, for example, 100 mm as shown in FIG. Can be.
[0051]
As described above, the gamma rays 12 can be of high energy, for example, 5 MeV or more, 50 MeV or less, and for example, 30 MeV or less, in order to reduce the interaction with the living body as described above. For example, a very thin and collimated material can be used. In order to generate very thin and collimated high-energy gamma rays, a giga-level (1 GeV or more) electron beam is generated by an accelerator (not shown), and a laser beam is made to collide with the electron beam, and the electron beam is generated by the inverse Compton effect. Get gamma rays of energy. The high-energy gamma rays (photon beams) obtained in this way are concentrated forward (in the traveling direction), and the energy depends on the angle. That is, high-energy gamma rays are localized within a certain angle from the traveling direction. Therefore, by using a thin collimator, it is possible to selectively extract only high-energy gamma rays.
[0052]
If the energy depends on the angle as described above, how much thinner collimated gamma rays should be adopted (how much the beam diameter of the gamma rays) should be selected. It depends on what you want to do. The choice of gamma ray at which energy value, as already mentioned, also depends on how safe the energy value is for the patient, in other words, how much interaction is acceptable.
[0053]
If the beam diameter of the gamma ray is too large, when the radiating material 13 is small, the gamma rays in the peripheral portion of the beam diameter do not enter the radiating material 13 and the efficiency is low. Therefore, the beam diameter of the gamma ray to be selected also changes depending on the size of the radiation material 13. For example, it is conceivable that the beam diameter of the gamma ray is reduced to substantially the same diameter as the cross section of the surface of the radiating material 13 on which the gamma ray 12 is incident.
[0054]
Next, an example of a simulation assuming the above configuration and a result thereof will be described.
[0055]
Here, as a method of generating gamma rays, a 2.8 GeV electron beam was bombarded with a carbon dioxide gas laser having a wavelength of 0.12 eV and a wavelength of 10.6 μm to obtain 15 MeV gamma rays. In this case, gamma rays having an energy higher than 7 MeV exist within 0.2 m radian.
[0056]
A platinum wire having a length of 10 mm and a diameter of 1 mm on the bottom surface was used as the radiating material 13, and the depth was set to 10 cm. That is, a platinum wire was placed between 95 mm and 105 mm from the skin. Positrons interact with electrons in the substance and emit 511 keV gamma rays. Since this is the same as that used for PET (positron tomography), the irradiation position can be confirmed using this PET.
[0057]
The simulation was performed by changing the energy E of the gamma rays 12 to 10 MeV (FIGS. 3 and 6), 20 MeV (FIGS. 4 and 7), and 40 MeV (FIGS. 5 and 8). The shape of the curve is the same in that the dose is concentrated near the platinum wire in any of E = 10 MeV, 20 MeV, and 40 MeV, but the higher the energy, the more the area can be irradiated. Therefore, by adjusting the energy, more effective treatment can be performed.
[0058]
3 to 5 show the relationship (discharged dose distribution) between the above depth and the amount of accumulated radiation dose. 6 to 8, the relationship between the amount of accumulated radiation dose and the radius is represented by the absorbed dose Gy (for 10 to the 10th power photons) at the depth with the highest dose.
[0059]
As shown in FIGS. 3 to 5, the accumulation of energy by the electromagnetic shower is concentrated near the platinum wire. Assuming that the length at which the dose becomes 1/10 is the range where the electron-positron pair is effectively irradiated, as shown in FIGS. 6 to 8, 0.8 cm at 10 MeV and 1 cm at 20 MeV. In the case of 0.2 cm and 40 MeV, it was 1.6 cm.
[0060]
The average energy of the electron-positron pair and the electromagnetic shower it evolved is higher when produced from high-energy gamma rays. Therefore, the penetrating power is strong, and the dose can be given to a wider area. FIGS. 3 to 5 and 6 to 8 show that the dose distribution is concentrated around the radiating material 13 first. FIGS. 3 to 5 show where the dose distribution is concentrated in the depth direction. When these results are combined, it can be said that the dose distribution spreads in the depth direction when the energy increases. 6 to 8 show the radial expansion. That is, by adjusting the energy, the dose distribution can be adjusted to the size of the affected part, and the spread of the dose distribution can be easily adjusted as compared with the case of heavy ions.
