Clinical Outcomes of Proton Beam Therapy for Unresectable Locally Advanced Pancreatic Cancer: A Single-Center Retrospective Study.

Purpose: We retrospectively researched the treatment outcome of proton beam therapy (PBT) and assessed its efficacy for inoperable locally advanced pancreatic cancer (LAPC) at our institution. Methods and Materials: Fifty-four patients (28 men and 26 women, median age 67 years ranging from 40-88 years) were diagnosed with unresectable stage III LAPC and administered PBT from April 2009 to March 2020. Patients who could not complete PBT, had new distant metastases during the treatment, or did not have enough follow-up time were excluded from this study. All patients were clinically staged based on the International Union of Cancer TNM staging system (eighth edition) using computed tomography, magnetic resonance imaging, and positron emission tomography and were diagnosed as stage III (histologic type: 18 patients with adenocarcinoma and 36 clinically diagnosed patients). PBT was performed using the passive method, with a median total dose of 67.5 GyE (range, 50-77 GyE/25-35 fractions).Chemotherapy was used in combination during PBT in 46 patients (85.2%). Overall survival (OS), local progression-free survival (LPFS), progression-free survival, and median OS time were analyzed by Kaplan-Meier and log-rank tests. Univariate and multivariate analyses were performed for the following factors: maximum standardized uptake value (SUVmax), Eastern Cooperative Group performance status (PS), tumor site, total irradiation dose, concurrent chemotherapy, and primary tumor site. Cutoff values for SUVmax and tumor diameter were estimated using receiver operating characteristic curves and the area under the curve based on OS. Multivariate analysis was evaluated using the Cox proportional hazards models. Adverse events were evaluated using the National Cancer Institute Common Terminology Criteria for Adverse Events version 5.0. Results: The median observation period was 17.4 months, ranging from 4.0 to 89.7 months. The median tumor diameter was 36.5 mm, ranging from 15 to 90 mm, the median SUVmax was 5.85 (range, 2.1-27.6), and their cutoff values were estimated to be 37 mm and 4.8 mm, respectively. The 1- and 2-year OS was 77.8% and 35.2%, respectively, with a median OS time of 18.2 months, and only one patient survived >5 years. Twelve patients (22.2%) developed local recurrence, and 1- and 2-year LPFS rates were 89.7% and 74.5%, respectively; progression-free survival at 1 year was 58.8%. The PS score, tumor site, and irradiation dose were the prognostic factors related to OS that showed a significant difference. On the other hand, there was a significant difference in factors involved in LPFS, at 96.7%/77.9% in the first year and 86.6%/54.4% in the second year in the groups with tumor dose ≥67.5 GyE and <67.5 GyE, respectively ( Conclusions: PBT achieving 67.5 Gy combined with standard chemotherapy showed excellent local control for unresectable LAPC. Total irradiation dose, tumor site, and PS score at an initial diagnosis could be important prognostic factors. In this study, the dose-effect relationship was found, so an increase in dose should be considered to improve prognosis.