University of Texas Southwestern Medical Center Dallas, TX
J. J. Shi1, A. Yen2, S. Wang2, B. Washington2, C. Kabat2, A. Pompos2, A. Garant2, N. B. Desai2, R. Hannan2, S. N. Badiyan3, M. H. Lin2, F. C. Su4, and D. X. Yang2; 1School of Medicine, University of Texas Southwestern Medical Center, Dallas, TX, 2Department of Radiation Oncology, University of Texas Southwestern Medical Center, Dallas, TX, 3Department of Radiation Oncology, UT Southwestern Medical Center, Dallas, TX, 4University of Texas Southwestern Medical Center, Dallas, TX
Purpose/Objective(s): Adaptive MRI-guided prostate stereotactic ablative radiotherapy (SAbR) may offer improved visualization of soft tissue anatomy and direct monitoring of intra- and inter-fraction prostate motion. Current MRI-guided online adaptive workflows range from daily plan adaptation based on patient positional shifts to full plan re-optimization based on updated re-contouring; however, the selection of an optimal adaptation strategy remains undefined. In this study, we aim to compare dosimetric differences between two online adaptation strategies used for dose-intensified prostate SAbR. Materials/
Methods: We retrospectively identified prostate cancer patients treated with MRI-guided SAbR 40 Gy with simultaneous integrated boost to 45 Gy in 5 fractions using an online “adapt-to-position” (ATP) workflow accounting for positional shift. We assessed the “delivered dose” of the adapt-to-position (dATP) workflow using updated physician-validated contours on daily MRI imaging. We also performed full plan re-optimization using the updated contours to simulate an online “adapt-to-shape” (ATS) workflow. Dosimetry differences were calculated. Results: Twenty prostate cancer patients undergoing 100 adaptive fractions were included for analysis. Patients had a median age of 68, average BMI of 28.5, and average prostate size of 47.3 mL. Average PTV V40 Gy coverage was 96.1%, 96.2%, 96.1%, and 95.9% for the reference, ATP, dATP, and ATS plans, respectively. Average PTV V45 Gy coverage was 89.7%, 89.2%, 87.9%, and 89.6%. Compared to the reference plan, the bladder maximum dose on average changed by 0.3% (IQR –0.05% to 0.84%), 1.5% (IQR –2.6% to 4.9%), and -1.3% (IQR –2.5% to 0.79%) for ATP, dATP, and ATS plans, respectively. The average change in rectum maximum dose was 0.3% (IQR –0.37% to 1.1%), 4.1% (IQR –0.01% to 8.0%), and -0.2% (IQR –1.9% to 1.7%). The average change in urethra maximum dose was 1.2% (IQR 0.41% to 1.8%), 2.4% (IQR 0.39% to 3.2%), -0.3% (IQR –0.82% to 0.45%). When comparing ATP and dATP plans, we identified one instance where bladder maximum dose increased by 15.9%, and one instance where rectum maximum dose increased by 25.3% due to inter-fraction anatomic differences. Strong correlations between anatomical volume changes and discrepancies in maximum doses between bladder-target (R=0.806) and rectum-target (R=0.820) interfaces were observed. Conclusion: ATP achieved comparable dosimetry results as ATS while maintaining an efficient positional shift adaptation workflow, though anatomic changes at the bladder- and rectum-target interface may be associated with maximum dose changes and serve as indicators for selection of an ATS strategy.