[0061]
FIG. 9 shows an example in which the above-described configuration is applied to a tumor 22 in a patient (treatment target) 21 such as a human body or an animal. The radiating material 13 is arranged near the affected part, that is, the tumor 22. For example, the radiating material 13 is embedded inside the tumor 22. In this state, the gamma ray 12 is irradiated from the outside of the patient 21, and when the gamma ray 12 hits the radiating material 13, an electron-positron pair and an electromagnetic shower developed therefrom are emitted from the radiating material 13. The electron-positron pair and the electromagnetic shower developed by the electron-positron pair pass through the inside of the radiating material 13, hit the tumor 22 near the radiating material 13, and damage the tumor 22. As a result, the tumor 22 is destroyed, and the affected part can be subjected to radiation treatment. The intensity of the electron-positron pair and the electromagnetic shower developed by the electron-positron pair decrease after a predetermined distance. Therefore, in consideration of the size of the tumor 22 and the like, the intensity of the gamma rays 12 and the material and shape of the radiating material 13 are reduced so that the intensity of the electron-positron pair and the electromagnetic shower developed by the electron-positron pair decrease sufficiently before exiting the tumor 22. By setting the size and the like, it is possible to suppress the influence of the electron positron pair and the electromagnetic shower developed by the pair on the normal tissue around the tumor 22.
[0062]
FIG. 11 shows a more specific example of the apparatus. The patient 21 is placed on the high-precision position control device (position control unit) 33 by lying down or the like. The high-precision position control device 33 controls the position of the patient 21 and thereby controls the position of the radiating material 13. The high-precision position control device 33 performs horizontal and vertical translation, and also uses a horizontal straight line and a vertical straight line as axes. It can be capable of rotational movement. The radiation material 13 is embedded in the affected part of the patient 21. A three-dimensional high-accuracy position detector (electron-positron pair emitting unit position detecting unit) 31 using a metal detector detects the position of the radiating material 13 (here, heavy metal) in the patient 21. The irradiation position confirmation device (gamma ray irradiation position detection unit) 32 can use, for example, a positron (PET) that detects 511 keV, and detects the incident position of the gamma ray 12 in the patient 21.
[0063]
In addition, a gamma ray beam generating device 36 is provided, and the gamma rays 12 emitted from the device are finely collimated by a collimator 37 using lead or the like and are incident on the patient 21. The gamma-ray beam generating device 36 can use, for example, an electron beam source that includes an electron linear accelerator and a converging lens and emits energy of 1 GeV or more, and a laser light source (both not shown). The electron beam from the electron beam source collides with the laser from the laser light source to extract gamma rays 12 as scattered light due to the inverse Compton effect. A gamma ray irradiation unit is configured by the gamma ray beam generating device 36 and the collimator 37.
[0064]
The controller (gamma ray irradiation control unit) 38 receives position information from the high-precision position detector 31 and the irradiation position confirmation device 32 and determines whether the position of the radiating material 13 matches the incident position of the gamma ray 12. If they do not match, they act on the gamma ray beam generator 36 to turn off the emission of the gamma rays 12, and if they match, they act on the gamma ray beam generator 36 to turn on the emission of the gamma rays 12. When receiving a command from the controller 38 to turn off the emission of the gamma ray 12, the gamma ray beam generating device 36 turns off the emission of the gamma ray 12 by turning off the laser of the laser light source. Thus, when the incident position of the gamma ray 12 is shifted from the position of the radiating material 13 due to the movement of the patient 21, the gamma ray 12 can be turned off.
[0065]
The high-precision position control device 33 receives the position information of the radiating material 13 and the position of the incident position of the gamma ray 12 from the high-precision position detector 31 and the irradiation position confirming device 32, respectively, and inputs the position of the radiating material 13 and the incidence of the gamma ray 12. It is possible to determine whether the positions match, and if they do not match, move parallel or rotationally in the matching direction, and stop the movement if they match.
[0066]
In the above method, a high-energy (10 to 50 MeV) gamma ray that is finely collimated and easy to turn on and off is used. As an example of this, a laser beam is made to collide with a giga-level electron beam to obtain a desired gamma ray by the inverse Compton effect. To obtain a giga-level electron beam, a large accelerator is required. On the other hand, a method of obtaining an effective gamma ray from a finely collimated high-energy gamma ray with an electron beam of several tens MeV level will be described below.
[0067]
In the above radiotherapy apparatus, as a gamma ray source, a heavy metal target and graphite are placed in the pores of a heavy metal, and a high energy electron beam is irradiated on the heavy metal target, and a gamma ray having an energy of 10 MeV or more is irradiated with an electron beam. The gamma rays pass through the graphite to absorb low energy components of the gamma rays, and the generated gamma rays are collimated by passing through heavy metal pores. Heavy metal pores act as collimators.
[0068]
The heavy metal irradiated with the electron beam is platinum or gold. It has a minute shape and a diameter of 1 mm or less and a length of 2 mm or less. Use a heavy metal with a large atomic number (short emission length). Tungsten is also possible. These are installed in the holes of heavy metal having pores.
[0069]
The incident electron beam is preferably 10 to 50 MeV, and more preferably 20 to 30 MeV.
