Clinical and Dosimetric Outcomes of MR Guided Prostate SBRT With Daily Online Plan Adaptation

Abstract

Stereotactic body radiotherapy (SBRT) is increasingly being utilized in the management of localized prostate cancer. MR guided adaptive radiotherapy may be a valuable tool for delivery of prostate SBRT as it allows for MR based soft tissue contouring, real time MR motion management and daily plan adaptation to account for interfraction anatomic variability. The goal of the present study is to identify dosimetric gains that can be achieved with an MR guided adaptive prostate SBRT platform and describe acute clinical outcomes.We reviewed our institutional experience treating patients with prostate cancer with MR guided adaptive SBRT to a dose of 36.25 Gy in 5 fractions. The adaptive workflow involves development of an "original" plan which is re-optimized daily based on anatomical variation for each fraction. "Predicted" plan dosimetry is generated for each fraction by re-calculating the original plan on the re-contoured daily anatomy, estimating what would have been delivered without adaptive capabilities. A "re-optimized" plan is developed based on the anatomy of the day to improve target coverage and organ-at-risk (OAR) metrics. Wilcoxon rank sum testing was used to compare predicted dosimetry to re-optimized dosimetry. Acute urinary and GI toxicity rates were assessed according to CTCAE v5.A cohort of 16 patients treated was analyzed with median follow up time of 3 months. Acute G2+ GU toxicity was observed in 19% and acute G2+ GI toxicity in 6%. No G3+ events were noted. Comparison of dosimetry metrics for PTV coverage, urethra, rectum and bladder are shown in table 1; 100% of fractions required plan adaptation due to either exceeding OAR metrics (68%) or sub-optimal PTV coverage (33%). Adaptive planning increased the proportion of fractions meeting prescription metrics for PTV coverage (V34.44 from 71% to 100%), urethra (V38.78 from 58% to 100%) and rectum (V38.06 from 80% to 100%). Urethral metrics were most frequently violated in predicted plans (42% of fractions) and re-optimized to acceptable dosimetry in 100% of fractions. Re-optimization significantly improved PTV coverage, rectal metrics and urethral metrics.Online adaptive re-optimization significantly improved plan dosimetry and achieved minimal acute toxicity. The clinical benefits of MR guided adaptive prostate SBRT for optimizing target coverage with urethral protection warrant further investigation.