JP2006000220A - Multi-leaf collimator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multi-leaf collimator which can increase the precision of the contour formation of an irradiation image which is projected on an isocenter. <P>SOLUTION: This multi-leaf collimator 10 is equipped with two stages of collimator blocks 11 and 12 which are constituted of a plurality of leaves 13 and 14 in the irradiation direction of an irradiation ray. In the multi-leaf collimator 10, the leaves constituting the collimator blocks in respective stages are constituted in a manner to be shifted by a specified pitch portion in the irradiation image of the applicable leaf which is projected on the isocenter by the irradiation ray. Thus, even when the irradiation ray such as a particle ray is not parallel but has a conical shape, the contour of an image which is actually irradiated can become more precise by making the irradiation images A1 and A2 projected on the isocenter 15 be shifted by a specified pitch. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a multi-leaf collimator, which is a multi-leaf collimator that regulates an irradiation field in cancer treatment using particle beams, and can improve the reproducibility of the contour of an irradiation image in consideration of the spread of the particle beams. .

  In recent years, treatments using particle beams such as proton beams and elementary particle beams have been carried out for cancer treatment, and the particle beam produced by the accelerator is processed into the shape of the cancer affected area using an irradiation field forming device. To irradiate.

  In this irradiation field forming apparatus, the particle beam emitted from the accelerator is subjected to the following operation to form a cancerous part.

 First, the particle beam emitted from the accelerator is vibrated by a wobbler electromagnet into a donut shape, and then the donut-shaped beam is made flatter by a scatterer.

  Then, the energy level is adjusted by a range shifter so as to adjust the dose peak so as to match the cancer affected area, and the beam is thickened by the ridge filter.

  Finally, the processing is completed by matching the processed beam with the shape of the cancer affected area using a collimator.

  A collimator is used to perform an operation for matching such a processed beam to the shape of the cancerous part, and one of them is a multi-leaf collimator.

  As shown in FIG. 7, for example, the multi-leaf collimator disclosed in the prior art in Patent Document 1 has a thin plate-like leaf 1 made of a material capable of shielding the particle beam B arranged vertically and arranged in a large number horizontally. While arranging the sheets, a pair of leaves 1 stacked side by side are opposed to each other.

 The space 2 formed by adjusting the distance between the opposing leaves 1 and the position thereof is used as an irradiation portion of the beam B, and the beam B is shielded by other leaf 1 portions, and a large number By changing the position of the leaf 1, the contour shape of the cancer affected part is reproduced.

  However, if there are gaps 3 for movement between the leaves 1, the beam B leaks from the gaps 3 and adversely affects healthy cells, which needs to be prevented.

 As one of the methods, as shown in FIG. 8, the leaves 4 and 5 are arranged in two stages in the irradiation direction of the beam B, and the upper and lower leaves 4 and 5 are arranged in a staggered pattern with a half pitch shift. It has been proposed to shield the beam B passing between the leaves 4 with the lower leaf 5 (see Patent Document 1).

  Similar multi-leaf collimators in which leaves are arranged in two upper and lower stages are also disclosed in Patent Documents 2 and 3 and the like.

In such a multi-leaf collimator in which the upper and lower two-stage leaves are shifted by half a pitch, the contour of the irradiation field is theoretically more accurate than the pitch in the case of using one-stage leaves. It is thought that it becomes high accuracy (twice) according to the pitch.
Japanese Examined Patent Publication No. 7-67491 JP 2003-210595 A JP 2004-16563 A

  However, even when trying to reproduce the required irradiation field using a multi-leaf collimator with the upper and lower leaves shifted by half a pitch, the contour reproducibility of the irradiation image actually projected on the isocenter is as high as the theoretical contour. There is a problem of not getting better.

  The present invention has been made in view of the problems of the prior art, and an object of the present invention is to provide a multi-leaf collimator capable of improving the accuracy of contour formation of an irradiation image projected on an isocenter.