[0070]
Graphite is the best absorber for low energy gamma rays. Aluminum is also possible.
[0071]
Lead is the best heavy metal having pores in terms of cost, and gold, platinum and tungsten are also possible.
[0072]
The neutron moderator is a substance containing hydrogen and can be water. Paraffin is good for ease of use.
[0073]
Cadmium is the best neutron absorber, shelf metals and lithium are also acceptable. Cadmium is the thinnest. The paraffin / cadmium combination is best.
[0074]
When an electron beam is irradiated on a radiating material, gamma rays generated generally have a wide spread and many low energy components. The present invention is a method for thinning a generated gamma ray and extracting high energy as a main component.
[0075]
Irradiation with an electron beam generates gamma rays having a wide energy distribution as shown in FIG. Gamma rays having an energy of 10 MeV or less are absorbed by passing through graphite.
[0076]
FIG. 12 shows the degree of absorption. In particular, 1 MeV or less can be reduced to a degree that is not perfect but does not affect the human body. Further, gamma rays leaking laterally are absorbed by heavy metal. Further, since the energy of the electron beam is high, a neutron beam (neutron) which is not preferable for the human body is emitted. Therefore, the neutron is decelerated with paraffin and absorbed with cadmium.
[0077]
Cancer treatment by radiation is roughly classified into a method using particle beams and a method using gamma rays (including electrons). In general, it is considered that a particle beam, particularly a heavy ion, can give a concentrated dose to a cancer tissue, so that treatment can be performed efficiently and safely.
[0078]
As described above, by combining a high-energy gamma ray and a radiating material, it is possible to realize a device for applying a dose to a tumor with an efficiency equal to or higher than that of a particle beam. The characteristics of the radiating material include (1) a substance having a short radiation length and (2) a thin shape.
[0079]
On the other hand, gamma rays are efficient if they are thin and collimated and have high energy.There are several possibilities for actually generating gamma rays.However, gamma rays obtained by laser inverse Compton scattering sufficiently satisfy the conditions. I have. However, this method has a practical problem that the device becomes too large. Therefore, here, as a supplement to this method, a method for obtaining a target gamma ray by using an electron beam accelerator such as a linac (linear accelerator) generally used as a medical gamma ray generator is presented. .
[0080]
The characteristics of gamma rays required for treatment using a radioactive material include the following three points.
(1) High energy gamma rays of 5 MeV or more (more preferably, 10 MeV or more) and several tens of MeV or less
(2) Collimated as thin as about millimeter
(3) Do not generate extra radiation
These conditions are for increasing the dose in the vicinity of the radiating material placed in the tumor part, while reducing the irradiation dose to other parts of the human body.
[0081]
When a high-energy electron beam is applied to a substance, gamma rays are generated by bremsstrahlung. Bremsstrahlung has been used in many cases as a method of generating gamma rays and X-rays, not only for research purposes. If a gamma ray according to the purpose can be generated by this very general method, a simple system should be able to be constructed.
[0082]
Gamma rays are obtained by applying accelerated electrons to a radiating material (heavy metal such as gold or platinum) to cause bremsstrahlung. The characteristics of the gamma ray energy distribution and angle distribution will be described below.
[0083]
It is known that the energy distribution of gamma rays obtained by bremsstrahlung of a sufficiently thin radiating material is given by the following equation.
[0084]
(Equation 1)

[0085]
Because the spectrum is inversely proportional to energy, low energy gamma rays are particularly prevalent in bremsstrahlung radiation. Since low-energy components have a large interaction with substances, when used for radiotherapy, they are absolutely highly absorbed at the surface and have a large effect on skin tissues.
[0086]
On the other hand, gamma rays emitted by bremsstrahlung are radiated forward in the same direction as the incident direction of the electron beam, but spread by a certain angle, which is given by the following equation.
[0087]
(Equation 2)

[0088]
Here, m is the mass of the electron, p is the energy of the electron beam, and θ is the angle at which gamma rays are emitted. The ratio of the gamma ray emitted within a certain angle θ to the total angle is m / p≪1 since the case where the electron energy is sufficiently larger than its mass m is assumed, and the distribution is concentrated forward. Since it is, sin θ to θ can be obtained. Therefore, it becomes as follows.
[0089]
(Equation 3)

[0090]
The amount obtained by integrating all angles is π · p²/ M²So the gamma ray is m²/ P²Half will be collected within the angle represented by. Typically, for 30 MeV electrons, the spread is 1/60 rad.
[0091]
A method of irradiating a radiation material with an electron beam accelerated to about 10 MeV and generating gamma rays by bremsstrahlung is generally used in cancer treatment. As described above, a linac (linear accelerator) is generally used as the accelerator. This is because if the energy is about 10 MeV, a sufficient penetrating power can be obtained, and the generation of unnecessary neutrons is suppressed.
