S2221
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
(NCT04939246) demonstrated that single-fraction stereotactic magnetic resonance-guided adaptive radiotherapy (SMART) for extracranial tumors was feasible, safe, and effective. Herein, we report single- fraction SMART plan quality from our institutional experience. Material/Methods: We performed a retrospective analysis of patients treated between 2020-2025 for primary or metastatic lesions with single-fraction SMART with step-and-shoot technique on a 0.35 Tesla MR Linac—largely in breath- hold but some free breathing with abdominal compression. GTV hotspots of 120-140% were used to improve dose fall-off. Online ART (oART) was performed when indicated to meet dosimetric constraints. Plan quality was quantified using PTV metrics for coverage (V100%/PTV volume), D95%, D90%, PITV (prescription isodose volume/PTV volume), dose fall-off (Dmax 2cm from PTV/Rx dose) and R50% (50%Rx IDL vol/PTV vol). Modulation and patient position were evaluated. Results: 71 patients and 79 total plans were evaluated: 56 (79%) treated to 1 lesion, 7 (10%) treated to 2 lesions in one plan, and 8 (11%) treated to 2 lesions in sequential plans. Target lesions were located in the lung (n=27, 30-34Gy), liver (n=25, 35-40Gy), pancreas (n=11, 20- 25Gy), adrenal gland (n=8, 25-34Gy), lymph nodes (n=6, 25-30Gy), celiac plexus (n=1, 25Gy), and kidney (n=1, 26Gy). Patients were positioned with 1 ipsilateral (51%), no (47%), both (1%), or 1 contralateral (1%) arm above the head. Median total in-room time was 64.5 minutes (Range: 39-138min), with 78.5% treated in <90 minutes. oART was used in 44.3% of cases. Median PTV volume was 16.9 cc (Range: 3.7-273.5cc). On average, plans had 16 beams (Range: 7-21) and 32 segments (Range: 11-60). Mean±SD PTV metrics were 96%±12% (D95%), 101%±7% (D90%), and 93%±9% (coverage). Mean conformality indices were 1.10±0.17 (PITV), 5.80±2.39 (R50%), and 53%±7% (D2cm). All mean plan quality metrics achieved RTOG 0813 requirements, except for R50% in lung and pancreas (expected due to simultaneous integrated boost). GTV Dmean, Dmax, V120%, and V130% were 124%±6%, 137%±5%, 6.7±10.1cc, and 2.7±7.5cc, respectively. Conclusion: This is the largest evaluation of single-fraction SMART treatment plans in which we report efficient delivery times, favorable target coverage, and superior plan quality—especially given that most target lesions were mobile and in proximity to critical OARs. Planning techniques reducing modulation should be considered for single-fraction SMART to optimize delivery time without compromising plan quality. References: Michael D. Chuong, Kathryn E. Mittauer, Michael F. Bassetti, et al. Stereotactic Magnetic Resonance
Conclusion: Despite limited data and short monitoring periods, prostate bed motion during SBRT appeared smaller than that of the intact prostate, following a similar posterior trend. Further data are required to validate these findings. Keywords: Prostate bed SBRT, Real-time ultrasound tracking Digital Poster Highlight 1440 Treatment Plan Quality and Dosimetric Outcomes of Ablative, Single-Fraction, Extracranial Stereotactic MR-Guided Adaptive Radiation Therapy Mattison J. Flakus 1 , Michael D. Chong 1 , Naseem Ud Din 1 , Nema Bassiri 2 , Tatiana Bejarano 1 , Yongsook C. Lee 1 , Rupesh Kotecha 1 , Noah Kalman 1 , Martin C. Tom 3 , Yonaton Weiss 1 , Adeel Kaiser 1 , Matthew D. Hall 1 , Minesh P. Mehta 1 , Alonso N. Gutierrez 1 , Kathryn E. Mittauer 1 1 Radiation Oncology, Miami Cancer Institute, Miami, USA. 2 Radiation Oncology, UT Health San Antonio, San Antonio, USA. 3 Radiation Oncology, MD Anderson Cancer Center, Houston, USA Purpose/Objective: Single-fraction SBRT is uncommonly employed for extracranial tumors but offers several advantages over multi-fraction SBRT. The SMART ONE trial
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