A Comparison of Treatment Plan Adaptation Methods on PTV Margin Coverage in MR-Linac Prostate SBRT.

Abstract

MRI-Linac guided radiotherapy is a novel treatment modality allowing for daily online adaptation of contours and treatment planning. MRI-Linac prostate SBRT (MRL-SBRT) has recently been shown to provide a tangible benefit in decreasing both acute and late toxicity, due to the ability to evaluate soft tissue contrast, directly monitor organ motion, and anatomical changes during the entire treatment course. Multiple MRI-Linac platforms are available, which provide different advantages with regards to imaging and motion management. We undertook this study to assess coverage of the clinical target volume with use of a standard PTV margin in patients receiving either adapt-to-position (ATP) only treatment or a combination of adapt-to-position and adapt-to-shape (ATP+ATS) on the Elekta Unity MR-Linac. Data was collected retrospectively in patients who underwent prostate SBRT using a 1.5 T MR-Linac (Unity, Elekta AB, Stockholm, Sweden). Prior to each fraction, determination of ATP or ATS planning was at the discretion of the treating physician. Prostate and organs at risk (OAR's) were contoured on the reference MRI and daily MR image sets acquired prior to each fraction. The prostate PTV consisted of a 5mm margin, except 3mm posteriorly. Contours were merged to account for target and OAR volumes during all fractions. Target volumes from the reference plan were then compared to the clinical target volume (CTV) on pre-treatment MR-Linac imaging as well as intersection with OAR's. Nineteen men, median age 72 years (range 67-75), with prostate cancer underwent treatment from May 2021 to January 2023 with MRL-SBRT. The average time for ATP vs ATP+ATS was 45 minutes (range 35-60) and 60 minutes (range 50-80), respectively. The mean CTV treated was 43.4cc (range 31-60) for ATP and 51.8cc (range 36-67) for ATP+ATS. Prior to plan adaptation, target volumes in 92.6% of fractions were not covered completely by the PTV. In the ATP cohort, 89% of fractions had a portion of the CTV uncovered with an average of 2.9cc. In the ATP+ATS cohort, 96.4% had a portion of the CTV uncovered, with an average of 6.9cc. There was no difference in the amount of PTV intersection for ATP or ATP+ATS plans with the rectum (1.1cc vs. 1.5cc) and bladder (9.2cc vs. 12.6cc), respectively. All patients completed the planned course of treatment. Adaptive radiation therapy for prostate cancer utilizing an MR-Linac based approach with the Elekta Unity was feasible for patients requiring either ATP only or ATP+ATS plans. As expected, ATS plans required longer treatment time, allowing for improved coverage of the target when deemed clinically necessary. Despite a larger overall volume treated in ATS plans, there was no difference in overlap with OARs between ATP and ATS plans. Adaption for each daily fraction and intrafraction motion monitoring improved CT coverage with our pre-specified PTV margin.