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

Abstract

Several 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. Overall, 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. Nine patients developed grade 3 or 4 late rectal complications. Linear regression analysis found that D_2cc in clinical dose was the sole risk factor for ≥grade 3 late rectal complications (p = 0.012). The receiver operating characteristic analysis found that D_2cc of 60.2 Gy (RBE) was a suitable cut-off value for predicting ≥grade 3 late rectal complications. Among 35 patients whose rectal D_2cc was ≥60.2 Gy (RBE), no correlations were found between severe rectal toxicities and LETd alone or physical dose per se. We demonstrated that severe rectal toxicities were related to the rectal D_2cc 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.