Pencil Beam Scanning Proton Therapy reirradiation for locally recurrent rectal cancer: clinical outcome, toxicity and dosimetric comparison from a single center experience

Purpose: Treatment for locally recurrence rectal cancer is challenging. Reirradiation can be a curative or palliative treatment, and part of multi modality treatment. However, it presents risk of late complications given the radiation sensitivity of nearby organs and tissues of the abdomen and pelvis. The aim of the present study is to report the outcome and toxicity of reirradiation with Pencil Beam Scanning Proton Therapy (PBS-PT) for patients with locally recurrent rectal cancer. Methods and materials: a single-institution, retrospective analysis of 15 patients with history of pelvic radiotherapy for rectal cancer receiving PBS-PT for local recurrence was performed. Data on patient, treatment characteristics, outcomes and acute and late toxicity were collected. Dosimetrical comparison between Photon and Proton for challenging plans was done. Univariate analyses (UVA) of several factors for outcomes including FDG-PET/CT median maximum standardized uptake value (SUVmax), changes in Neutrophil/Lymphocytes ratio after PT was performed. Results: The median reirradiation dose was: 50 GyRBE (range 50,4-39,6 GyRBE). Two patients received concurrent chemotherapy. The 1-year local control, progression-free survival, and overall survival rates were 66.7%, 33,3%, 93,3% respectively. Acute grade >3 toxicity rate was 6,6%, while late grade >3 toxicity was 13,3%. At UVA Cox proportional hazards model analyses no one of the variables showed a significant difference in all outcomes. Dosimetrical comparison showed a substantial advantage in favor of PT for pre-sacral and sacral region of relapse. Conclusions: in our study PBS-PTfor locally recurrent rectal cancer demonstrated low acute toxicity rates and acceptable late toxicity supporting the use of PBS-PT as an option for this patient population.