[0092]
By the way, a gamma ray of about 10 to 50 MeV is suitable for treatment using a radiation material. In order to obtain a thin collimated gamma ray, higher energy is more efficient. This requires an electron beam with high energy. When the energy of the electron beam begins to exceed 10 MeV, the generation of neutrons cannot be ignored. Therefore, higher energy is better, but about 20 to 30 MeV is practically appropriate.
[0093]
Since the gamma rays are narrowed down, it is necessary to find conditions necessary for obtaining sufficient intensity. The thickness of the radiating material that causes bremsstrahlung to electrons is 0.3L_RThe degree is appropriate. At this thickness, the conversion efficiency to gamma rays is about 30%, and the probability of the generated gamma rays interacting and being lost is about 15%, so that about 25% of the total electron energy is converted to gamma rays. As an actual substance, gold, platinum, tungsten, or the like having a thickness of about 1 mm is placed.
[0094]
In order to extract a gamma-ray beam as thin as the size of the radiating material placed inside the body, the distance must be as short as possible. Considering the distance between the collimator and the body, a distance of 40 cm from the radiating material causing the bremsstrahlung to the radiating material in the body will suffice. Assuming that the diameter of the radiating material in the body is 1 mm, the angular spread is 1.2 mrad. The gamma ray entering this angle is (0.0012 × 60) at 30 MeV.²= 5 × 10^-3It is. As a result, if the current of the 30 MeV electron beam is 1 μA, 2 × 10⁹/ Sec gamma rays are obtained. The required dose is typically obtained in seconds to tens of seconds. If this is 10 MeV, (0.0012 × 20)²= 6 × 10^-4Then, when the energy is reduced, the amount of gamma rays that can be irradiated to the thin forward radiating material is relatively reduced, and the efficiency is reduced by almost one digit compared to the case of 30 MeV. In practice, it can be covered by increasing the current or increasing the time, so it would be of great significance to be able to treat with an accelerator that is relatively easily available.
[0095]
The following apparatus can be used to obtain a gamma ray optimal for a treatment using a radiation material by this method. A collimator is installed to reduce the gamma rays generated by bremsstrahlung to a small beam size. Since gamma rays spread with the distance they fly, shortening the distance from the collimator to the irradiation site allows gamma rays to be used efficiently. Therefore, a material having a short radiation length is suitable as a material. Platinum and gold are optimal, but in view of cost, heavy metal containing tungsten as a main component is the most realistic. Since the radiation length of heavy metal is about 3.7 mm, a few centimeters has a sufficient shielding effect against gamma rays.
[0096]
Gamma rays generated by bremsstrahlung are more common at lower energies, but this gives unwanted doses, especially to the surface of living organisms. To eliminate this, carbon is filled in the gamma ray path and the hole in the collimator. FIG. 12 shows an energy spectrum of gamma rays obtained by bremsstrahlung radiation. When there is no absorbing material, it can be seen that the spectrum inversely proportional to the energy represented by the equation (1) decreases from the low energy region as the carbon thickness increases. 30g / cm along the way²If the thickness is about the same, gamma rays of 1 MeV or less that affect the surface of the human body are reduced by almost one digit, so that the level can be almost harmless. Since the density of graphite is about 2.2, it may be about 15 cm. Using industrial diamonds can make it even shorter.
[0097]
The shape of the collimator is as shown in FIGS. At the center is a collimator for extracting thin gamma rays made of heavy metal. It also plays a role in stopping gamma rays leaking around. Outside it is a neutron moderator. Materials containing much hydrogen, such as paraffin and polyethylene, are excellent. The thickness of the moderator layer can be determined by how much the neutron flux is desired to be reduced. Typically, the thickness of the layer is about 10 cm. However, when the amount of neutrons originally generated is small, the layer can be made small and compact.
[0098]
A cadmium metal sheet is spread around the moderator. If the thickness is about 0.5 to 1 mm, most of the decelerated neutrons can be absorbed. Note that cadmium that has absorbed neutrons emits gamma rays, so lead shielding should be placed outside of it. The gamma rays generated at this time have relatively low energy, so that about 5 cm of lead can be sufficiently shielded.
[0099]
A configuration example of the gamma ray irradiation unit 50 will be described with reference to FIGS.
[0100]
The tungsten (unnecessary gamma ray absorbing portion) 54 is a column, and the inside thereof is hollowed out concentrically. Inside this, from the left, tungsten (unnecessary gamma ray absorbing portion) 55, platinum plate (gamma ray emitting portion) 56, collimator piece 57, collimator piece 58, platinum plate (low energy gamma ray absorbing portion) 61, collimator piece 59, collimator piece 60 is fitted. The collimator pieces 57 to 60 are made of tungsten. A collimator is formed by the collimator pieces 57 to 60, and has a hole through which only the gamma rays emitted from the gamma ray emitting portion having a predetermined radiation angle or less are passed. Although four collimator pieces are used here, the present invention is not limited to this, and one or more, for example, two or more pieces can be used according to the setting.
