3282 - Clinical Efficacy of Localized Salvage Prostate Cancer Retreatments Using CBCT-Guided Proton Scanning Beams with Over 2-Year Follow-Ups

C. Y. Shang¹, S. Ramirez², M. Estrada¹, G. Evans¹, and T. R. Williams¹; ¹South Florida Proton Therapy Institute, Delray Beach, FL, ²SFPTI, Delray Beach, FL

Purpose/Objective(s): To retrospectively analyze the clinical efficacy of localized salvage prostate cancer retreatment by CBCT and surface mapping guided intensity modulated proton therapy (IMPT). Materials/

Methods: 46 out of 50 cases underwent local salvage radiation retreatments utilizing CBCT and surface-mapping guided IMPT with proton beam scanning, were retrospectively analyzed over a follow-up period of 28.4 ± 13.0 months. All had previous definitive external radiation therapy or brachytherapy for localized prostate cancer. After 8.7 ± 7.9-year remission, the patients experienced consecutively raised PSA and identifiable FDG-PET (n=33)/PSMA-PET (n=10) or positive local biopsy (n=3). Gross tumor volume (GTV) is outlined by using fused PET and MR images. Depending on the proximity to rectum and bladder, the planning tumor volume (PTV) is expanded from GTV by variable margins of 2-3 mm, measuring 10.8 cm³ in average. Usually, two coplanar oblique beams are used to deliver a total of 30.06 - 60.00 GyE intensity-modulated proton to PTV over 17 - 30 fractions. All the treatments plans were computed on a treatment planning system using AcurosPT (Monte Carlo alternative) dose algorithm with 3-5 mm/3.5% dosimetry robustness.

Results: In this series, the volume ratio of PTV (10.8 ± 15.9 cm³) to GTV (4.4 ± 7.6 cm³) is 3.1. The plans are normalized D95% to 95% prescribed dose. 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. Due to proximity of GTV to the organs at risk, calculated doses to the rectum and bladder noticeably vary between cases. With a mean follow-up of 21.4 ± 11.4 months, the results of PSA value reduction, suggesting the local tumor control, and the International Prostate Symptom Scores, suggesting the side effects of the organ at risk, are tabulated in table 1. In this serial more than 78% cases showed PSA reduction while over half (52.2%) PSA’s were reduced by more than 0.8 ng/mL. Overall 71.7% cases experienced mild or no symptoms and 95.6% experienced mild to moderate symptoms. In this investigated group, four cases reported grade II and one for grade III symptoms of proctitis.

Conclusion: The findings of this retrospective study from a single institute with over two-year follow-ups indicate that employing CBCT and surface mapping guided IMPT for salvage retreatment shows promising clinical efficacy for localized recurrent prostate cancer subsequent to initial definitive regional radiation therapy. Note: The groups of IPSS (International Prostate Symptom Score) respectively implies the mildly, moderately, and severely symptomatic grades.

Abstract 3282 – Table 1

	PSA Reduction, ng/mL			IPSS Scored Groups
Values	by > 0.8	Yes	No	0-7	8-19	20-35
Cases	24	36	10	33	11	1
Percent	52.2%	78.3%	21.7%	71.7%	23.9%	2.2%