Damen E M, Brugmans M J, van der Horst A, Bos L, Lebesque J V, Mijnheer B J, McShan D L, Fraass B A, Kessler M L
Radiotherapy Division, The Netherlands Cancer Institute/Antoni van Leeuwenhoek Huis, Amsterdam, The Netherlands.
Int J Radiat Oncol Biol Phys. 2001 Mar 15;49(4):1183-95. doi: 10.1016/s0360-3016(00)01525-x.
To develop and verify a multisegment technique for prostate irradiation that results in better sparing of the rectal wall compared to a conventional three-field technique, for patients with a concave-shaped planning target volume (PTV) overlapping the rectal wall.
Five patients have been selected with various degrees of overlap between PTV and rectal wall. The planned dose to the ICRU reference point is 78 Gy. The new technique consists of five beams, each having an open segment covering the entire PTV and several smaller segments in which the rectum is shielded. Segment weights are computer-optimized using an algorithm based on simulated annealing. The score function to be minimized consists of dose-volume constraints for PTV, rectal wall, and femoral heads. The resulting dose distribution is verified for each patient by using point measurements and line scans made with an ionization chamber in a water tank and by using film in a cylindrical polystyrene phantom.
The final number of segments in the five-field technique ranges from 7 to 9 after optimization. Compared to the standard three-field technique, the maximum dose to the rectal wall decreases by approximately 3 Gy for patients with a large overlap and 1 Gy for patients with no overlap, resulting in a reduction of the normal tissue complication probability (NTCP) by a factor of 1.3 and 1.2, respectively. The mean dose to the PTV is the same for the two techniques, but the dose distribution is slightly less homogeneous with the five-field technique (Average standard deviation of five patients is 1.1 Gy and 1.7 Gy for the three-field and five-field technique, respectively). Ionization chamber measurements show that in the PTV, the calculated dose is in general within 1% of the measured dose. Outside the PTV, systematic dose deviations of up to 3% exist. Film measurements show that for the complete treatment, the position of the isodose lines in sagittal and coronal planes is calculated fairly accurately, the maximum distance between measured and calculated isodoses being 4 mm.
We developed a relatively simple multisegment "step-and-shoot" technique that can be delivered within an acceptable time frame at the treatment machine (Extra time needed is approximately 3 minutes). The technique results in better sparing of the rectal wall compared to the conventional three-field technique. The technique can be planned and optimized relatively easily using automated procedures and a predefined score function. Dose calculation is accurate and can be verified for each patient individually.
针对计划靶区（PTV）呈凹形且与直肠壁重叠的患者，开发并验证一种前列腺照射的多野技术，与传统的三野技术相比，该技术能更好地保护直肠壁。
选择了5例PTV与直肠壁有不同程度重叠的患者。ICRU参考点的计划剂量为78 Gy。新技术由五束射野组成，每束射野都有一个覆盖整个PTV的开放野以及几个屏蔽直肠的较小子野。子野权重通过基于模拟退火的算法进行计算机优化。要最小化的评分函数包括PTV、直肠壁和股骨头的剂量体积约束。通过在水箱中使用电离室进行点测量和线扫描以及在圆柱形聚苯乙烯体模中使用胶片，对每位患者得到的剂量分布进行验证。
优化后，五野技术的最终子野数量在7至9个之间。与标准三野技术相比，对于重叠程度大的患者，直肠壁的最大剂量降低约3 Gy，对于无重叠的患者降低1 Gy，导致正常组织并发症概率（NTCP）分别降低1.3倍和1.2倍。两种技术对PTV的平均剂量相同，但五野技术的剂量分布均匀性稍差（三位患者的平均标准差，三野技术为1.1 Gy，五野技术为1.7 Gy）。电离室测量表明，在PTV内，计算剂量一般在测量剂量的1%以内。在PTV外，存在高达3%的系统剂量偏差。胶片测量表明，对于整个治疗过程，矢状面和冠状面等剂量线的位置计算相当准确，测量等剂量线与计算等剂量线之间的最大距离为4 mm。
我们开发了一种相对简单的多段子野“步进式”技术，该技术可在治疗机的可接受时间范围内实施（额外所需时间约为3分钟）。与传统三野技术相比，该技术能更好地保护直肠壁。使用自动化程序和预定义的评分函数，该技术相对容易进行计划和优化。剂量计算准确，且可为每位患者单独验证。
