Session Item

To analyze the dosimetric planning parameters and organ volumetric doses for 34 localized salvage prostate cancer retreatments using intensity-modulated proton scanning beams with a near-term recurrence-free survival, PSA response, and toxicity assessment.

A total of 34 salvage radiation retreatments using proton beam scanning for locally recurrent prostate cancers for 30.6 -60 GyE over 17 - 30 fractions with a daily doses of 1.8 or 2.0 GyE were retrospectively analyzed. All the patients, at age of 77.4 (ranging from 56.4 to 88.3) years old with previous radiation treatments of either EBRT (30), Brachytherapy (3), or HIFU (1), were treated on a ProBeam Compact™ (Varian Medical, CA) with two coplanar oblique proton scanning beams under daily CBCT guidance. The local recurrences were confirmed pathologically except for one being diagnosed with Axumin-PET for elevated PSA. 32 cases had positive Axumin/PSMA-PET (7) or FDG-PET (25) which assisted in outlining GTVs. All the treatment plans employed 3-5 mm/3.5% dosimetry robustness and computed on a Varian Eclipse planning system using the single-field optimization and AcurosPT (Monte Carlo alternative) dose algorithm.  PTVs, expanded from GTVs by about 3 mm based on the proximities to the rectum and/or bladder, are used for prescribed dose evaluations.

In this series, PTV (11.89 ± 19.17 cm3) to GTV (4.78 ± 9.20 cm3) ratio is 3.18 ± 1.32. The D95% of PTV is (96.8 ± 2.1) % of prescribed doses. The volumetric doses of the organs at risk are listed in table 1, where values are grossly comparable with those treated with other modalities as published. In some complicated cases, the maximal volumetric dose values appear noticeably higher than the average. With a mean follow-up time of 16.4 ± 10.3 months, only mild GU and GI toxicities and no local recurrence were identified. 28 (or 82.4%) cases showed more than 50% of PSA reduction post-proton retreatments.  

Table 1. Volumetric Doses of Organ at Risks for Prostate Cancer Radiation Retreatments using Proton Beam Scanning (n =34)

Our preliminary results, with over 16-month follow-up, suggest a good clinical efficacy in salvage proton scanning beam retreatment for localized recurrent prostate cancer post definitive localized radiation therapy using the dosimetric parameters as presented. The volumetric dosimetric values to the rectum, bladder, and urethra analyzed in this study may imply a practical reference for future prostate retreatments using intensity-modulated proton scanning beams.