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

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.

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.

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 (¹⁸F-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.

The results showed that PBT may have potential to be a good treatment option for LRRC. ¹⁸F-FDG-PET/CT before and after PBT may be useful for assessing tumor response and predicting outcomes.