  As a result of intensive studies to solve the above-mentioned problems of the prior art, the beam irradiated to the multi-leaf collimator is not a theoretical parallel beam, and the actual beam B is a wobbler magnet as shown in FIG. Since it has a conical shape as a vertex, the irradiation image 6 shielded by the upper leaf 4 near the vertex O and the irradiation image 7 shielded by the lower leaf 5 away from the vertex O are different in size and have the same pitch. In the leaves 4 and 5, the irradiation image 6 of the upper leaf 4 becomes large.

  For this reason, the outer side of the irradiation image 6 formed by the upper leaf 4 is blocked by the lower leaf 5 as shown in FIG. 2B, so that the irradiation image 6 actually projected onto the isocenter 8. It was found that the contour reproducibility of was not improved. In particular, in a wide-spread small particle beam cancer treatment apparatus, the distance from the wobbler electromagnet to the isocenter is even shorter, and the influence of the angle of the apex O of the conical beam becomes large.

A specific configuration of the present invention completed from such examination results is as follows.
That is, the multi-leaf collimator according to claim 1 of the present invention is a multi-leaf collimator provided with a plurality of collimator blocks composed of a plurality of leaves in the irradiation direction of the irradiation line, and constitutes the collimator block of each stage. The leaf is configured to be shifted by a predetermined pitch in an irradiation image of the leaf projected on the isocenter by the irradiation line.

  According to this multi-leaf collimator, in the multi-leaf collimator provided with a plurality of collimator blocks composed of a plurality of leaves in the irradiation direction of the irradiation line, the leaves constituting the collimator block of each stage are The irradiation image of the leaf projected on the isocenter is configured to deviate by a predetermined pitch, so that the irradiation projected on the isocenter even if the irradiation beam such as a particle beam is not parallel but conical. By shifting the image by a predetermined pitch, the contour of the actually irradiated image can be made highly accurate.

  A multi-leaf collimator according to claim 2 of the present invention, in addition to the configuration according to claim 1, is configured such that the leaf of the collimator block on the downstream side in the irradiation direction of the irradiation line is connected to the collimator block on the upstream side. The leaf pitch in the same irradiation direction on the isocenter is configured to have the same size by widening the leaf pitch compared to the leaf.

  According to this multi-leaf collimator, the leaf pitch of the collimator block on the downstream side in the irradiation direction of the irradiation line is made wider than the leaf of the collimator block on the upstream side so that the leaf pitch is increased on the isocenter. The irradiation image of the leaf in the same irradiation direction is configured to have the same size, and the irradiation image of the leaf on the upstream side and the downstream side has the same size on the isocenter, so that the angle of the irradiation line The contour shape of the irradiation field can be made with high accuracy without being influenced by the above.

  Furthermore, in the multi-leaf collimator according to claim 3 of the present invention, in addition to the configuration according to claim 1 or 2, the irradiation line is constituted by a beam extending conically from the apex, and the leaf is spread of the beam. On the basis of the above, the irradiation image on the isocenter is configured so that the pitch is deviated by 1 / stage.

  According to this multi-leaf collimator, the irradiation line is constituted by a beam that spreads in a conical shape from the apex, and the leaf is shifted in pitch by one step in the irradiation image on the isocenter based on the spread of the beam. Even if the irradiation line has a conical shape, the contour of the irradiation field by the multi-stage leaf can be obtained with the highest accuracy by setting the multi-stage leaf to a pitch that is shifted by a step number on the irradiation image. I can do it.

  Moreover, in addition to the structure in any one of Claims 1-3, the multi-leaf collimator of Claim 4 of this invention changes the pitch and width | variety of the said leaf, It is characterized by having the same irradiation image width.

  According to this multi-leaf collimator, the pitch and width of the leaf are changed, the pitch is shifted by one step, and the irradiation image width is the same, and the irradiation line is conical. However, the contour of the irradiation field by the multi-stage leaf can be made with the highest accuracy.

