JP4076869B2 - Radiation therapy equipment - Google Patents

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・ｐ²／ｍ²なので、ガンマ線はｍ²／ｐ²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、２×１０⁹／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／ｃｍ²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／ｃｍ²）程度なので、５ｃｍ（１１ｇ／ｃｍ²）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）＝Ｎ₀・ｅｘｐ（−σｔ）
と表される。ただし、物質を通る前の個数をＮ₀、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：ｃｍ²／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／ｃｍ²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＝６ｃｍ²／ｇ
Ｅ＝１ＭｅＶのとき σ＝０．０６ｃｍ²／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／ｃｍ²
の厚さとなる。よって
Ｎ（０．１ＭｅＶ）＝ｅｘｐ（−６×２．２４５）Ｎ₀＝１．４×１０^-6Ｎ₀
Ｎ（１ＭｅＶ）＝ｅｘｐ（−０．０６×２．２４５）Ｎ₀＝０．８７Ｎ₀
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／ｃｍ²で、図１５より
Ｅ＝０．１ＭｅＶのときσ＝０．１６ｃｍ²／ｇ、Ｎ＝０．０３Ｎ₀
Ｅ＝１ＭｅＶのとき σ＝０．０６ｃｍ²／ｇ、Ｎ＝０．２７Ｎ₀
Ｅ＝１０ＭｅＶのとき σ＝０．０２ｃｍ²／ｇ、Ｎ＝０．６４Ｎ₀
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]
BACKGROUND OF THE INVENTION
The present invention relates to a radiotherapy apparatus for a tumor or the like using gamma rays.
[0002]
[Prior art]
X-rays and gamma rays are used for the treatment of tumors (cancer) by radiation, and among them, gamma rays are widely used.
[0003]
Tumor radiation therapy utilizes the loss of energy as radiation passes through the body, causing biological damage (hereinafter simply referred to as damage) to the tumor tissue. As shown in the upper part of FIG. 17, radiation 41 is irradiated from the outside to the tumor 22 in the patient 21 such as a person or 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 effects of radiation are not limited to tumors but also to normal tissues, the basis of radiation therapy is to maximize the dose in tumors and minimize the dose in normal tissues. 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 exists, and that the damage does not occur at other depths.
[0005]
Known techniques include Patent Documents 1 and 2.
[0006]
[Patent Document 1]
Japanese Patent Laid-Open No. 11-151310 (publication date: June 8, 1999)
[0007]
[Patent Document 2]
JP 2000-331799 A (publication date November 30, 2000)
[0008]
[Problems to be solved by the invention]
Although gamma rays can be easily obtained, as shown in FIG. 18, normal tissues are also damaged. In FIG. 18, the horizontal axis represents the depth as described above, and the vertical axis represents the degree of damage as described above.
[0009]
For this reason, as shown in FIG. 19, a method (gamma knife) in which gamma rays 12 are irradiated from four directions and focused on the tumor 22 has been developed. However, if the tumor 22 is sufficiently damaged, the dose integrated into the normal tissue is increased.
[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 respect. The Bragg peak means that the maximum radiant energy is released immediately before the radiation stops. However, charged particles such as heavy ions and protons require a high-energy accelerator, and in reality, a dedicated beam line is required. Moreover, since the effective range is small, there is little merit in a large affected part.
[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 tissue.
[0012]
[Means for Solving the Problems]
In order to solve the above problems, the radiotherapy apparatus according to the present invention emits an electron-positron pair to emit an electromagnetic shower when irradiated with gamma rays, and irradiates the affected part in the treatment target body with an electron-positron pair emitting part, And a gamma ray irradiation unit that irradiates gamma rays from the outside of the treatment target and hits the electron-positron pair emission unit.
[0013]
With the above configuration, the gamma ray irradiation unit irradiates gamma rays from the outside of the treatment object (human body or the like) and hits the electron-positron pair emission unit. Then, an electron positron pair is emitted from the electron positron pair emitting part, which develops into an electromagnetic shower and is irradiated to the affected part (tumor or the like).
[0014]
Therefore, by arranging the electron-positron pair emitting part 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 treated with radiation. When the affected part exists inside the treatment object, 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 therefrom decreases as it travels a predetermined distance. Therefore, the influence on the normal tissue around the affected part can be suppressed.
[0015]
Therefore, it is possible to irradiate a large amount of radiation to the affected area such as a tumor while suppressing damage to normal tissue as much as possible, and to perform radiotherapy efficiently.
[0016]
In addition to the above-described configuration, the radiotherapy apparatus according to the present invention includes an electron positron pair emission part position detection unit that detects the position of the electron positron pair emission part in the treatment object, and a gamma ray in the treatment object. Detected by the gamma-ray irradiation position detector for detecting the irradiated position, the position of the electron-positron pair emitter in the treatment object detected by the electron-positron pair emitter position detector and the gamma-ray irradiation position detector. A position control unit that returns the position of the electron-positron pair emission unit to the position irradiated with gamma rays when information indicating that the position irradiated with gamma rays is shifted in the treatment target is input. And a gamma ray irradiation control unit for controlling the gamma ray irradiation unit so that the gamma ray irradiation is turned off while the position of the electron-positron pair emission unit is shifted from the position where the gamma ray is irradiated. It is characterized.
[0017]
With the above configuration, the generation of gamma rays is turned off while the position of the gamma ray irradiation unit in the treatment object is shifted from the position where the gamma ray is irradiated in the treatment object.
[0018]
Therefore, in addition to the effect of the above configuration, gamma rays can be irradiated without waste.
[0019]
In addition to the above-described configuration, the radiotherapy apparatus according to the present invention includes a gamma ray emitting unit that emits gamma rays by bremsstrahlung when an electron beam hits, and a gamma ray emitted from the gamma ray emitting unit. Among them, the collimator having a hole through which only one having a predetermined radiation angle or less passes, and the gamma ray emitting unit and the collimator are placed between the gamma ray emitting unit and the gamma ray emitted from the gamma ray emitting unit has an energy of a predetermined value or less. It is characterized by having a low-energy gamma ray absorber that absorbs things.
