Y. Huang, B. Zhao, B. S. H. Noora, I. Gallagher, A. J. Doemer, K. Thind, and A. M. Feldman; Department of Radiation Oncology, Henry Ford Health, Detroit, MI
Purpose/Objective(s): Intrafraction motion needs to be managed in adaptive radiotherapy due to longer treatment times. Cone beam CT guided stereotactic adaptive radiotherapy (CT-STAR) to prostate has been implemented since October 2023 in our department. The purpose of this retrospective study is to assess the adequacy of CTV-PTV margin in ensuring appropriate target coverage during intrafraction radiotherapy delivery within the CT-STAR regimen. Materials/
Methods: Between October 2023 and February 2024, CT-STAR treatment was prescribed to 15 prostate cancer patients on a HyperSight-equipped Ethos Linac (Varian Medical Systems, Palo Alto, CA). The prescription ranged from 36.25 to 40 Gy in 5 fractions with simultaneous boost to 42.5 Gy to dominant intraprostatic lesion for select patients. Prior to the treatment, patients were instructed to drink 8 oz of water after emptying bladder. At each fraction, target and organs-at-risk (OARs) volumes were updated based on an initial CBCT (CBCT1). Next, scheduled and adaptive IMRT plans were generated as per the planning directive to achieve the prescribed clinical goals. Following the selection of either scheduled or adaptive plan, a verification CBCT (CBCT2) was acquired and registered to CBCT1. Any patient motion observed with CBCT2 was corrected prior to treatment delivery. Halfway through the delivery, another CBCT (CBCT3) was acquired and registered to CBCT1 to assess and correct intrafraction motion. Besides intrafraction correction, evaluation metrics includes quantitative and qualitative change in size and shape of bladder and rectum. Specifically, bladder volume doubling was noted, quantified by its largest dimension increasing by =25% from CBCT1 to CBCT2.Change in shape of rectum was noted, attributed to potential gas pocket from CBCT1 to CBCT2. Lastly, time between CBCT1 and CBCT2 and between CBCT2 and CBCT3 were recorded. Results: A total of 58 fractions were treated following the adaptive workflow described above, with the remainder fractions deemed appropriate for delivery using the scheduled plan. Total procedure time for CT-STAR from CBCT1 to end of treatment was 32.9±6.9 min. The time from CBCT1 to CBCT2, which accounts for segmentation, adaptive planning, and quality assurance was 19.4±5.7 min. At CBCT2, 16 out of 58 sessions were observed to have bladder volume doubling, and 11 sessions showed gas pocket shifted or developed in rectum. The time from CBCT2 to mid-treatment CBCT3 was 6.0±1.3 min. Intrafraction motion management following CBCT3 led to corrections of -0.2±1.7, -0.3±1.0, and 0.1±0.5 mm in vertical, longitudinal, and lateral directions, respectively. This corresponds to 1.5±1.5 mm overall with the 95th percentile being 4.1 mm. Conclusion: A CTV-PTV margin of 4 mm is appropriate for intrafraction motion management in CT-STAR to prostate. A more rigorous approach in managing bladder and rectum is under investigation to ensure the quality of adaptive treatment.