  Furthermore, the multi-leaf collimator according to claim 5 of the present invention comprises the collimator block in two stages in addition to the structure according to any of claims 1 to 4, and the leaf is irradiated on the isocenter. The irradiation image by the two-stage leaf is shifted by half a pitch in the image.

According to this multi-leaf collimator, the collimator block is configured in two stages, and the leaf is configured such that an irradiation image on the isocenter is shifted by half a pitch from the irradiation image on the isocenter. As a result, even if the irradiation line is conical, the same accuracy of the outline shape of the irradiation field as in the case of a parallel beam can be secured.
Here, the leaf pitch refers to the distance between the width centers of adjacent leaves, and the pitch on the irradiated surface refers to the distance between the width centers in the irradiated image of the leaf.

  According to the multi-leaf collimator according to claim 1 of the present invention, in the multi-leaf collimator provided with a plurality of collimator blocks composed of a plurality of leaves in the irradiation direction of the irradiation line, the collimator blocks of the respective stages are configured. Since the leaf is configured to be shifted by a predetermined pitch in the irradiation image of the leaf projected onto the isocenter by the irradiation line, even if the irradiation line such as a particle beam is not parallel but conical, The projected image can be shifted by a predetermined pitch, and the contour of the actually irradiated image can be made highly accurate.

  According to the multi-leaf collimator according to claim 2 of the present invention, the leaf of the collimator block at the downstream stage in the irradiation direction of the irradiation line is more preferably compared with the leaf of the collimator block at the upstream stage. Since the irradiation image of the leaves in the same irradiation direction on the isocenter is configured to have the same size by widening the pitch, the irradiation images of the upstream and downstream leaves should be the same size on the isocenter. Therefore, the contour shape of the irradiation field can be made with high accuracy without being affected by the angle of the irradiation line.

  Furthermore, according to the multi-leaf collimator according to claim 3 of the present invention, the irradiation line is constituted by a beam extending conically from the apex, and the leaf is an irradiation image on the isocenter based on the spread of the beam. Since the pitch is shifted by 1 / stage, even if the irradiation line is conical, the multi-stage leaf is set to a pitch that is shifted by 1 / stage on the irradiation image, thereby irradiating the multi-stage leaf. The contour of the field can be made most accurate.

  Moreover, according to the multi-leaf collimator according to claim 4 of the present invention, the pitch and width of the leaf are changed, and the pitch is shifted by a fraction of the number of stages, and the same irradiation image width is configured. By changing the pitch and width of the leaf, the contour of the irradiation field by the multi-stage leaf can be made with the highest accuracy even when the irradiation line is conical.

  Furthermore, according to the multi-leaf collimator according to claim 5 of the present invention, the collimator block is configured in two stages, and the irradiation image on the isocenter is an irradiation image by the two-stage leaf at half pitches. Since the configuration is deviated, the accuracy of the contour shape of the irradiation field shifted by a half pitch as in the case of a parallel beam can be ensured even if the irradiation line is conical due to the two-stage leaf.

Hereinafter, an embodiment of a multi-leaf collimator of the present invention will be described in detail with reference to the drawings.
1 and 2 relate to an embodiment in which the multi-leaf collimator of the present invention is configured by a two-stage leaf collimator block, and FIG. 1 is a view of the leaf end face on one side of the leaf collimator block from the direction perpendicular to the axis of the irradiation line. FIG. 2 is an explanatory view showing the case viewed from the side of the leaf collimator block (when the leaves on both sides are viewed from the direction perpendicular to the plane of FIG. 1) in comparison with the conventional example (b).

  In the multi-leaf collimator 10 of the present invention, the irradiation images 16 and 17 on the isocenter 15 formed by the respective leaves 13 and 14 constituting the upper and lower two-stage leaf collimator blocks 11 and 12 are vertically shifted so as to be shifted by a half pitch. The leaf pitches p1 and p2 are different.

