ESTRO 2024 - Abstract Book

S2347

Clinical - Urology

ESTRO 2024

708

Digital Poster

Clinical Efficacy & Dose Analysis of Salvage Prostate Cancer Retreatments using Proton Beam Scanning

Charles Shang, Salina Ramirez, Grant Evans, Timothy R Williams

South Florida Proton Therapy Institute, Radiation Oncology, Delray Beach, USA

Purpose/Objective:

The local salvage treatment for locally recurrent prostate cancer has still been a challenging task[i]. This study is designed to retrospectively analyze the clinical efficacy of salvage retreatment for recurrent localized prostatic cancer with daily CBCT-guided proton beam scanning (PBS), exclusively for those who previously received definitive localized radiation therapy. This study also summarizes the dosimetric planning parameters and volumetric doses to the organs at risk using two-oblique intensity modulated proton therapy (IMPT) beams.

Material/Methods:

A total of 44 salvage prostate cancer retreatments using CBCT-guided proton beam scanning were collected for retrospective analysis from a single institution. The average length of posttreatment follow-ups is 21.4 ± 11.4 months. All patients, ranging from 56.4 to 88.3 years old, previously had definitive external radiation therapy or brachytherapy for localized prostate cancer (n = 40) or for post-prostatectomy (n = 4). After 8.5 ± 7.9-year (1.9 to 49.9 years) remission, all experienced consecutively raised levels of prostate-specific antigen (PSA) and identifiable FDG-PET (n = 31)/PSMA-PET (prostate-specific membrane antigen PET, n = 9) or positive local biopsy (n = 4). Gross tumor volume (GTV) is outlined using fused PET and MR images. Depending on the proximity to the rectum and bladder, the planning tumor volume (PTV) is expanded from GTV with variable margins of 2-3 mm. Out of 44 cases, 35 patients were inserted with a thin urinary catheter during CT simulation to identify the urethra. Using two coplanar oblique proton beams, a total of 30.06 - 60.00 GyE is planned and delivered to PTV over 17 - 30 fractions. All the IMPT plans employed 3-5 mm/3.5% dosimetry uncertainty margins with about 6 mm dosimetric robustness, which is computed by the Varian Eclipse planning system (Varian Medical System, Palo Alto, CA) using AcurosPT (Monte Carlo alternative) dose algorithm.

Results:

In this series, the volume ratio of PTV (11.2 ± 16.8 cm3) to GTV (4.59 ± 8.15 cm3) is 2.44 ± 5.68. All the plans are normalized from 95% of PTV to 95% of prescribed doses. The volumetric doses of the organs at risk are listed in Table 1, where values are noticeably lower than those reported for localized proton treatment for prostate cancer [ii] [iii]. Due to the 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, as tabulated in Table 2, show a reduction in PSA, suggesting the local tumor control, and favorite International Prostate Symptom Scores (IPSC), suggesting low side effects for the organs at risk. In this series, 50% of cases showed some reductions in PSA, even as soon as 7 days after the completion of an 18-fractionated prostate retreatment course. 29.5% of patients demonstrated PSA reduction by more than 1.0 ng/mL after the retreatment. Within our follow-up period, no local or marginal