[0101]
The diameter of the platinum plates 56 and 61 is the same as the inner diameter of the tungsten 54.
[0102]
Tungsten 57 to 60 have a cylindrical shape whose outer shape is congruent with each other, and the diameter of the bottom surface is the same as the inner diameter of tungsten 54. Each of the tungsten 57 to 60 has a hole which is hollowed out in a concentric shape passing through an axis and penetrated therethrough. The hole diameter is constant for each of the tungsten 57 to 60. And the diameter of this hole is smaller as it is closer to the platinum plate 56,
Collimator piece 57 <collimator piece 58 <collimator piece 59 <collimator piece 60.
[0103]
The inner diameter of the tungsten 55 can be set larger than the diameter of the hole of the collimator piece 57 having the smallest hole diameter (closest to the platinum plate 56).
[0104]
55 to 61 are filled in the tungsten 54 so that there is no gap between the bottom surface of the tungsten 54 and the bottom surface.
[0105]
The holes of the collimator pieces 57 to 60 are filled with graphite (low energy gamma ray absorbing portion) 64.
[0106]
Tungsten 54 and 55 are covered with paraffin (neutron moderator) 53. This is to slow down neutrons emitted together with gamma rays from the gamma ray emitting section.
[0107]
The paraffin 53 is covered with a cadmium sheet (neutron absorbing part) 52. This absorbs neutrons emitted together with gamma rays from the gamma ray emitting section.
[0108]
The cadmium sheet 52 is covered with lead (secondary gamma ray absorbing portion) 51. This absorbs gamma rays emitted from the neutron absorber.
[0109]
The lead 51 has a columnar shape, and the gamma ray irradiation unit 50 as a whole has a columnar shape.
[0110]
In FIG. 14, the lead 51 is illustrated as a separate member such as 51 a, 51 b, and 51 c, but it may be a separate member as described above, or these three may be integrated as the lead 51. It may be a member. The same applies to the cadmium sheet 52 (52a, 52b, 52c) and the paraffin 53 (53a, 53b).
[0111]
The platinum plate 56 receives an electron beam 62 from a source (irradiation source) of an electron beam (not shown). As described above, a linac (linear accelerator) or the like generally used as a medical gamma ray generator can be used as a source of the electron beam.
[0112]
When the electron beam 62 hits the platinum plate 56, gamma rays are generated from the platinum plate 56. Of these gamma rays, those with low energy are absorbed by the graphite 64 and the platinum plate 61. Further, of the gamma rays, those that do not pass through the hole of the collimator are absorbed by the collimator pieces 57 to 60. Therefore, the gamma rays 63 that exit the gamma ray irradiating unit 50 through the graphite 64 contain many high energy components.
[0113]
The collimator is formed by stacking collimator pieces 57 to 60 having different inner diameters. That is, in this configuration, the diameter of the collimator changes stepwise and discontinuously, and a separate member is provided for each change position. However, besides this, for example, one tungsten member having a conical inner diameter may be used as the collimator. In this case, the collimator diameter changes continuously.
[0114]
Gamma rays produced by bremsstrahlung are more common at lower energies, but this gives unwanted doses, especially to the surface of living organisms. In order to eliminate this, an absorbing material (low-energy gamma ray absorbing part) is packed in a path through which the gamma ray passes and a hole in the collimator. Assuming that the length of the collimator hole is 10 cm, typically, the first 5 cm is filled with graphite (carbon) 64, and then a 1 mm thick platinum (or gold, tungsten) plate 61 is packed, and the next 5 cm is filled. A configuration in which graphite 64 is packed is possible. That is, in this example, the lengths (5 cm) of the two graphites 64 are equal to each other and are half the length (10 cm) of the hole of the collimator.
[0115]
First, an electron beam is applied to a platinum (or gold, tungsten) plate 56 (converter, conversion material) (gamma ray emitting portion) having a thickness of about 1 mm to generate gamma rays. Having the point of origin as small as possible improves the efficiency of the collimator. Typically, it is desirable that the distance be about a fraction of a millimeter or less. The gamma rays generated here are narrowed down by a collimator.
[0116]
At the point of the conversion material, there are electrons with reduced energy, but if the gamma-ray emitting portion is a heavy metal, gamma rays are newly generated from these electrons. Many of these are unwanted low-energy components that we want to remove. Therefore, an absorber is placed behind the place where the gamma ray is generated, and the low energy component of the generated gamma ray is absorbed and removed while suppressing the decrease of the high energy component. For this, carbon such as graphite 64 is used as described above.
