Outcomes and Prognostic Factors for Locally Recurrent Rectal Cancer Treated With Proton Beam Therapy

PURPOSE: Our objective was to report the outcome and prognostic factors for patients with locally recurrent rectal cancer (LRRC) treated with proton beam therapy (PBT) at our institution. METHODS AND MATERIALS: The study included PBT-treated patients with LRRC between December 2008 and December 2019. Treatment response was stratified using an initial imaging test after PBT. Overall survival (OS), progression-free survival (PFS), and local control (LC) were estimated using the Kaplan-Meier method. Each outcome's prognostic factors were verified using the Cox proportional hazards model. RESULTS: Twenty-three patients were enrolled (median follow-up, 37.4 months). There were 11 patients with complete response (CR) or complete metabolic response (CMR), 8 with partial response or partial metabolic response, 2 with stable disease or stable metabolic response, and 2 with progressive disease or progressive metabolic disease. Three- and 5-year OS, PFS, and LC were 72.1% and 44.6%, 37.9% and 37.9%, and 55.0% and 47.2%, respectively, with 54.4 months’ median survival time. The maximum standardized uptake value of fluorine-18-fluorodeoxyglucose-positron emission tomography–computed tomography ((18)F-FDG-PET/CT) before PBT (cutoff value, 10) showed significant differences in OS (P = .03), PFS (P = .027), and LC (P = .012). The patients who achieved CR or CMR after PBT had significantly better LC than those with non-CR or non-CMR (hazard ratio, 4.49; 95% confidence interval, 1.14-17.63; P = .021). Older patients (aged ≥65 years) had significantly higher LC and PFS rates. Patients with pain before PBT and larger tumors (≥30 mm) also had significantly lower PFS. Of 23 patients, 12 (52%) experienced further local recurrence after PBT. One patient developed grade 2 acute radiation dermatitis. Regarding late toxicity, grade 4 late gastrointestinal toxic effects were recorded in 3 patients, in 2 of whom reirradiation was associated with further local recurrence after PBT. CONCLUSIONS: The results showed that PBT may have potential to be a good treatment option for LRRC. (18)F-FDG-PET/CT before and after PBT may be useful for assessing tumor response and predicting outcomes.