A Single-Institution Prospective Study To Evaluate the Safety and Efficacy of Real- Time Image-Gated Spot-Scanning Proton Therapy (RGPT) for Prostate Cancer

Abstract

BackgroundIn Real-time-image Gated spot-scanning Proton Therapy (RGPT), the dose distribution is distorted by gold fiducial markers placed in the prostate. Distortion can be suppressed by using small markers and more than two fields, but additional fields may increase the dose to organs at risk. Therefore, we conducted a prospective study to evaluate the safety and short-term clinical outcome of RGPT for prostate cancer. MethodsBased on the previously reported frequency of early adverse events (AE) and the non-inferiority margin of 10%, the required number of cases was calculated to be 43 using the one-sample binomial test by the SWOG statistical tools with the one-sided significance level of 2.5% and the power 80%. Patients with localized prostate cancer were enrolled and 3-4 pure gold fiducial markers of 1.5 mm diameter were inserted in the prostate. The prescribed dose was 70 Gy(RBE) in 30 fractions, and treatment was performed with three fields from the left, right, and the back, or four fields from either side of slightly anterior and posterior oblique fields. The primary endpoint was the frequency of early AE (≥ Grade 2) and the secondary endpoint was the biochemical relapse-free survival rate (BRFS) and the frequency of late AE. ResultsForty-five cases were enrolled between 2015 and 2017 and all patients completed the treatment protocol. The median follow-up period was 63.0 months. The frequency of early AE (≥ Grade 2) was observed in 4 cases (8.9%), therefore the non-inferiority was verified. The overall 5-year BRFS was 88.9%. As late AE, Grade 2 rectal bleeding were observed in 8 cases (17.8%). ConclusionsThe RGPT for prostate cancer with 1.5 mm markers and 3- or 4- fields was as safe as conventional proton therapy in early AE, and its efficacy was suggested comparable with previous studies.