  Since the beam B of the particle beam, which is an irradiation beam, has a conical shape with a wobbler electromagnet as a vertex, as shown in FIG. 1, the irradiation image 16 shielded by the upper leaf 13 is an irradiation image shielded by the lower leaf 14. 17 is larger than 17, but the pitch p2 of the lower leaf 14 is made wider than the pitch p1 of the upper leaf 13 in accordance with the distance from the vertex O of the wobbler electromagnet, thereby irradiating images from the respective leaves 13, 14. 16 (size A1) and 17 (size A2) are set to have the same size A1 = A2 on the isocenter 15.

  Further, in the multi-leaf collimator 10, the upper leaf 13 and the lower leaf 14 are arranged so that the lower leaf 14 is placed between the leaves 13 and the opposing surfaces of the leaves 14 as shown in FIG. The beam B is positioned outside in accordance with the spread of the conical beam from the leaf 13 and the sizes A1 and A2 of the irradiation images 16 and 17 to be regulated, as in the conventional example shown in FIG. The upper and lower leaves 4 and 5 in the case of parallel irradiation are arranged at the same position so that the outer side of the irradiation image 6 formed by the upper leaf 4 is not blocked by the lower leaf 5.

  In the multi-leaf collimator 10 configured in this way, the beam B emitted in a conical shape from the center O of the wobbler electromagnet is shielded by the upper leaf 13 and the lower leaf 14, and between the opposing leaves 13 and the leaves 14. By adjusting the interval and position, it is formed into a spatial shape corresponding to the shape of the cancer affected area.

  At this time, the lower leaf 14 is positioned outward from the upper leaf 13 by a certain distance in the moving direction of the leaf 14 as shown in FIG.

  Further, since the pitch p2 of the lower leaf 14 is larger than the pitch p1 of the upper leaf 13, (in the conventional multi-leaf collimator, the irradiation image 7 formed by the lower leaf 4 is the irradiation image 6 formed by the upper leaf 3). The beam formed by the upper leaf 13 and the lower leaf 14, respectively, forms the irradiation images 16 and 17 with the same size A 1 = A 2 on the isocenter 15.

  Since the upper leaf 13 and the lower leaf 14 are arranged so that the irradiation images 16 and 17 are shifted by a half pitch in the irradiation image, the accuracy of the outline of the irradiation image is improved as compared with the case of one step, Double accuracy.

  In the multi-leaf collimator, not only the pitches p1 and p2 are changed using the leaves 13 and 14 having the same thickness but also the thicknesses of the pitches p1 and p2 and the leaves 13 and 14 are changed. Thus, a predetermined pitch such as a half pitch may be shifted.

  Further, the collimator blocks 11 and 12 are not limited to the upper and lower two stages, and may be further multistaged. In this case, if the pitch is shifted on the irradiation image by one of the number of stages, the contour is further increased. Accuracy can be improved.

Next, a specific pitch calculation method for such multi-stage leaves will be described by taking the upper and lower two-stage leaves as an example.
Here, a condition for obtaining a projection image with a 1/2 leaf pitch is obtained in consideration of the spread angle of the particle beam.

(1) Leaf Arrangement Method First, as a leaf arrangement method, a condition for projecting the shape A onto the patient irradiation surface (isocenter) 15 corresponding to the spread of the beam B is obtained.

  As shown in FIG. 3, the irradiation shapes A3 and A4 created by the upper and lower leaves 13 and 14 are projected onto the irradiation surface 15 with a size A enlarged by the beam divergence angle.

  Here, the part forming the contour is not the lower end of the leaves 13 and 14 but the central part in the thickness direction of the leaves 13 and 14.

  In order to obtain the shape A on the isocenter 15, it is necessary to make the projected images of A3 and A4 have the same size on the isocenter.

  Therefore, the shapes of A3 and A4 are combined as a reduced irradiation shape A according to the distance from the beam apex. When the respective reduction ratios are r1 and r2, the following equation 1 is obtained.

