3336 - Clinical Efficacy of Salvage Re-Irradiation for Recurrent Localized Prostate Cancer Using CBCT- and Surface Mapping-Guided Proton Scanning Beams: A Single Institute Study with 32.5-Month Follow-Ups

Materials/Methods: A total of 56 prostate salvage proton retreatments from 46 patients, between 56.4 to 88.3 years old, are retrospectively analyzed. All had definitive localized radiotherapy for prostate cancer (n=53) or the prostatectomy bed (n=3). The local recurrencies were diagnosed by consecutively raised PSA and identifiable PSMA-PET (n=52)/FDG-PET (n=2) or positive biopsy (n=2) after a remission period of 7.2 ± 6.0 years, as short as 7 to 18 months (n=9). Eight patients underwent two or three (n=2) salvage proton retreatments for different prostate recurrencies. Depending on the target proximity to the urethra, rectum and bladder neck, 2-3 mm and 3.5% range uncertainties are employed to compensate the variations of beam setup and target location. 30.60 - 60.00 GyE over 17 - 30 fractions were delivered by two coplanar oblique beams, guided by CBCT and surface-mapping, on a technology company ProBeam™. The International Prostate Symptom Scores (IPSS) are used for evaluating the adverse effects of prostate radiation treatments.

Results: The results of PSA value reduction and, suggesting the side effects of the organ at risk, are tabulated in table 1. In this serial 43 out of 56 cases (76.8%) showed a noticeable (>1.0 ng/mL) PSA reduction, while 30 out of 56 cases (57.1%) PSA were reduced by >2.0 mg/mL post the retreatments. Over 57% of cases reported mild or no symptoms, while only 4 out of 56 cases exhibited severe symptomatic grades. Notably, symptoms improved over time in all cases without the need for comprehensive interventions. Due to the location proximity of the recurrencies to the organs at risk, doses to the rectum (D_mean = 2.01 ±1.76 GyE) and bladder (D_15cc 2.58 ±5.33 GyE) vary noticeably between cases.

Conclusion: This single-institutional result suggests the efficacy of salvage re-irradiation with proton scanning beams for locally recurrent prostate cancer, showing effective local control through rapid PSA reduction and limited short-term side effects. The findings also support repeated proton localized retreatment in selected cases, highlighting its potential as an impactful therapy for recurrent prostate cancer.

Abstract 3336 - Table 1: Short-term results of localized recurrency control (PSA Reduction) and adverse effects (IPSS Score) after salvage prostate retreatments, N=56

(Note: The groups of IPSS respectively imply the mildly, moderately, and severely symptomatic grades defined by NHS-Royal United Hospital Bath.)