[0020]
With the above configuration, when a high energy electron beam is applied to the gamma ray emission part in order to obtain a high energy gamma ray, a high energy gamma ray is emitted over a wide range (large radiation angle). High energy gamma rays can be emitted only in a desired narrow range. As a result, it is possible to prevent high-energy gamma rays from being irradiated to an irrelevant part of the patient without entering the electron-positron pair emission part. Even if the emission range is narrowed in this way, the efficiency is poor if low-energy gamma rays are mixed, 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 accelerators at the giga level. The affected area can be sufficiently treated with an electromagnetic shower.
[0022]
The radiotherapy apparatus according to the present invention is characterized in that, 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.
[0023]
With the above configuration, an unnecessary gamma ray absorbing portion that absorbs gamma rays emitted from the gamma ray emitting portion and not passing through the hole of the collimator is provided.
[0024]
Therefore, in addition to the effects of the above configuration, unnecessary gamma rays can be absorbed.
[0025]
The radiotherapy apparatus according to the present invention is characterized in that, in addition to the above-described configuration, a neutron absorber that absorbs neutrons emitted together with gamma rays from the gamma ray emitter is provided around the gamma ray emitter. Yes.
[0026]
With the above configuration, a neutron absorber that absorbs neutrons emitted together with gamma rays from the gamma ray emitter is provided around the gamma ray emitter.
[0027]
Therefore, in addition to the effects of the above configuration, unnecessary neutrons can be absorbed.
[0028]
In addition to the above-described configuration, the radiotherapy apparatus according to the present invention further includes a neutron moderating unit that decelerates neutrons emitted together with gamma rays from the gamma ray emitting unit between the gamma ray emitting unit and the neutron absorbing unit. It is characterized by having prepared.
[0029]
With the above configuration, a neutron moderating unit that decelerates neutrons emitted together with gamma rays from the gamma ray emitting unit is provided between the gamma ray emitting unit and the neutron absorbing unit.
[0030]
Therefore, unnecessary neutrons can be decelerated in addition to the effects of the above configuration.
[0031]
In addition to the above configuration, the radiotherapy apparatus according to the present invention has a property that the neutron absorber emits gamma rays when it absorbs neutrons, and a neutron absorber around the neutron absorber A secondary gamma ray absorbing portion that absorbs gamma rays emitted from the device is provided.
[0032]
With the above configuration, the neutron absorber has a property of emitting gamma rays when absorbing neutrons, and a secondary gamma ray absorber that absorbs gamma rays emitted from the neutron absorber around the neutron absorber. Is provided.
[0033]
Therefore, in addition to the effects of the above configuration, unnecessary gamma rays can be absorbed.
[0034]
In addition to the above-described configuration, the radiotherapy apparatus according to the present invention may be configured such that the collimator is a cylindrical object having a congruent outer shape, and is a cylindrical gamma ray path penetrating about an axis. A plurality of collimator pieces having different diameters are arranged on the same axis 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 collimator diameter changes stepwise and discontinuously, and is a separate member for each changing position. As a result, if the collimator pieces are combined in various ways, the inclination of the collimator can be easily changed. For example, the diameter of the collimator can be reduced by combining many different hole diameters. Conversely, if a combination of large hole diameters is combined, the diameter of the collimator increases abruptly as it progresses, so the diameter of the collimator can be increased.
[0036]
Therefore, the gamma ray emission angle range can be easily changed by simply changing the combination of the collimator pieces. Therefore, in addition to the effects of the above configuration, the necessary gamma ray intensity can be obtained more easily.
[0037]
DETAILED DESCRIPTION OF THE INVENTION
An embodiment of the present invention will be described below with reference to FIGS.
[0038]
First, the basic concept in this embodiment will be described.
[0039]
Gamma rays with high energy above a certain level have a low interaction with water and living bodies such as human bodies and animals, and pass through with little influence on water and living bodies. That is, there is little energy loss in the living body. Here, the interaction refers to an effect that causes a result of destroying a tumor tissue in the case of a tumor, and more generally refers to an effect that results in giving high energy to an object. This embodiment pays attention to this, and effectively performs minimally invasive radiation therapy on a living body using gamma rays.
[0040]
The energy value of gamma rays to be used is determined by, for example, the ability of the gamma rays of that energy value to inherently cause interactions and other equipment and devices to prevent interaction with the living body. It can be determined in consideration of the presence or absence and the effects thereof. For example, those of 5 MeV or more and 50 MeV or less have a low interaction with water or a living body as described above, and are considered to pass through with little influence on water or the living body. In addition, those having 5 MeV or more mainly generate electron-positron pairs as a result of the interaction. Therefore, it is also possible to employ a value having such a value.
[0041]
The energy value of the gamma ray to be used can also be determined in consideration of the characteristics of the radiation material that emits an electron-positron pair described later.
[0042]
As shown in FIG. 1, a radiation material (electron positron pair emitting part) 13 is placed inside water 11 corresponding to a living body such as a human body or an animal. Then, gamma rays 12 are irradiated from the outside of the water 11. By doing in this way, the radiation material 13 discharge | releases an electron positron pair.
[0043]
That is, as shown in FIG. 1 and FIG. 2, a thin collimated high energy gamma ray 12 is formed on the bottom surface of the columnar heavy metal radiation material 13 (when it is formed linearly and cut along a plane perpendicular to the length direction). When irradiated to the cross section, the radiating material 13 generates an electron positron pair, and emits the electron positron pair and the electromagnetic shower 14 in which the electron positron pair is developed mainly from the surface (side surface) in the radial direction (vertical direction in the drawing). The affected part is irradiated with this electron-positron pair and the electromagnetic shower 14 developed from the electron-positron pair, and treatment is performed.