  In an actual apparatus, the position of the multi-leaf collimator (MLC) can be changed, and the position can be changed within a range of, for example, 200 to 700 mm from the isocenter.

  In such a multi-leaf collimator, the difference in the enlargement rate in the leaf feed direction can be dealt with by changing the feed amounts of the upper and lower leaves 13 and 14, but in the leaf pitches p1 and p2, the irradiation surface 15 is affected by the difference in the enlargement rate. Since the leaf pitches are different, an interference pattern is generated in the combined irradiation shape.

  For this reason, it is impossible to form a half pitch contour near both ends of the irradiation field.

  Therefore, it was found from the drawing based on the above conditions that the position where the interference occurs slightly moves depending on the distance from the isocenter 15 of the multi-leaf collimator (MLC).

(2) Interference Analysis Next, an interference pattern is analyzed to obtain a dimensional condition that does not cause interference in the projected image on the isocenter 15.

  As shown in FIG. 4, the pitches of the upper and lower leaves 13 and 14 projected on the irradiation surface are P1 and P2. From the reduction ratio in the previous item, P1 and P2 are as follows. In addition, p1 and p2 are the pitches of the upper and lower leaves themselves and can be expressed by the following formula 2.

P1 is larger than P2 by δ due to the difference in magnification.
Therefore,
δ = P1-P2
∴P1 = P2 + δ
It becomes.

  In the center of the multi-leaf collimator (MLC), as shown in FIG. 5, if the upper and lower leaves 13 and 14 are shifted by 1/2 pitch, the upper and lower leaves are displaced in the leaf pitch direction. , A portion that is not 1/2 pitch occurs.

  Therefore, as shown in FIG. 6, assuming that the positions of the n-th upper and lower leaves from the center are P1 (n) and P2 (n), the respective positions can be expressed by the following Equation 3.

As shown in FIG. 6, interference occurs at a position where P1 (n)> P2 (n).
Therefore, n where interference occurs is obtained with an apparatus having the following layout conditions.

Layout conditions
L = 4748.82 mm
l = 4425.82 / 3925.82 mm (MLC position 200/700)
h = 88 mm
p1 = p2 = 4.5mm
Where L is the distance from the apex of the beam spread (the center of the downstream electromagnet) to the isocenter,
l is the position from the top of the beam spread (downstream electromagnet center) to the MLC
h is the leaf pitch between the top and bottom
p1 and p2 are the leaf pitches of the upper and lower stages.
Using these conditions and solving for n using the condition P1 (n)> P2 (n) that causes interference, the following equation 4 can be obtained.

  As a result, although the interference position changes depending on the position of the multi-leaf collimator (MLC), it can be seen that interference occurs at the 23rd to 24th sheets from the center.

(3) Avoidance of interference Next, a condition where the projection pitches of the upper and lower leaves are the same on the isocenter is obtained as a condition in which interference does not occur.
Since the position of interference differs depending on the position of the multi-leaf collimator (MLC), it is determined from the figure, and a condition for avoiding interference is obtained when the multi-leaf collimator (MLC) having a large influence of interference is at a position of 700 mm.
The condition for obtaining an irradiation image of 1/2 pitch on the isocenter can be expressed by the following equation 5.

  Then, when the upper leaf pitch p1 is 4.5 mm and the lower leaf pitch p2 where no interference occurs is obtained, the following equation 6 can be obtained.

  From this result, the interference pattern can be avoided if the lower leaf pitch p2 = 4.6 with respect to the upper leaf pitch p1 = 4.5.

  When the position of the multi-leaf collimator is 200 mm, the leaf pitch cannot be changed, so the interference avoidance conditions are different. Even when the pitch is 700 mm, the interference is almost avoided by drawing. Make sure you can.

  In addition, depending on the size of the irradiation field, the upper and lower leaf gaps may overlap on the outer periphery, but in this case, in addition to arranging the upper and lower leaves in a staggered pattern, What is necessary is just to combine a shape (labyrinth shape, corrugation structure, etc.).