[0117]
The energy loss of electrons in carbon is 2 MeV / (g / cm²), So 5cm (11g / cm²) If it is around, the electrons almost stop.
[0118]
Next, as described above, a thin platinum plate 61 having a thickness of about 1 mm is placed. This effectively lowers low energy gamma rays. Further, about 5 cm of carbon is used as an absorber after that. A heavy metal has a property of lowering its absorption efficiency at an energy lower than the K absorption edge (about 80 keV), so that portion is absorbed again by carbon.
[0119]
The platinum (or gold or tungsten) plate 61 absorbs low energy components (when thick, absorbs high energy components), and the graphite 64 mainly absorbs low energy components and converts electrons into gamma rays. Can be decelerated without generation.
[0120]
FIGS. 15 and 16 show the interaction cross-sections (total absorption areas) with gamma rays for carbon and platinum, respectively. It can be considered to correspond approximately to the probability of absorption.
[0121]
In FIG. 15, the interaction cross section with gamma rays decreases slowly with the gamma ray absorption cross section energy of carbon.
[0122]
In FIG. 16, the interaction cross section with the gamma ray sharply decreases with the gamma ray absorption cross section energy of platinum. Also, when the energy exceeds a few MeV, it starts increasing. Further, there is a point where the cross-sectional area decreases at about 80 keV.
[0123]
The extent to which gamma rays are reduced when passing through a substance with a thickness t is determined using the absorption coefficient σ.
N (t) = N₀・ Exp (-σt)
It is expressed as Where the number before passing through the substance is N₀, N (t), and the base x of the natural logarithm to the power x is expressed as exp (x).
[0124]
15 and 16, the vertical axis represents σ (unit: cm)²/ G). The thickness of the material is g / cm taking into account the density of the material, not the unit of length.²It is convenient to decide in.
[0125]
Taking platinum (specific gravity 21.45) as an example, from FIG.
Σ = 6cm when E = 0.1MeV²/ G
Σ = 0.06 cm when E = 1 MeV²/ G
It can be seen that it is. Here, 1mm thick platinum
t = 22.45 × 0.1 = 2.245 g / cm²
Of thickness. Therefore
N (0.1 MeV) = exp (−6 × 2.245) N₀= 1.4 × 10^-6N₀
N (1MeV) = exp (−0.06 × 2.245) N₀= 0.87N₀
Thus, the gamma ray of 0.1 MeV can be almost completely dropped, while the gamma ray of 1 MeV almost remains.
[0126]
For carbon 10 cm (specific gravity of graphite is about 2.2), t = 22 g / cm²And from FIG.
Σ = 0.16cm when E = 0.1MeV²/ G, N = 0.03N₀
Σ = 0.06 cm when E = 1 MeV²/ G, N = 0.27N₀
Σ = 0.02 cm when E = 10 MeV²/ G, N = 0.64N₀
It can be seen that the energy dependence is not extreme, but the high energy side can be left.
[0127]
【The invention's effect】
As described above, the radiation therapy apparatus according to the present invention includes an electron positron pair emission unit that emits an electron positron pair when a gamma ray hits, generates an electromagnetic shower, and irradiates an affected part in the treatment target with an electron positron pair. And a gamma ray irradiating unit for irradiating a gamma ray from outside to the electron-positron pair emitting unit.
[0128]
As a result, the gamma rays emitted from the gamma ray irradiating section hit the electron positron pair emitting section, and the electron positron pair emitting section emits the electron positron pair. This electron-positron pair develops into an electromagnetic shower and irradiates the affected part near the gamma ray irradiating part, so that the affected part can be subjected to radiation treatment.
[0129]
Therefore, it is possible to irradiate an affected part such as a tumor with a large amount of radiation while minimizing damage to a normal tissue, thereby providing an effect that radiation treatment can be efficiently performed.
[0130]
Further, in addition to the above configuration, the radiation therapy apparatus according to the present invention further includes an electron positron pair emission unit position detection unit that detects a position of the electron positron pair emission unit in the treatment target, and a gamma ray in the treatment target. A gamma ray irradiation position detecting unit that detects a position where irradiation is performed, a position of the electron positron pair emitting unit in the treatment target detected by the electron positron pair emitting unit position detecting unit, and a position detected by the gamma ray irradiation position detecting unit. When the information indicating that the position where the gamma ray is irradiated is shifted in the treatment target body is input, the position of the electron-positron pair emitting unit is returned to the position where the gamma ray is irradiated, and a position control unit is returned. A configuration including a gamma-ray irradiation control unit that controls the gamma-ray irradiation unit so as to turn off the gamma-ray irradiation while the position of the electron-positron pair emission unit and the position where the gamma ray is irradiated are shifted. A.