[0044]
When irradiated with gamma rays, the radiating material 13 emits electron-positron pairs, which develop into an electromagnetic shower 14 (see FIG. 2). The electromagnetic shower is radiation including gamma rays, electrons, and positrons emitted from an object (here, the radiation material 13). Radiation length L of radiation material_RIs shorter, because it is possible to irradiate only the affected part with an electromagnetic shower, and 5 mm or less is preferable. The radiation length represents a material-specific length necessary for converting gamma rays into electron-positron pairs at a constant conversion rate. The radiation 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 generation rate of electron positron pairs 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 increase a sufficient therapeutic effect, it is preferable that most of the gamma rays are converted into electron-positron pairs with the size of the affected area. Gamma rays are 3L in the radiation material_RAfter passing a certain distance, it can be said that most gamma rays are converted into electron-positron pairs. Therefore, as the radiation material, the distance through which gamma rays pass in the radiation material is 3L._RThe length of the radiating material is about the size of the affected area (when the radiating material is arranged in the affected area, the size of the radiating material can be embedded in the affected area). Substances are preferred. As an example of the radiation material 13, heavy metal is advantageous in that the radiation length is short. Examples of heavy metals include platinum, gold, tungsten, lead, and mercury. In consideration of putting in the human body, lead and mercury are not preferable, and platinum is preferable.
[0045]
Heavy metals are usually used as radiation shielding materials. In the present embodiment, the treatment is performed by extracting the attenuated radiation (electron positron pair) and irradiating the affected area by using the property in reverse.
[0046]
As the radiation material 13, for example, a columnar (needle) shape can be used. In this case, for example, a material having a diameter (up and down direction in FIG. 1) of 2 mm or less and a length (left and right direction in FIG. 1) of 5 mm or more can be used. For example, when the radiation 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]
Gamma rays 12 are applied to the radiation material 13. For example, the gamma rays 12 can be applied to a surface (bottom surface) perpendicular to the length direction of the column (or needle) of the radiation material 13.
[0048]
When the radiating material 13 is irradiated with the gamma rays 12, the gamma rays 12 pass through the gamma rays 12 in the respective portions inside the radiating material 13, that is, in the case of a column (needle) -shaped radiating material 13 in the length direction ( From the surface along the horizontal direction in FIGS. 1 and 2 (the side surface in the case of a pillar (needle) as shown in FIG. 2), the electron-positron pair and the electromagnetic shower 14 in which the electron-positron pair has evolved are exposed to the outside of the radiating member 13. And released. Therefore, from the viewpoint of enabling efficient emission of such electron-positron pairs, the shape of the radiating material 13 is long in the passing direction of the gamma rays 12 and the passing direction of the gamma rays 12 as shown in FIG. The distance between the gamma ray 12 and the surface of the radiating material 13 in the direction perpendicular to the surface, that is, the column (needle) -shaped radiating material 13, preferably has a short thickness direction (vertical direction in FIGS.
[0049]
As shown in FIG. 10, the longitudinal direction of the columnar (needle) -shaped radiation material 13 is perpendicular to the incident direction of the gamma rays 12, and the surface along the length direction (vertical direction in FIG. 10) of the radiation material 13, When gamma rays 12 are irradiated on the side surfaces of the columnar (needle) -shaped radiation material 13, the ratio (conversion rate) of conversion to electron-positron pairs when the gamma rays 12 pass through the radiation material 13 is extremely reduced, In addition, even if an electron-positron pair is generated from each gamma ray passage site of the radiation material 13, the distance of the radiation material 13 that passes through the radiation material 13 before reaching the outside, that is, the affected area such as a tumor, is. Since it is long, the attenuation becomes remarkable, and there is a possibility that a sufficient amount of electron-positron pairs will not reach the affected area. Therefore, when such a column (needle) -shaped radiation material 13 is used, it is preferable to irradiate the bottom surface with gamma rays 12.
[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 radiation material 13 corresponding to the position of the tumor part is, for example, 100 mm as shown in FIG. Can do.
[0051]
As described above, the gamma ray 12 can be high energy in order to reduce the interaction with the living body, for example, 5 MeV or more, 50 MeV or less, or 30 MeV or less, for example. Also, for example, a very finely collimated material can be used. In order to generate very fine collimated high-energy gamma rays, a giga-level (1 GeV or higher) electron beam is created by an accelerator (not shown), and the electron beam is made to collide with the laser beam to increase the energy by the inverse Compton effect. Get energy gamma rays. The high energy gamma rays (photon beam) obtained in this way are concentrated in the front (traveling direction), and the energy depends on the angle. That is, high-energy gamma rays are localized in a range within a certain angle from the traveling direction. Therefore, it is possible to selectively extract only high energy gamma rays by using a thin collimator.
[0052]
If the energy depends on the angle as described above, the energy value of the gamma ray to be selected (how much the beam diameter of the gamma ray is to be adopted) It depends on what you want to do. The energy value of the gamma ray to be selected depends on how safe the energy value is for the patient, in other words, how much interaction can be allowed.
[0053]
If the beam diameter of the gamma rays is too large, if the radiating material 13 is small, the gamma rays around the beam diameter do not enter the radiating material 13 and the efficiency is poor. 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 rays is reduced to substantially the same diameter as the cross section of the surface of the radiation material 13 on which the gamma rays 12 are incident.
[0054]
Next, an example of the simulation assuming the above configuration and the result thereof will be shown.
[0055]
Here, as a method of generating gamma rays, a carbon dioxide gas laser having a wavelength of 10.2 μV and a wavelength of 10.6 μm was collided with an electron beam of 2.8 GeV to obtain 15 MeV gamma rays. In this case, gamma rays with energy higher than 7 MeV exist within 0.2 m radians.
[0056]
A platinum wire having a length of 10 mm and a diameter of 1 mm on the bottom surface was used as the radiation member 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. The positron interacts with electrons in the substance and emits 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 ray 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 the case where E = 10 MeV, 20 MeV, and 40 MeV in which the dose is concentrated in the vicinity of the platinum wire. However, the higher the energy, the wider the area can be irradiated. Therefore, more effective treatment is possible by adjusting the energy.
