Pitcher Garrett M, Hogstrom Kenneth R, Carver Robert L
Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA, USA.
Mary Bird Perkins Cancer Center, Baton Rouge, LA, USA.
J Appl Clin Med Phys. 2017 Sep;18(5):259-270. doi: 10.1002/acm2.12155. Epub 2017 Aug 12.
Prototype 10 × 10 and 20 × 20-cm electron collimators were designed for the Elekta Infinity accelerator (MLCi2 treatment head), with the goal of reducing the trimmer weight of excessively heavy current applicators while maintaining acceptable beam flatness (±3% major axes, ±4% diagonals) and IEC leakage dose. Prototype applicators were designed initially using tungsten trimmers of constant thickness (1% electron transmission) and cross-sections with inner and outer edges positioned at 95% and 2% off-axis ratios (OARs), respectively, cast by the upstream collimating component. Despite redefining applicator size at isocenter (not 5 cm upstream) and reducing the energy range from 4-22 to 6-20 MeV, the designed 10 × 10 and 20 × 20-cm applicator trimmers weighed 6.87 and 10.49 kg, respectively, exceeding that of the current applicators (5.52 and 8.36 kg, respectively). Subsequently, five design modifications using analytical and/or Monte Carlo (MC) calculations were applied, reducing trimmer weight while maintaining acceptable in-field flatness and mean leakage dose. Design Modification 1 beveled the outer trimmer edges, taking advantage of only low-energy beams scattering primary electrons sufficiently to reach the outer trimmer edge. Design Modification 2 optimized the upper and middle trimmer distances from isocenter for minimal trimmer weights. Design Modification 3 moved inner trimmer edges inward, reducing trimmer weight. Design Modification 4 determined optimal X-ray jaw positions for each energy. Design Modification 5 adjusted middle and lower trimmer shapes and reduced upper trimmer thickness by 50%. Design Modifications 1→5 reduced trimmer weights from 6.87→5.86→5.52→5.87→5.43→3.73 kg for the 10 × 10-cm applicator and 10.49→9.04→8.62→7.73→7.35→5.09 kg for the 20 × 20-cm applicator. MC simulations confirmed these final designs produced acceptable in-field flatness and met IEC-specified leakage dose at 7, 13, and 20 MeV. These results allowed collimation system design for 6 × 6-25 × 25-cm applicators. Reducing trimmer weights by as much as 4 kg (25 × 25-cm applicator) should result in easier applicator handling by the radiotherapy team.
为医科达Infinity加速器（MLCi2治疗头）设计了10×10厘米和20×20厘米的原型电子准直器，目的是减轻当前过重的限束器重量，同时保持可接受的射野平坦度（长轴±3%，对角线±4%）和符合国际电工委员会（IEC）规定的泄漏剂量。最初设计原型限束器时使用了厚度恒定（电子透射率1%）的钨制限束器，其横截面的内边缘和外边缘分别位于离轴比（OAR）的95%和2%处，由上游准直部件铸造而成。尽管在等中心（而非上游5厘米处）重新定义了限束器尺寸，并将能量范围从4 - 22兆电子伏降低到6 - 20兆电子伏，但设计的10×10厘米和20×20厘米限束器重量分别为6.87千克和10.49千克，超过了当前限束器的重量（分别为5.52千克和8.36千克）。随后，应用了五种基于解析计算和/或蒙特卡罗（MC）计算的设计修改方案，在保持可接受的射野平坦度和平均泄漏剂量的同时减轻了限束器重量。设计修改1将限束器外边缘做成斜角，利用仅低能束能使原电子充分散射到达限束器外边缘这一特性。设计修改2优化了限束器距等中心的上部和中部距离，以使限束器重量最小。设计修改3将限束器内边缘向内移动，减轻限束器重量。设计修改4确定了每种能量下X射线光阑的最佳位置。设计修改5调整了中部和下部限束器的形状，并将上部限束器厚度减少了50%。对于10×10厘米的限束器，设计修改1→5使限束器重量从6.87千克→5.86千克→5.52千克→5.87千克→5.43千克→3.73千克，对于20×20厘米的限束器，限束器重量从10.49千克→9.04千克→8.62千克→7.73千克→7.35千克→5.09千克。MC模拟证实，这些最终设计产生了可接受的射野平坦度，并在7、13和20兆电子伏时符合IEC规定的泄漏剂量。这些结果为6×6 - 25×25厘米限束器的准直系统设计提供了依据。将限束器重量减轻多达4千克（25×25厘米限束器）应会使放疗团队操作限束器更加轻松。
