RE-IRRADIATION FOR RECTAL CANCER USING PENCIL-BEAM SCANNING PROTON THERAPY: A SINGLE INSTITUTIONAL EXPERIENCE

Abstract Purpose Re-irradiation (Re-RT) for rectal cancer (RC) after prior pelvic RT has been shown to be safe and effective. However, limited data exists for proton therapy (PT), including Pencil-Beam Scanning Proton Therapy (PBS-PT). We hypothesize that PT is safe and feasible for re-treatment and may allow for decreased toxicity and treatment escalation. Methods and Materials A single-institution retrospective IRB-approved analysis of all RC patients with prior pelvic RT receiving PBS-PT Re-RT was performed. Data regarding patient and treatment characteristics; and outcomes were collected. Local Progression (LP), Progression Free Survival (PFS), Overall Survival (OS), and late Grade 3+ toxicity (G3+Tox) were estimated using Kaplan-Meier. Results Twenty-eight patients (median follow-up 28.6 months) received PBS-PT Re-RT between 2016-2019: 18 recurrent RC [median prior dose 54.0 Gy] and 10 de novo RC with variable prior RT. Median Re-RT dose was 44.4 Gy [(16.0-60.0); 21/28 BID], and 24/28 received concurrent chemotherapy. Six underwent surgical resection. Three (10.7%) experienced grade 3 acute toxicities, and one did not complete RT due to toxicity. Four (14.2%) had late G3+Tox, including one grade 5 toxicity in a patient with prior RT-related injury. One-year LP, PFS, and OS were 33.7% (95% CI 14.5-52.9%), 45.0% (95% CI 26.2-63.8%), and 81.8% (95% CI 67.3-96.3%), respectively. Conclusions This is the largest series using PT for Re-RT for RC and the first using PBS-PT. Low acute toxicity rates and acceptable late toxicity support PBS-PT as an option for this high risk patient population, with a need for continued follow up.