[0058]
3 to 5 show the relationship between the depth and the accumulated amount of radiation dose (emitted dose distribution). 6 to 8, the relationship between the radiation dose accumulation amount and the radius is represented by the absorbed dose Gy (for 10 to the 10th power photons) at the deepest depth.
[0059]
As shown in FIGS. 3 to 5, energy accumulation by the electromagnetic shower is concentrated in the vicinity of the platinum wire. Considering the length where the dose becomes one-tenth as the range in which the electron-positron pair is effectively irradiated, as shown in FIGS. 6 to 8, it is 0.8 cm in the case of 10 MeV, and 1 in the case of 20 MeV. In the case of 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 made from high-energy gamma rays. Therefore, the penetrating power is strong and a dose can be given to a wider area. FIGS. 3 to 5 and FIGS. 6 to 8 first show that the dose distribution is concentrated around the radiation material 13. 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 as the energy increases. 6 to 8 show the radial spread. That is, the dose distribution can be adjusted to the size of the affected part by adjusting the energy, 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 configuration is applied to a tumor 22 in a patient (treatment object) 21 such as a human body / animal. The radiation material 13 is arranged in the vicinity of the affected part, that is, the tumor 22. For example, the radiation material 13 is embedded inside the tumor 22. In this state, gamma rays 12 are irradiated from the outside of the patient 21, and when it hits the radiating material 13, the electron positron pair and the electromagnetic shower in which the electron positron pair is developed are emitted from the radiating material 13. The electron-positron pair and the electromagnetic shower in which the electron-positron pair has developed pass through the inside of the radiation material 13 and hit the tumor 22 in the vicinity of the radiation material 13 to damage the tumor 22. As a result, the tumor 22 is destroyed and the affected area can be treated with radiation. The intensity of the electron-positron pair and the electromagnetic shower in which the electron-positron pair is developed decreases as it travels 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 radiation material 13 are set so that the intensity of the electron-positron pair and the electromagnetic shower in which the electron positron pair is developed sufficiently decreases 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 in which the electron-positron pair has developed on the normal tissue around the tumor 22.
[0062]
FIG. 11 shows a more specific device example. The patient 21 is placed on the high-accuracy position control device (position control unit) 33 such as lying down. The high-accuracy position control device 33 controls the position of the patient 21 and thereby controls the position of the radiation member 13. The high-accuracy position control device 33 controls horizontal and vertical translations, and uses horizontal and vertical straight lines as axes. It may be capable of rotational movement. The radiation material 13 is embedded in the affected area of the patient 21. A three-dimensional high-accuracy position detector (electron positron pair emission position detector) 31 using a metal detector detects the position of the radiation material 13 (heavy metal in this case) 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 rays 12 in the patient 21.
[0063]
Further, a gamma ray generating device 36 is provided, and the gamma ray 12 emitted therefrom is collimated finely by a collimator 37 using lead or the like and enters the patient 21. The gamma ray beam generator 36 can use, for example, an electron beam source that emits energy of 1 GeV or more, and a laser light source (both not shown), which are composed of an electron linear accelerator and a converging lens. The electron beam from the electron beam source and the laser from the laser light source collide, and the gamma rays 12 are extracted as scattered light by the inverse Compton effect. The gamma ray generator 36 and the collimator 37 constitute a gamma ray irradiation unit.
[0064]
A controller (gamma ray irradiation control unit) 38 receives position information from the high-accuracy position detector 31 and the irradiation position confirmation device 32, and determines whether the position of the radiation material 13 and the incident position of the gamma ray 12 match. If they do not match, it acts on the gamma ray beam generator 36 to turn off the emission of the gamma rays 12, and if they match, it acts on the gamma ray beam generator 36 to turn on the emission of the gamma rays 12. When receiving a command to turn off the emission of the gamma ray 12 from the controller 38, the gamma ray beam generator 36 turns off the emission of the gamma ray 12 by turning off the laser of the laser light source. As a result, the gamma ray 12 can be turned off when the incident position of the gamma ray 12 deviates from the position of the radiation member 13 due to the patient 21 moving.
[0065]
The high-accuracy position controller 33 receives the position information of the radiation material 13 and the incident position of the gamma rays 12 from the high-accuracy position detector 31 and the irradiation position confirmation device 32, respectively. It is possible to determine whether or not the positions match, and if they do not match, it is possible to move in parallel or rotationally in the matching direction, and to stop the movement if they match.
[0066]
In the above method, high energy (10 to 50 MeV) gamma rays that are finely collimated and easily turned on and off are used. As an example for this purpose, a laser beam collides with a giga-level electron beam to obtain a desired gamma ray by the inverse Compton effect. In order to obtain a giga-level electron beam, a large-scale accelerator is required. On the other hand, a method for obtaining effective gamma rays with an electron beam of several tens MeV level will be described below.
[0067]
In the above radiotherapy apparatus, as a gamma ray generation source, a heavy metal target and graphite are installed in a heavy metal pore, a high energy electron beam is irradiated to the heavy metal target, and a gamma ray containing energy of 10 MeV or more is emitted as an electron beam. Is generated from the opposite side, the gamma rays are passed through the graphite to absorb low energy components of the gamma rays, and the generated gamma rays are collimated by passing through the heavy metal pores. Heavy metal pores act as a collimator.
[0068]
The heavy metal that irradiates the electron beam is platinum or gold. It has a minute shape and has a diameter of 1 mm or less and a length of 2 mm or less. Use heavy metals with large atomic numbers (short emission length). Tungsten is also possible. These are installed in the hole of the heavy metal which has a pore.
[0069]
The incident electron beam is preferably 10 to 50 MeV, and more preferably 20 to 30 MeV.
[0070]
Graphite is the best low energy gamma ray absorber. Aluminum is also acceptable.
[0071]
The heavy metal having pores is best in terms of cost, and gold, platinum, and tungsten are also acceptable.