  As described above, in the two-stage multi-leaf collimator having the above layout conditions, the irradiation beam becomes conical by setting the upper leaf pitch p1 to 4.5 mm and the lower leaf pitch p2 to 4.6 mm. However, it is possible to obtain a contour shape shifted by a half pitch in the irradiation image on the isocenter 15 and improve the shape accuracy of the irradiation field.

  Similarly, if the layout conditions are determined, it is possible to obtain the leaf pitch that ensures the contour accuracy on the isocenter considering the case where the irradiation beam spreads in a conical shape.

The multi-leaf collimator according to the present invention is constituted by a two-stage leaf collimator block. An explanatory view when the leaf end surface on one side of the leaf collimator block is viewed from the direction perpendicular to the axis of the irradiation line according to the embodiment and irradiation on the isocenter It is explanatory drawing of image shift | offset | difference. A conventional example in which the multi-leaf collimator of the present invention is viewed from the side of the leaf collimator block according to the embodiment in which the leaf collimator block of two stages is configured (the leaves on both sides are viewed from the direction perpendicular to the sheet of FIG. 1) ( It is explanatory drawing shown in comparison with b). It is explanatory drawing of the arrangement | positioning of the leaf concerning one Embodiment which comprised the multi-leaf collimator of this invention with the leaf collimator block of 2 steps | paragraphs, and irradiation shape. It is explanatory drawing of the leaf pitch concerning one Embodiment which comprised the multi-leaf collimator of this invention by the leaf collimator block of two steps, and the pitch of an irradiation image. It is explanatory drawing of arrangement | positioning in the center part when the upper and lower leaf pitch concerning one Embodiment which comprised the multi-leaf collimator of this invention with the leaf collimator block of 2 steps | paragraphs differs. It is explanatory drawing of the nth arrangement | positioning from the center in case the upper and lower leaf pitches concerning one Embodiment which comprised the multi-leaf collimator of this invention by the two-stage leaf collimator block. It is the side view and top view of the conventional one-stage leaf collimator. It is the side view and top view of the conventional 2 step | paragraph leaf collimator. It is explanatory drawing of the relationship between the leaf pitch of the conventional 2 steps | paragraph of leaf collimator, and the magnitude | size of an irradiation image.

Explanation of symbols

10 Multi-leaf collimator 11, 12 Upper and lower leaf collimator blocks 13, 14 Upper and lower leaves 15 Isocenter 16, 17 Irradiation images p1, p2 Upper and lower leaf pitches A1, A2 Size of upper and lower leaf irradiation images O Vertex of wobbler electromagnet

Claims (5)

In a multi-leaf collimator equipped with multiple stages of collimator blocks composed of multiple leaves in the irradiation direction of the irradiation line,
A multi-leaf collimator configured such that the leaves constituting the collimator blocks of the respective stages are shifted by a predetermined pitch in the irradiation image of the leaves projected onto the isocenter by irradiation lines. Irradiation of leaves in the same irradiation direction on the isocenter with the leaves of the collimator block at the downstream stage in the irradiation direction of the irradiation beam having a wider leaf pitch than the leaves of the collimator block at the upstream stage. The multi-leaf collimator according to claim 1, wherein the images have the same size. The irradiation line is composed of a beam extending conically from the apex, and the leaf is configured so that the pitch is shifted by a fraction of the number of steps in the irradiation image on the isocenter based on the spread of the beam. The multi-leaf collimator according to claim 1 or 2. The multi-leaf collimator according to any one of claims 1 to 3, wherein the pitch and width of the leaf are changed, and the pitch is shifted by a fraction of the number of steps, and the same irradiation image width is obtained. . 5. The collimator block according to any one of claims 1 to 4, wherein the collimator block is configured in two stages, and the leaf is configured so that an irradiation image by the two-stage leaf is shifted by a half pitch from an irradiation image on the isocenter. A multi-leaf collimator as described in 1.

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