Vatanen T, Traneus E, Lahtinen T
Department of Oncology, Kuopio University Hospital, Box 1777, FIN-70211, Kuopio, Finland.
Phys Med Biol. 2009 Apr 21;54(8):2407-19. doi: 10.1088/0031-9155/54/8/010. Epub 2009 Apr 1.
Use of a water-equivalent bolus in electron-beam radiotherapy is sometimes impractical and non-hygienic. Therefore, the feasibility of applying adjacent narrow beams for producing high surface dose electron beams without a bolus was investigated. Depth dose curves and profiles in water were calculated and measured for 6 and 9 MeV electron-beam segments (width 0.3-1.5 cm, length 10 cm) for source-to-surface distances (SSD) 102 and 105 cm. Segment shaping was performed with an add-on electron multi-leaf collimator prototype attached to the Varian 2100 C/D linac. Dose calculations were performed with the Voxel Monte Carlo++ algorithm. Resulting dose distributions in typical clinical cases were compared with the bolus technique. With a composite segmental field with 1.0 cm wide segments the surface dose was over 90% of the depth dose maximum for both energies. The build-up area practically disappeared with a 0.5 cm wide single beam. This led to decrease in the therapeutic range for composite fields with segment widths smaller than 1.0 cm. The new technique yielded similar surface doses as the bolus technique. The photon contamination was 4% with a 9 x 10 cm(2) field (1.0 cm wide segments) compared to 1% for the respective open field with 9 MeV with a bolus. The calculated dose agreed within 2 mm and 3% of the measured dose in 93.7% and 85.2% of the voxels. Adjacent narrow eMLC beams with a 1.0 cm width are suitable to produce electron fields with high surface dose. Despite a slight nonuniformity in the surface profiles in the lateral part of the field at SSD 102 cm, surface dose and target coverage are comparable with the bolus technique.
在电子束放射治疗中使用水等效填充物有时不切实际且不卫生。因此，研究了应用相邻窄束来产生无填充物的高表面剂量电子束的可行性。计算并测量了源皮距（SSD）为102和105 cm时，6和9 MeV电子束段（宽度0.3 - 1.5 cm，长度10 cm）在水中的深度剂量曲线和剂量分布。使用连接到Varian 2100 C/D直线加速器的附加电子多叶准直器原型进行射野成形。使用体素蒙特卡罗++算法进行剂量计算。将典型临床病例中的所得剂量分布与填充物技术进行比较。对于两种能量，使用1.0 cm宽射野段的复合射野时，表面剂量超过深度剂量最大值的90%。对于0.5 cm宽的单束，剂量建成区实际上消失了。这导致射野宽度小于1.0 cm的复合射野的治疗范围减小。新技术产生的表面剂量与填充物技术相似。与使用9 MeV填充物的相应开放射野相比，9×10 cm²射野（1.0 cm宽射野段）的光子污染为4%，而前者为1%。在93.7%的体素中，计算剂量与测量剂量在2 mm范围内一致，在85.2%的体素中，二者相差在3%以内。宽度为1.0 cm的相邻窄电子多叶准直器射野适合产生高表面剂量的电子射野。尽管在SSD为102 cm时射野横向部分的表面剂量分布略有不均匀，但表面剂量和靶区覆盖与填充物技术相当。