[0072]
Neutron moderator is a substance containing hydrogen, which can be water. Paraffin is good for ease of use.
[0073]
The best neutron absorber is cadmium, and shelf elements and lithium are also acceptable. Cadmium can be thinnest. The paraffin / cadmium combination is the best.
[0074]
When an electron beam is irradiated onto a radiation material, the generated gamma rays are generally broad and have many low energy components. The present invention is a method of narrowing generated gamma rays and extracting high energy as a main component.
[0075]
When an electron beam is irradiated, gamma rays with a wide energy distribution are generated as shown in FIG. Passes through the graphite and absorbs gamma rays with an energy of 10 MeV or less.
[0076]
The degree of absorption is shown in FIG. In particular, 1 MeV or less can be lowered to an extent that is not perfect but does not affect the human body. Further, gamma rays that leak laterally are absorbed by heavy metal. In addition, since the energy of the electron beam is high, a neutron beam (neutron) which is undesirable for the human body is emitted. For this reason, the neutron is decelerated with paraffin and absorbed with cadmium.
[0077]
There are two types of cancer treatment using radiation: a method using particle beams and a method using gamma rays (including electrons). In general, particle beams, particularly those called heavy ions, are thought to be able to treat cancer tissue efficiently and safely because they can concentrate doses on cancer tissues.
[0078]
As described above, by combining high-energy gamma rays and a radiation material, it is possible to realize a device that gives a dose to a tumor site with an efficiency equal to or higher than that of particle beams. The characteristics of the radiation material are (1) a substance with a short radiation length and (2) a thin shape.
[0079]
On the other hand, it is efficient to use gamma rays that are thin and high in collimated energy, and there are several possibilities for the device to actually generate them. However, the gamma rays obtained by laser inverse Compton scattering sufficiently satisfy the conditions. Yes. However, this method has a practical problem that the apparatus becomes too large. Therefore, here, as a supplement to this method, a method for obtaining a target gamma ray 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 radiation material include the following three points.
(1) A high energy gamma ray of 5 MeV or more (more preferably 10 MeV or more) and several tens MeV or less.
(2) It should be collimated as thin as millimeters.
(3) Do not generate extra radiation
These conditions are for reducing the irradiation dose to other parts of the human body while increasing the dose in the vicinity of the radiation material placed on the tumor part.
[0081]
When a high-energy electron beam is applied to a material, gamma rays are generated by Bremsstrahlung. Braking radiation has been used not only for research purposes but also in many cases as a method of generating gamma rays and X-rays. If a gamma ray meeting the purpose can be generated by this very common method, a simple system should be constructed.
[0082]
Gamma rays are obtained by applying accelerated electrons to a radiation material (heavy metal such as gold or platinum) to cause bremsstrahlung. The characteristics of the gamma ray energy distribution and angular distribution are described below.
[0083]
It is known that the energy distribution of gamma rays obtained by the bremsstrahlung of a sufficiently thin radiation material is given by the following equation.
[0084]
[Expression 1]

[0085]
Because the spectrum is inversely proportional to energy, bremsstrahlung has particularly high levels of low energy gamma rays. Since the low energy component has a large interaction with the substance, when used for radiotherapy, the absorption is inevitably large on the surface and has a great influence on the skin tissue.
[0086]
On the other hand, gamma rays emitted by bremsstrahlung are radiated in the front as much as the incident direction of the electron beam, but there is a spread by a certain angle, which is given by the following equation.
[0087]
[Expression 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 rays emitted within a certain angle θ to the total angle is assumed to be m / p << 1 because the energy of the electrons is sufficiently larger than its mass m, and the distribution is concentrated forward. Therefore, sin θ can be set to θ. Therefore, it becomes as follows.
[0089]
[Equation 3]

[0090]
The amount integrated over all angles is π · p²/ M²So gamma rays are m²/ P²Half will be gathered within the angle represented by. Typically, for an electron of 30 MeV, the spread is 1/60 rad.
[0091]
A method of accelerating an electron beam to about 10 MeV and irradiating a radiation material and generating gamma rays by bremsstrahlung is generally used in cancer treatment. As the accelerator, a linac (linear accelerator) is generally used as described above. This is because if the energy is about 10 MeV, sufficient transmission power can be obtained and generation of unnecessary neutrons can be suppressed.
[0092]
Now, gamma rays of about 10 to 50 MeV are suitable for treatment using a radiation material. Higher energy is more efficient for obtaining finely collimated gamma rays. For this purpose, an electron beam with high energy is required. When the electron beam energy starts to exceed 10 MeV, the generation of neutrons cannot be ignored. Therefore, higher energy is better, but in reality, about 20 to 30 MeV would be appropriate.
[0093]
Since the gamma rays are narrowed down, it is necessary to search for conditions necessary to obtain sufficient intensity. The thickness of the radiation material that causes braking radiation to electrons is 0.3L_RThe degree is appropriate. At this thickness, the conversion efficiency to gamma rays is about 30%, and the probability that the generated gamma rays interact and be lost is about 15%, so 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]
It is necessary to make the distance as short as possible in order to extract a thin gamma ray beam so as to correspond to the size of the radiation material placed in the body. Considering the distance through the collimator and the body, it would be sufficient to consider 40 cm as the distance from the radiating material causing the bremsstrahlung to the radiating material in the body. When the diameter of the radiation material in the body is 1 mm, the spread of the angle is 1.2 mrad. The gamma rays that fall within this angle are (0.0012 × 60) for 30 MeV.²= 5 × 10^-3It is. As a result, if the current of the electron beam of 30 MeV is 1 μA, 2 × 10⁹/ Sec gamma rays can be obtained. The required dose is typically obtained in seconds to tens of seconds. When this is 10 MeV (0.0012 × 20)²= 6 × 10^-FourWhen the energy is reduced, the amount of gamma rays that can be irradiated to the thin material in front is relatively reduced, and the efficiency is reduced by almost an order of magnitude compared to the case of 30 MeV. In practical use, it can be covered by increasing the current or taking time, so it would be meaningful to enable treatment with an accelerator that is relatively easy to obtain.
