Underwood Tracy S A, Voog Justin C, Moteabbed Maryam, Tang Shikui, Soffen Edward, Cahlon Oren, Lu Hsiao-Ming, Zietman Anthony L, Efstathiou Jason A, Paganetti Harald
a Department of Radiation Oncology , Massachusetts General Hospital, Harvard Medical School , Boston , MA , USA.
b Department of Medical Physics and Bioengineering , University College London , London , UK.
Acta Oncol. 2017 Apr;56(4):575-581. doi: 10.1080/0284186X.2016.1275781. Epub 2017 Jan 11.
Anterior-oblique (AO) proton beams can form an attractive option for prostate patients receiving external beam radiotherapy (EBRT) as they avoid the femoral heads. For a cohort with hydrogel prostate-rectum spacers, we asked whether it was possible to generate AO proton plans robust to end-of-range elevations in linear energy transfer (LET) and modeled relative biological effectiveness (RBE). Additionally we considered how rectal spacers influenced planned dose distributions for AO and standard bilateral (SB) proton beams versus intensity-modulated radiotherapy (IMRT).
We studied three treatment strategies for 10 patients with rectal spacers: (A) AO proton beams, (B) SB proton beams and (C) IMRT. For strategy (A) dose and LET distributions were simulated (using the TOPAS Monte Carlo platform) and the McNamara model was used to calculate proton RBE as a function of LET, dose per fraction, and photon α/β. All calculations were performed on pretreatment scans: inter- and intra-fractional changes in anatomy/set-up were not considered.
For 9/10 patients, rectal spacers enabled generation of AO proton plans robust to modeled RBE elevations: rectal dose constraints were fulfilled even when the variable RBE model was applied with a conservative α/β = 2 Gy. Amongst a subset of patients the proton rectal doses for the planning target volume plans were remarkably low: for 2/10 SB plans and 4/10 AO plans, ≤10% of the rectum received ≥20 Gy. AO proton plans delivered integral doses a factor of approximately three lower than IMRT and spared the femoral heads almost entirely.
Typically, rectal spacers enabled the generation of anterior beam proton plans that appeared robust to modeled variation in RBE. However, further analysis of day-to-day robustness would be required prior to a clinical implementation of AO proton beams. Such beams offer almost complete femoral head sparing, but their broader value relative to IMRT and SB protons remains unclear.
前斜（AO）质子束可为接受外照射放疗（EBRT）的前列腺癌患者提供一个有吸引力的选择，因为它们可避开股骨头。对于使用水凝胶前列腺 - 直肠间隔物的队列，我们探讨了是否有可能生成对射程末端线性能量转移（LET）升高和模拟相对生物效应（RBE）具有鲁棒性的AO质子计划。此外，我们考虑了直肠间隔物如何影响AO和标准双侧（SB）质子束与调强放疗（IMRT）的计划剂量分布。
我们研究了10例使用直肠间隔物患者的三种治疗策略：（A）AO质子束，（B）SB质子束和（C）IMRT。对于策略（A），模拟了剂量和LET分布（使用TOPAS蒙特卡罗平台），并使用麦克纳马拉模型根据LET、分次剂量和光子α/β计算质子RBE。所有计算均基于治疗前扫描进行：未考虑解剖结构/设置中的分次间和分次内变化。
对于9/10的患者，直肠间隔物能够生成对模拟RBE升高具有鲁棒性的AO质子计划：即使应用保守的α/β = 2 Gy的可变RBE模型，直肠剂量限制也能满足。在一部分患者中，计划靶体积计划的质子直肠剂量非常低：对于2/10的SB计划和4/10的AO计划，≤10%的直肠接受≥20 Gy的剂量。AO质子计划的积分剂量比IMRT低约三倍，并且几乎完全避开了股骨头。
通常，直肠间隔物能够生成对模拟RBE变化具有鲁棒性的前束质子计划。然而，在AO质子束临床应用之前，需要进一步分析其日常鲁棒性。这种束几乎完全避开了股骨头，但其相对于IMRT和SB质子的更广泛价值仍不清楚。