[0131]
Accordingly, in addition to the effect of the above configuration, there is an effect that gamma rays can be irradiated without waste.
[0132]
Further, in addition to the above configuration, the radiation therapy apparatus according to the present invention, wherein the gamma ray irradiator, the gamma ray emitter that emits a gamma ray by bremsstrahlung when hit by an electron beam, and the gamma ray emitted from the gamma ray emitter. Among them, a collimator having a hole through which only those having a predetermined emission angle or less, and placed between the gamma-ray emitting unit and the collimator, the energy of the gamma rays emitted from the gamma-ray emitting unit is less than or equal to a predetermined value. And a low-energy gamma ray absorbing unit that absorbs light.
[0133]
Thereby, in addition to the effect of the above configuration, it is possible to safely and efficiently obtain high-energy gamma rays using an existing accelerator that is already available for cancer treatment or the like without using an expensive accelerator at the giga level. The effect is that the affected part can be sufficiently treated with the electromagnetic shower.
[0134]
Further, the radiotherapy apparatus according to the present invention has a configuration in which, in addition to the above-described configuration, an unnecessary gamma ray absorbing unit that absorbs gamma rays emitted from the gamma ray emitting unit and not passing through the hole of the collimator is provided.
[0135]
Thereby, in addition to the effect of the above configuration, there is an effect that unnecessary gamma rays can be absorbed.
[0136]
In addition, the radiotherapy apparatus according to the present invention has a configuration in which, in addition to the above configuration, a neutron absorbing unit that absorbs neutrons emitted together with gamma rays from the gamma ray emitting unit around the gamma ray emitting unit.
[0137]
Thereby, in addition to the effect of the above configuration, there is an effect that unnecessary neutrons can be absorbed.
[0138]
In addition, the radiotherapy apparatus according to the present invention, in addition to the above configuration, between the gamma ray emitting section and the neutron absorbing section, a neutron moderating section that slows down neutrons emitted together with gamma rays from the gamma ray emitting section. It is a configuration provided.
[0139]
Thereby, in addition to the effect of the above configuration, there is an effect that unnecessary neutrons can be decelerated.
[0140]
Further, in addition to the above configuration, the radiation therapy apparatus according to the present invention has a property that the neutron absorbing section emits gamma rays when absorbing neutrons, and the neutron absorbing section is provided around the neutron absorbing section. And a secondary gamma ray absorbing unit that absorbs gamma rays emitted from the device.
[0141]
Thereby, in addition to the effect of the above configuration, there is an effect that unnecessary gamma rays can be absorbed.
[0142]
In addition, in the radiotherapy apparatus according to the present invention, in addition to the above-described configuration, the collimator is a cylindrical object having a congruent outer shape, and a hole having a cylindrical gamma ray path penetrating around an axis. In this configuration, a plurality of collimator pieces having different diameters are coaxially arranged without gaps in such an order that the hole diameter increases as the distance from the gamma-ray emitting section increases.
[0143]
Thus, the emission angle range of gamma rays can be easily changed only by changing the combination of the collimator pieces. Therefore, in addition to the effect of the above configuration, there is an effect that the required gamma ray intensity can be more easily obtained.
[Brief description of the drawings]
FIG. 1 is a view showing the concept of radiation therapy according to the present invention.
FIG. 2 is a plan view showing a relationship between an incident position of a gamma ray and an arrangement of a radiating material.
FIG. 3 is a graph showing the relationship between the depth of a radiation material and the amount of accumulated radiation dose when the radiation material is irradiated with gamma rays.
FIG. 4 is a graph showing the relationship between the depth of the radiating material and the amount of accumulated radiation dose when the radiating material is irradiated with gamma rays.
FIG. 5 is a graph showing the relationship between the depth of the radiating material and the amount of accumulated radiation dose when the radiating material is irradiated with gamma rays.
FIG. 6 is a graph showing the relationship between the radius at the depth where the dose is the highest and the amount of accumulated radiation dose when the gamma ray is irradiated onto the radiation material.
FIG. 7 is a graph showing the relationship between the radius at the depth where the dose is highest and the amount of accumulated radiation dose when the gamma ray is irradiated onto the radiation material.
FIG. 8 is a graph showing the relationship between the radius at the depth where the dose is highest and the amount of accumulated radiation dose when the radiation material is irradiated with gamma rays.
FIG. 9 is a view showing the concept of radiation therapy according to the present invention.
FIG. 10 is a plan view showing a relationship between an incident position of a gamma ray and an arrangement of a radiating material.
FIG. 11 is a plan view showing a configuration example of a radiotherapy apparatus according to the present invention.
FIG. 12 is a graph showing an energy spectrum of a gamma ray.