[0095]
In order to obtain the optimal gamma ray for the treatment using the radiation material in this method, the following apparatus can be used. A collimator is installed to reduce the gamma rays generated by the bremsstrahlung to a small beam size. Since gamma rays spread with the flight distance, shortening the distance from the collimator to the irradiation site allows efficient use of gamma rays. Therefore, a material having a short radiation length is suitable as a material. Platinum or gold is optimal, but considering the cost, heavy metal based on tungsten is the most realistic. Since the radiation length of heavy metal is about 3.7 mm, it has a sufficient shielding effect against gamma rays at several centimeters.
[0096]
The gamma rays produced by bremsstrahlung are higher at lower energies, but this gives unwanted doses to the body, especially the surface. In order to remove this, carbon is filled in the path of gamma rays and the holes in the collimator. FIG. 12 shows an energy spectrum of gamma rays obtained by braking radiation. When there is no absorber, it can be seen that the spectrum inversely proportional to the energy expressed by Equation (1) decreases from the low energy region as the carbon thickness is increased. 30 g / cm along the path²If the thickness is enough, gamma rays of 1 MeV or less that affect the surface of the human body are reduced by almost an order of magnitude, so that the level can be made almost harmless. Since graphite has a density of 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. The center is a collimator that draws out thin gamma rays made of heavy metal. It also plays a role in stopping gamma rays that leak around. Outside that is a neutron moderator. Substances rich in 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. The thickness of the layer is typically about 10 cm, but it can be made small and compact when the original amount of neutron generation is small.
[0098]
A cadmium metal sheet is wound around the moderator. If the thickness is about 0.5 to 1 mm, most of the decelerated neutrons can be absorbed. Since cadmium that has absorbed neutrons emits gamma rays, lead shielding is placed outside the cadmium. The gamma rays generated at this time are relatively low in energy, and can be sufficiently shielded with about 5 cm of lead.
[0099]
A configuration example of the gamma ray irradiation unit 50 will be described with reference to FIGS. 13 and 14.
[0100]
Tungsten (unnecessary gamma ray absorbing portion) 54 is a cylinder, and the inside thereof is cut out concentrically. Inside, 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 inserted. 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 a gamma ray emitted from the gamma ray emission portion has a predetermined radiation angle or less. Although four collimator pieces are used here, the present invention is not limited to this, and one or more, for example, two or more can be used depending on the setting.
[0101]
The platinum plates 56 and 61 have the same diameter as that of the tungsten 54.
[0102]
Tungsten 57 to 60 are cylindrical shapes whose outer shapes are 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 that is hollowed out in a concentric shape passing through the shaft. For each of the tungsten 57 to 60, the hole diameter is constant. And the diameter of this hole is so small that it is near the platinum plate 56,
The collimator piece 57 <the collimator piece 58 <the collimator piece 59 <the 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 packed in the tungsten 54 so that there is no gap between the bottom surface and the bottom surface of the tungsten 54.
[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 decelerates the neutrons emitted from the gamma ray emitting portion together with the gamma rays.
[0107]
The paraffin 53 is covered with a cadmium sheet (neutron absorption part) 52. This absorbs neutrons emitted together with gamma rays from the gamma ray emitting portion.
[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 cylindrical shape, and the gamma ray irradiation unit 50 as a whole has a cylindrical shape.
[0110]
In FIG. 14, lead 51 is drawn as a separate member such as 51 a, 51 b, and 51 c, but may be separate members as described above, or these three may be integrated as lead 51. It is good also as a member. The same applies to the cadmium sheets 52 (52a, 52b, 52c) and paraffin 53 (53a, 53b).
[0111]
The electron beam 62 strikes the platinum plate 56 from an electron beam generation source (irradiation source) (not shown). As described above, a linac (linear accelerator) or the like generally used as a medical gamma ray generator can be used as the electron beam generation source.
[0112]
When the electron beam 62 hits the platinum plate 56, gamma rays are generated from the platinum plate 56. Of these gamma rays, low energy ones are absorbed by the graphite 64 and the platinum plate 61. Further, gamma rays that do not pass through the holes of the collimator are absorbed by the collimator pieces 57-60. Therefore, the gamma ray 63 that exits the graphite 64 and exits the gamma ray irradiation unit 50 contains a large amount of high energy components.
[0113]
The collimator is made by stacking collimator pieces 57 to 60 having different inner diameters. That is, in this configuration, the collimator diameter changes stepwise and discontinuously, and is a separate member for each changing position. However, other than 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]
The gamma rays generated by bremsstrahlung are higher at lower energies, but this gives unnecessary doses especially to the surface of the living body. In order to remove this, an absorbing material (low energy gamma ray absorbing portion) is packed in the path through which the gamma ray passes and the hole in the collimator. If the length of the collimator hole is 10 cm, typically the first 5 cm is packed with graphite (carbon) 64, then the 1 mm thick platinum (or gold, tungsten) plate 61 is packed and the next 5 cm. 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 of the collimator hole (10 cm).
[0115]
First, an electron beam is irradiated onto a platinum (or gold, tungsten) plate 56 (converter) (gamma ray emitting portion) having a thickness of about 1 mm to generate gamma rays. The origin is as small as possible, improving the efficiency of the collimator. Typically, it is desirable to be about 1 / mm or less. The gamma rays generated here are narrowed down by a collimator.
[0116]
At the point of the conversion material, electrons with reduced energy exist, but if the gamma ray emitting part is a heavy metal, new gamma rays are generated from the electrons. Many of these are unwanted low energy components that we want to remove. Therefore, an absorbing material is placed behind the place where the gamma rays are generated, and the low energy components are absorbed and removed while suppressing the decrease in the high energy components of the generated gamma rays. 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) Is about 5cm (11g / cm)²) If it is around, electrons will almost stop.
