Department of Radiation Oncology, Mayo Clinic Arizona, Phoenix, Arizona, USA.
Med Phys. 2022 Dec;49(12):7428-7437. doi: 10.1002/mp.16008. Epub 2022 Oct 18.
Parallel-opposed lateral beams are the conventional beam arrangements in proton therapy for prostate cancer. However, when considering linear energy transfer (LET) and RBE effects, alternative beam arrangements should be investigated.
To investigate the dose and dose averaged LET (LET ) impact of using new beam arrangements rotating beams 5°-15° posteriorly to the laterals in prostate cancer treated with pencil-beam-scanning (PBS) proton therapy.
Twenty patients with localized prostate cancer were included in this study. Four proton treatment plans for each patient were generated utilizing 0°, 5°, 10°, and 15° posterior oblique beam pairs relative to parallel-opposed lateral beams. Dose-volume histograms (DVHs) from posterior oblique beams were analyzed. Dose-LET -volume histogram (DLVH) was employed to study the difference in dose and LET with each beam arrangement. DLVH indices, , defined as the cumulative absolute volume that has a dose of at least d (Gy[RBE]) and a LET of at least l (keV/µm), were calculated for both the rectum and bladder to the whole group of patients and two-sub groups with and without hydrogel spacer. These metrics were tested using Wilcoxon signed-rank test.
Rotating beam angles from laterals to slightly posterior by 5°-15° reduced high LET volumes while it increased the dose volume in the rectum and increased LET in bladders. Beam angles rotated five degrees posteriorly from laterals (i.e., gantry in 95° and 265°) are proposed since they achieved the optimal balance of better LET sparing and minimal dose increase in the rectum. A reduction of V(50 Gy[RBE], 2.6 keV/µm) from 7.41 to 3.96 cc (p < 0.01), and a slight increase of V(50 Gy[RBE], 0 keV/µm) from 20.1 to 21.6 cc (p < 0.01) were observed for the group without hydrogel spacer. The LET sparing was less effective for the group with hydrogel spacer, which achieved the reduction of V(50 Gy[RBE], 2.6 keV/µm) from 4.28 to 2.10 cc (p < 0.01).
Posterior oblique angle plans improved LET sparing of the rectum while sacrificing LET sparing in the bladder in the treatment of prostate cancer with PBS. Beam angle modification from laterals to slightly posterior may be a strategy to redistribute LET and perhaps reduce rectal toxicity risks in prostate cancer patients treated with PBS. However, the effect is reduced for patients with hydrogel spacer.
在前列腺癌的质子治疗中，平行对向侧射束是常规的射束排列。然而，当考虑线性能量传递（LET）和相对生物效应（RBE）的影响时，应该研究替代的射束排列。
研究在使用铅笔束扫描（PBS）质子治疗前列腺癌时，将射束向后旋转 5°-15°的新射束排列对剂量和剂量平均 LET（LET）的影响。
本研究纳入了 20 名局部前列腺癌患者。对每位患者生成了 4 种质子治疗计划，分别为平行对向侧射束和相对的 0°、5°、10°和 15°后斜对射束。分析了后斜射束的剂量体积直方图（DVH）。使用剂量-LET-体积直方图（DLVH）研究了每种射束排列的剂量和 LET 差异。对于整个患者组和没有水凝胶间隔器的两个亚组，计算了直肠和膀胱的 DLVH 指数 ，定义为具有至少 d（Gy[RBE]）剂量和至少 l（keV/µm）LET 的累积绝对体积。使用 Wilcoxon 符号秩检验对这些指标进行了测试。
将射束角度从侧向向后旋转 5°-15°，降低了高 LET 体积，同时增加了直肠的剂量体积，并增加了膀胱的 LET。从侧向向后旋转 5 度的射束角度（即 95°和 265°的机架）是最佳的，因为它们在更好的 LET 节约和直肠中剂量增加最小之间实现了最佳平衡。在没有水凝胶间隔器的组中，V(50 Gy[RBE]，2.6 keV/µm)从 7.41 减少到 3.96 cc（p < 0.01），V(50 Gy[RBE]，0 keV/µm)从 20.1 增加到 21.6 cc（p < 0.01）。对于有水凝胶间隔器的组，LET 节约效果较差，V(50 Gy[RBE]，2.6 keV/µm)从 4.28 减少到 2.10 cc（p < 0.01）。
在使用 PBS 治疗前列腺癌时，后斜角计划改善了直肠的 LET 节约，同时牺牲了膀胱的 LET 节约。从侧向到稍向后的射束角度修改可能是一种重新分配 LET 的策略，也许可以降低接受 PBS 治疗的前列腺癌患者的直肠毒性风险。然而，对于有水凝胶间隔器的患者，这种效果会降低。
