Retrospective Analysis of Proton Beam Therapy for Postoperative Locally Recurrent Rectal Cancer

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<h3>Purpose/Objective(s)</h3> The outcomes of postoperative locally recurrent rectal cancer (PLRRC) are unsatisfactory in the multimodal treatment era. We retrospectively analyzed the outcomes of proton beam therapy (PBT) for PLRRC at our institution. <h3>Materials/Methods</h3> We included 79 PBT-treated PLRRC patients between December 2008 to December 2019. We excluded patients with prior history of salvage surgery, radiotherapy, or distant metastasis before PBT, and those treated with postoperative adjuvant PBT. We included patients who had history of chemotherapy. We estimated the overall survival (OS), disease-free survival (DFS), and local control (LC) using the Kaplan–Meier method. The prognostic factor for each outcome was verified using a Cox proportional hazards model. <h3>Results</h3> We enrolled 23 patients. The median follow-up time was 36.8 months. The median age was 64 (range, 34–83) years. The median time from surgery to recurrence was 18.1 months. Twenty patients (87%) had adenocarcinoma, two had carcinoid, and one had neuroendocrine carcinoma. The postoperative recurrent sites were presacral (12 patients), pelvic side wall (seven patients), and anastomoses (four patients). The median recurrent tumor size was 37 (range, 16–130) mm. The median maximum standardized uptake value (SUV_max) of Fluorine-18-fluorodeoxyglucose-positron emission tomography/computed tomography (18^F-FDG-PET/CT) before PBT was 7.4. Twenty-one patients (91%) received PBT alone. The median total dose was 72.6 (range, 60–87) Gy relative biological effectiveness. The treatment response was stratified by initial imaging (median, two months) after PBT. Of 23 patients, 16 (70%) received 18^F-FDG-PET/CT for initial imaging modality. Eleven patients had complete response (CR), eight had partial response, two had stable disease, and two had progressive disease. Eleven patients (48%) had lumbar or anal pain before PBT and had symptomatic pain improvement after PBT. Three- and five-year OS, DFS, and LC were 71.5%/43.3%, 37.3%/37.3%, and 54.5%/46.7%, respectively. The SUV_max of 18^F-FDG-PET/CT before PBT (cut-off value:10) showed significant difference for OS (<i>P</i>=0.0135), DFS (<i>P</i>=0.047), and LC (<i>P</i>=0.0239). The patients who achieved CR after PBT had significantly better LC compared with those with non-CR (HR 4.71, 95% CI 1.18–18.65, <i>P</i>=0.0177). Elderly patients (≧65 years) had significantly better LC and DFS. The patients with pain before PBT and larger tumor (≧30 mm) also had significantly lower DFS. Of 23 patients, 12 (52%) had further local recurrence after PBT. One patient developed grade 2 acute radiation dermatitis. Grade 4 late gastrointestinal toxicity (perforation) was recorded in three patients, two of which were associated with reirradiation for further local recurrence after PBT. <h3>Conclusion</h3> The outcomes of PBT for PLRRC are better than those of conventional photon therapy, and are comparable with those of surgery. 18^F-FDG-PET/CT before/after PBT correctly reflected the treatment effect and prognosis.