[0118]
Next, a thin platinum plate 61 having a thickness of about 1 mm is placed as described above. This effectively reduces low energy gamma rays. In addition, about 5 cm of carbon is used as an absorbent material behind it. A heavy metal has a property that the absorption efficiency is lowered at an energy below the K absorption edge (about 80 keV), so that portion is again absorbed by carbon.
[0119]
The platinum (or gold or tungsten) plate 61 absorbs a low energy component (when thick, it absorbs even a high energy component), and the graphite 64 mainly absorbs a low energy component and also absorbs electrons by gamma rays. It can be decelerated without generating.
[0120]
FIGS. 15 and 16 show the cross-sectional area (total absorption area) of interaction with gamma rays for carbon and platinum, respectively. It can be considered that it almost corresponds to the probability of absorption.
[0121]
In FIG. 15, the interaction cross section with gamma rays slowly decreases with the gamma ray absorption cross section energy of carbon.
[0122]
In FIG. 16, the interaction cross section with gamma rays decreases rapidly with the gamma ray absorption cross section energy of platinum. Also, it starts to increase when the energy is above several MeV. In addition, there is a point where the cross-sectional area decreases around 80 keV.
[0123]
The extent to which gamma rays decrease when passing through a substance with thickness t is determined using the absorption coefficient σ.
N (t) = N₀Exp (-σt)
It is expressed. However, the number before passing through the substance is N₀, Let N (t) be the number after passing, and express x to the power of the base e of the natural logarithm as exp (x).
[0124]
15 and 16, the vertical axis represents σ (unit: cm²/ G). The thickness of the material is g / cm considering the density of the material, not the unit of length.²It is convenient to decide in
[0125]
Considering platinum (specific gravity 21.45) as an example, from FIG.
When E = 0.1 MeV, σ = 6 cm²/ G
When E = 1 MeV σ = 0.06 cm²/ G
It can be seen that it is. Here, 1mm thick platinum
t = 22.45 × 0.1 = 2.245 g / cm²
It becomes the thickness of. 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 0.1 MeV gamma ray can be almost completely reduced, while the 1 MeV gamma ray remains almost completely.
[0126]
For carbon 10 cm (the specific gravity of graphite is about 2.2), t = 22 g / cm²From FIG.
When E = 0.1 MeV, σ = 0.16 cm²/ G, N = 0.03N₀
When E = 1 MeV σ = 0.06 cm²/ G, N = 0.27N₀
When E = 10 MeV σ = 0.02 cm²/ G, N = 0.64N₀
Thus, the energy dependence is not extreme, but it can be seen that the high energy side can be left.
[0127]
【The invention's effect】
As described above, the radiotherapy apparatus according to the present invention emits an electron-positron pair to generate an electromagnetic shower when irradiated with gamma rays, and irradiates the affected area in the treatment object, and the treatment object. And a gamma ray irradiation unit that irradiates the electron positron pair emission unit by irradiating gamma rays from the outside.
[0128]
Thereby, the gamma ray emitted from the gamma ray irradiation part hits the electron positron pair emission part, and the electron positron pair emission part is emitted. This electron-positron pair is developed into an electromagnetic shower, and can be irradiated to the affected area in the vicinity of the gamma ray irradiating section to treat the affected area with radiation.
[0129]
Therefore, it is possible to irradiate a diseased part such as a tumor with a lot of radiation while suppressing damage to normal tissue as much as possible, and there is an effect that radiation therapy can be performed efficiently.
[0130]
In addition to the above-described configuration, the radiotherapy apparatus according to the present invention includes an electron positron pair emission part position detection unit that detects the position of the electron positron pair emission part in the treatment object, and a gamma ray in the treatment object. Detected by the gamma-ray irradiation position detector for detecting the irradiated position, the position of the electron-positron pair emitter in the treatment object detected by the electron-positron pair emitter position detector and the gamma-ray irradiation position detector. A position control unit that returns the position of the electron-positron pair emission unit to the position irradiated with gamma rays when information indicating that the position irradiated with gamma rays is shifted in the treatment target is input. A configuration including a gamma ray irradiation control unit that controls the gamma ray irradiation unit so that the gamma ray irradiation is turned off while the position of the electron-positron pair emission unit is shifted from the position where the gamma ray is irradiated. A.
[0131]
Thereby, in addition to the effect by said structure, there exists an effect that a gamma ray can be irradiated without waste.
[0132]
In addition to the above-described configuration, the radiotherapy apparatus according to the present invention includes a gamma ray emitting unit that emits gamma rays by bremsstrahlung when an electron beam hits, and a gamma ray emitted from the gamma ray emitting unit. Among them, the collimator having a hole through which only one having a predetermined radiation angle or less passes, and the gamma ray emitting unit and the collimator are placed between the gamma ray emitting unit and the gamma ray emitted from the gamma ray emitting unit has an energy of a predetermined value or less. It is the structure provided with the low energy gamma ray absorption part which absorbs things.
[0133]
As a result, 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, without using expensive accelerators at the giga level. It is possible to produce an effect that the affected area can be sufficiently treated with an electromagnetic shower.
[0134]
In addition to the above-described configuration, the radiotherapy apparatus according to the present invention includes 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.
[0135]
Thereby, in addition to the effect by said structure, there exists an effect that an unnecessary gamma ray can be absorbed.
[0136]
In addition to the above-described configuration, the radiotherapy apparatus according to the present invention includes a neutron absorbing unit that absorbs neutrons emitted from the gamma ray emitting unit together with gamma rays around the gamma ray emitting unit.
[0137]
Thereby, in addition to the effect by said structure, there exists an effect that an unnecessary neutron can be absorbed.
[0138]
In addition to the above-described configuration, the radiotherapy apparatus according to the present invention further includes a neutron moderating unit that decelerates neutrons emitted together with gamma rays from the gamma ray emitting unit between the gamma ray emitting unit and the neutron absorbing unit. This is a configuration provided.