FIG. 13 is a sectional view showing a gamma ray irradiation unit.
FIG. 14 is a perspective view showing members constituting a gamma ray irradiation unit.
FIG. 15 is a graph showing the relationship between the energy of gamma rays in carbon and the cross-sectional area of interaction with gamma rays.
FIG. 16 is a graph showing the relationship between the energy of gamma rays in platinum and the cross-sectional area of interaction with gamma rays.
FIG. 17 is a diagram showing the concept of radiation therapy.
FIG. 18 is a graph showing changes in damage caused by gamma rays in radiation therapy.
FIG. 19 is a diagram showing an example of radiation therapy using gamma rays.
[Explanation of symbols]
11 water
12 gamma ray
13 radiation material (electron positron pair emission part)
14 electromagnetic shower
21 patient (subject to be treated)
22 tumor
31 ° high-accuracy position detector (electron positron pair emission position detector)
32 ° irradiation position confirmation device (gamma ray irradiation position detection unit)
33 ° high-accuracy position controller (position controller)
36 gamma ray beam generator (gamma ray irradiation unit)
37 ° collimator (gamma ray irradiator)
38 controller (gamma ray irradiation controller)
50 ° gamma ray irradiation unit
51, 51a, 51b, 51c lead (secondary gamma ray absorbing part)
52, 52a, 52b, 52c cadmium sheet (neutron absorbing part)
53, 53a, 53b paraffin (neutron moderator)
54 ° tungsten (unnecessary gamma ray absorbing part)
55 ° tungsten (unnecessary gamma ray absorbing part)
56mm platinum plate (gamma ray emission part)
57, 58, 59, 60 Tungsten (collimator)
61% platinum plate (low energy gamma ray absorbing part)
62 electron beam
63 gamma rays
64% graphite (low energy gamma ray absorbing part)

Claims (8)

An electron positron pair emitting section that emits an electron positron pair when gamma rays hit and generates an electromagnetic shower to irradiate an affected part in a treatment target body,
A gamma ray irradiating unit for irradiating a gamma ray from the outside of the treatment target and applying the gamma ray to the electron-positron pair emitting unit. An electron-positron pair emission unit position detection unit that detects the position of the electron-positron pair emission unit in the treatment target;
A gamma ray irradiation position detection unit that detects a position where gamma rays are irradiated in the treatment target,
The position of the electron positron pair emission unit in the treatment target detected by the electron positron pair emission unit position detection unit and the position irradiated with gamma rays in the treatment target detected by the gamma ray irradiation position detection unit are When the information indicating the shift is input, a position control unit that returns the position of the electron-positron pair emitting unit to the position where the gamma rays are irradiated,
A gamma-ray irradiation control unit that controls the gamma-ray irradiation unit so as to turn off the gamma-ray irradiation while the position of the electron-positron pair emitting unit and the position where the gamma rays are irradiated are shifted. The radiotherapy device according to claim 1. The gamma ray irradiation unit,
A gamma ray emitting unit that emits gamma rays by bremsstrahlung when the electron beam hits,
Among the gamma rays emitted from the gamma ray emitting unit, a collimator having a hole that passes only those having a predetermined radiation angle or less,
A low-energy gamma-ray absorbing unit that is disposed between the gamma-ray emitting unit and the collimator and absorbs gamma rays emitted from the gamma-ray emitting unit whose energy is equal to or less than a predetermined value. Item 3. The radiotherapy apparatus according to item 1 or 2. 4. The radiation therapy apparatus according to claim 3, further comprising an unnecessary gamma ray absorbing section for absorbing gamma rays emitted from the gamma ray emitting section and not passing through a hole of the collimator. The radiation therapy apparatus according to claim 3, further comprising a neutron absorbing unit around the gamma ray emitting unit for absorbing neutrons emitted together with the gamma rays from the gamma ray emitting unit. 5. The radiotherapy apparatus according to claim 4, further comprising a neutron moderating unit between the gamma ray emitting unit and the neutron absorbing unit, for slowing down neutrons emitted together with the gamma rays from the gamma ray emitting unit. The neutron absorbing portion has a property of emitting gamma rays when absorbing neutrons,
The radiation therapy apparatus according to claim 5, further comprising a secondary gamma ray absorbing section that absorbs gamma rays emitted from the neutron absorbing section around the neutron absorbing section. The collimator is
A plurality of collimator pieces whose outer shapes are congruent cylindrical shapes and have different diameters of holes serving as cylindrical gamma ray paths penetrating around the axis, such that the hole diameter increases as the distance from the gamma ray emission part increases The radiotherapy apparatus according to any one of claims 3 to 7, wherein the radiotherapy apparatuses are coaxially arranged without any gap in the order.

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