Dose-averaged linear energy transfer per se does not correlate with late rectal complications in carbon-ion radiotherapy

Abstract

Background and purposeSeveral studies have focused on increasing the linear energy transfer (LET) within tumours to achieve higher biological effects in carbon-ion radiotherapy (C-ion RT). However, it remains unclear whether LET affects late complications. We assessed whether physical dose and LET distribution can be specific factors for late rectal complications in C-ion RT. Materials and methodsOverall, 134 patients with uterine carcinomas were registered and retrospectively analysed. Of 134 patients, 132 who were followed up for >6 months were enrolled. The correlations between the relative biological effectiveness (RBE)-weighted dose based on the Kanai model (the ostensible “clinical dose”), dose-averaged LET (LETd), or physical dose and rectal complications were evaluated. Rectal complications were graded according to the Radiation Therapy Oncology Group/European Organization for Research and Treatment of Cancer criteria. ResultsNine patients developed grade 3 or 4 late rectal complications. Linear regression analysis found that D2cc in clinical dose was the sole risk factor for ≥grade 3 late rectal complications (p = 0.012). The receiver operating characteristic analysis found that D2cc of 60.2 Gy (RBE) was a suitable cut-off value for predicting ≥grade 3 late rectal complications. Among 35 patients whose rectal D2cc was ≥60.2 Gy (RBE), no correlations were found between severe rectal toxicities and LETd alone or physical dose per se. ConclusionWe demonstrated that severe rectal toxicities were related to the rectal D2cc of the clinical dose in C-ion RT. However, no correlations were found between severe rectal toxicities and LETd alone or physical dose per se.