[0139]
Thereby, in addition to the effect by said structure, there exists an effect that an unnecessary neutron can be decelerated.
[0140]
In addition to the above configuration, the radiotherapy apparatus according to the present invention has a property that the neutron absorber emits gamma rays when it absorbs neutrons, and a neutron absorber around the neutron absorber It is the structure provided with the secondary gamma ray absorption part which absorbs the gamma ray emitted from.
[0141]
Thereby, in addition to the effect by said structure, there exists an effect that an unnecessary gamma ray can be absorbed.
[0142]
In addition to the above-described configuration, the radiotherapy apparatus according to the present invention may be configured such that the collimator is a cylindrical object having a congruent outer shape, and is a cylindrical gamma ray path penetrating about an axis. A plurality of collimator pieces having different diameters are arranged on the same axis without gaps in such an order that the hole diameter increases with increasing distance from the gamma ray emitting portion.
[0143]
Thus, the gamma ray emission angle range can be easily changed by simply changing the combination of the collimator pieces. Therefore, in addition to the effect of the above-described configuration, there is an effect that a necessary gamma ray intensity can be obtained more easily.
[Brief description of the drawings]
FIG. 1 is a diagram showing the concept of radiation therapy according to the present invention.
FIG. 2 is a plan view showing the relationship between the incident position of gamma rays and the arrangement of radiation materials.
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 radiation material and the accumulated dose of radiation when the radiation material is irradiated with gamma rays.
FIG. 5 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. 6 is a graph showing the relationship between the radius at the depth with the highest dose and the accumulated dose of radiation dose when the radiation material is irradiated with gamma rays.
FIG. 7 is a graph showing the relationship between the radius and the accumulated dose of radiation dose at the depth with the highest dose when the radiation material is irradiated with gamma rays.
FIG. 8 is a graph showing the relationship between the radius at the depth with the highest dose and the accumulated dose of radiation dose when the radiation material is irradiated with gamma rays.
FIG. 9 is a diagram showing the concept of radiotherapy according to the present invention.
FIG. 10 is a plan view showing the relationship between the incident position of gamma rays and the arrangement of radiation materials.
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 gamma rays.
FIG. 13 is a cross-sectional view showing a gamma ray irradiation unit.
FIG. 14 is a perspective view showing each member constituting the 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 radiotherapy.
FIG. 19 is a diagram showing an example of radiation therapy using gamma rays.
[Explanation of symbols]
11 Water
12 Gamma rays
13 Emissive material (electron positron emission part)
14 Electromagnetic shower
21 patients (subjects to be treated)
22 Tumor
31 High-accuracy position detector (electron positron emission position detector)
32 Irradiation position confirmation device (gamma ray irradiation position detector)
33 High-precision position control device (position control unit)
36 Gamma-ray beam generator (gamma-ray irradiation unit)
37 Collimator (gamma irradiation unit)
38 Controller (Gamma irradiation control unit)
50 Gamma irradiation unit
51, 51a, 51b, 51c Lead (secondary gamma ray absorption part)
52, 52a, 52b, 52c Cadmium sheet (neutron absorber)
53, 53a, 53b Paraffin (neutron moderator)
54 Tungsten (unnecessary gamma ray absorption part)
55 Tungsten (unnecessary gamma ray absorption part)
56 Platinum plate (gamma emission part)
57, 58, 59, 60 Tungsten (collimator)
61 Platinum plate (low energy gamma ray absorber)
62 electron beam
63 Gamma rays
64 Graphite (low energy gamma ray absorber)

Claims (8)

An electron positron pair emitting part that emits an electron positron pair when it hits a gamma ray to generate an electromagnetic shower and irradiates the affected part in the treatment object,
A radiotherapy apparatus comprising: a gamma ray irradiation unit configured to irradiate gamma rays from the outside of a treatment target and hit the electron positron pair emission unit. An electron positron pair emission part position detector for detecting the position of the electron positron pair emission part in the treatment object;
A gamma ray irradiation position detection unit for detecting a position where gamma rays are irradiated in the treatment object;
The position of the electron-positron pair emission part in the treatment object detected by the electron-positron pair emission part position detection part and the position where the gamma ray is irradiated in the treatment object detected by the gamma-ray irradiation position detection part. When the information indicating that there is a shift is input, a position control unit that returns the position of the electron-positron pair emission unit to the position irradiated with gamma rays,
A gamma ray irradiation control unit for controlling 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. The radiotherapy apparatus according to claim 1. The gamma irradiation unit is
A gamma ray emitting part that emits gamma rays by bremsstrahlung when hit by an electron beam,
A collimator having a hole through which only a gamma ray emitted from the gamma-ray emitting portion passes 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 that have an energy of a predetermined value or less. Item 3. The radiotherapy apparatus according to Item 1 or 2. 4. The radiotherapy apparatus according to claim 3, further comprising an unnecessary gamma ray absorbing portion that absorbs gamma rays emitted from the gamma ray emitting portion and not passing through a hole of a collimator. The radiotherapy apparatus according to claim 3 or 4, further comprising a neutron absorber that absorbs neutrons emitted together with gamma rays from the gamma ray emitter around the gamma ray emitter. The radiotherapy apparatus according to claim 4, further comprising a neutron moderating unit that decelerates neutrons emitted together with gamma rays from the gamma ray emitting unit between the gamma ray emitting unit and the neutron absorbing unit. The neutron absorber has the property of emitting gamma rays when it absorbs neutrons,
The radiotherapy apparatus according to claim 5 or 6, further comprising a secondary gamma ray absorbing portion that absorbs gamma rays emitted from the neutron absorbing portion around the neutron absorbing portion. The collimator is
Cylindrical objects with congruent outer shapes, and a plurality of collimator pieces with different hole diameters that become cylindrical gamma ray paths penetrating around the axis, the hole diameters increase as they move away from the gamma ray emitting portion The radiotherapy apparatus according to any one of claims 3 to 7, wherein the radiotherapy apparatuses are arranged in order on the same axis without gaps.

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