Main Session
Sep
30
SS 39 - Radiation and Cancer Physics 6: Innovations in Planning Algorithm and Quality Assessment
326 - Plan Quality Reporting of the Phase II Stereotactic MR-Guided Adaptive Radiation Therapy in One Fraction (SMART ONE) Trial
Presenter(s)
Kathryn Mittauer, PhD - Miami Cancer Institute- Baptist Health South Florida, Miami, FL
K. E. Mittauer1,2, N. Ud Din1, A. Shepard3, N. Bassiri4, A. Gutierrez1,2, C. Glide-Hurst3, J. Crosby3, R. Kotecha1,2, N. S. Kalman5, M. C. Tom6, M. P. Mehta1,5, A. Burr3, M. F. Bassetti3, and M. D. Chuong1,2; 1Department of Radiation Oncology, Miami Cancer Institute, Baptist Health South Florida, Miami, FL, 2Florida International University, Herbert Wertheim College of Medicine, Miami, FL, 3Department of Human Oncology, University of Wisconsin School of Medicine and Public Health, Madison, WI, 4University of Texas Health Science Center at San Antonio, San Antonio, TX, 5Herbert Wertheim College of Medicine, Florida International University, Miami, FL, 6Department of CNS Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX
Purpose/Objective(s):
Single-fraction (1-fx) SABR has advantages of improved patient and provider convenience, reduced healthcare costs, and potential synergy with immunotherapy. While 1-fx SBRT has historically been limited to favorable anatomic locations, stereotactic MR-guided adaptive RT (SMART) offers markerless gating and increased accessibility to tumors near dose-limiting organs. While the phase II SMART ONE trial prospectively demonstrated that 1-fx SBRT on an MR Linac is feasible, safe, and effective for extracranial visceral tumors, the technical aspects of the trial have yet to be fully characterized. Therefore, our primary objective was to evaluate the planning technique and plan quality from the SMART One trial.Materials/Methods:
The SMART One trial enrolled 30 patients (32 lesions) at 2 U.S. institutions between 2021 and 2023 for primary or metastatic lesions of the lung (30-34 Gy, n=11), liver (35-40 Gy, n=5), pancreas (25 Gy, n=1), adrenal (25 Gy, n=9), and abdominal/pelvic lymph nodes (LNs) (25 Gy, n=6). Two patients had 2 lesions. All patients were treated with step and shoot IMRT on a 0.35 T MR Linac with online adaptive RT (oART) performed when needed to meet OAR constraints. Retrospective analysis was performed for patient position, modulation, and plan quality. Plan quality was quantified for target coverage (TC = PTV V100%/vol), PTV D95% and D90%, homogeneity index (HI = PTV D2%/D98%), prescription (Rx) isodose/target volume (PITV), conformity (D2cm = max dose at 2 cm from PTV/Rx dose), and gradient index (R50%=50% Rx isodose vol/PTV vol). Ablative dose was evaluated as GTV Dmean, Dmax, V120%, and V130%.Results:
Patient position was simulated and treated with both (11, 34%), 1 contralateral (20, 63%), or no (1, 3%) arms down. Median beams and segments were 16 (Range: 11-20) and 28 (Range: 20-60) respectively, with nominal median beam on-time (ungated) of 18 min (Range: 9.6-33.1). Most liver, adrenal, and lung lesions (20/22 lesions) had 1 contralateral arm down, indicating priority to maximize dose rate of FFF beam for lateralized targets. The median total in-room time was 53 min (Range: 39-195), with 87.1% of lesions <90 min. oART was used 53.1%. Median PTV volume was 13.9 cc (Range: 5.3-91.8). Mean ± SD coverage was 100% ± 9% (PTV D95%), 103% ± 6% (PTV D90%), and 94% ± 6% (TC), respectively. Mean homogeneity and conformality were 1.2 ± 0.2 (PITV), 5.8 ± 1.7 (R50%), 1.4 ± 0.3 (HI), and 52% ± 9% (D2cm), respectively. The GTV mean ± SD ablative dose was substantial at 126% ± 6% (Dmean), 140% ± 8% (Dmax), 5.4 ± 5.3 cc (V120%), 2.1 ± 2.3 cc (V130%). Conclusion: We demonstrate that 1-fx SABR plan quality in the SMART ONE trial was excellent and treatment delivery was made efficient by purposefully minimizing plan modulation (beams/segments) and maximizing dose rate (patient position). When combined with the previously reported favorable clinical toxicity profile, the dose escalation demonstrated in this phase II trial while maintaining OAR constraints supports further 1-fx SABR dose escalation strategies.