BACKGROUND AND PURPOSE Proton therapy (PT) is currently being studied to improve normal tissue (NT) sparing beyond what can be achieved with conventional photon-based therapy. Compared to photons, PT dose distributions have a reduced NT low-to-intermediate 'dose bath' and a different biological effectiveness, questioning the applicability of photon-based NT complication probability (NTCP) models to PT. The aim of this study was to assess the applicability of photon-based NTCP models to rectum morbidity outcomes following PT. MATERIALS AND METHODS Treatment planning and morbidity data from 1151 prostate cancer patients treated with passive scattering PT and from 159 patients treated with conventional 3D conformal four-field photon therapy were analysed. Prospectively scored gastrointestinal morbidities (grade >=2) were analysed, with a total of 184 events (protons; medical and procedural) and 12 events (photons; procedural only), respectively. Rectal dose volume histograms were extracted for all patients in both cohorts and used as input to two different NTCP models, with up to six different published photon-based parameter sets. RESULTS Photon-based rectal NTCP models either over- or underestimated the clinically observed gastrointestinal morbidity when used on the proton cohort, depending on the choice of endpoint (p < 0.05 for all parameter sets, for both morbidity classifications). Four of the six photon-based NTCP models showed a good fit to the photon outcome data (p > 0.05). CONCLUSION There were large differences in morbidity predictions between cohorts and modalities, indicating that the validity of NTCP models and parameters across institutions and treatment modalities should be carefully investigated prior to clinical application.

中文翻译：

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从光子到质子放疗的直肠正常组织并发症概率模型的跨模式适用性

背景和目的 目前正在研究质子治疗 (PT)，以改善正常组织 (NT) 的保留，超出常规基于光子的治疗所能达到的效果。与光子相比，PT 剂量分布具有降低的 NT 中低“剂量浴”和不同的生物有效性，质疑基于光子的 NT 并发症概率 (NTCP) 模型对 PT 的适用性。本研究的目的是评估基于光子的 NTCP 模型对 PT 后直肠发病率结果的适用性。材料和方法 分析了 1151 名接受被动散射 PT 治疗的前列腺癌患者和 159 名接受常规 3D 适形四场光子治疗的患者的治疗计划和发病率数据。分析了前瞻性评分的胃肠道疾病（等级 >=2），总共有 184 个事件（质子；医学和程序）和 12 个事件（光子；仅程序）。提取了两个队列中所有患者的直肠剂量体积直方图，并将其用作两个不同 NTCP 模型的输入，其中包含多达六个不同的已发布的基于光子的参数集。结果 当用于质子队列时，基于光子的直肠 NTCP 模型高估或低估了临床观察到的胃肠道发病率，这取决于终点的选择（对于两种发病率分类，所有参数集的 p < 0.05）。六个基于光子的 NTCP 模型中有四个显示出与光子结果数据的良好拟合（p > 0.05）。结论 队列和方式之间的发病率预测存在很大差异，