Adaptive skin radiotherapy (ASRT): Clinical outcomes and dosimetric evaluation of a time-efficient approach for online adaptive MRI-guided radiotherapy in thorax.

Introduction: The thorax presents unique challenges in radiotherapy: although stereotactic ablative radiotherapy (SABR) achieves high local control in NSCLC and pulmonary metastases, further improvements are desirable, particularly dose escalation in radioresistant metastases and planning target volume (PTV) margin reduction to minimize toxicity. Online adaptive Magnetic Resonance Imaging-guided Radiotherapy (MRIgART) supports these strategies by ensuring daily dose accuracy through compensation of Electron Density (ED) variations. However, prolonged treatment times remain the main obstacle to clinical implementation. To address this limitation, we introduce Adaptive Skin Radiotherapy (ASRT), a time-efficient MRIgART workflow designed to maintain accuracy while enhancing feasibility. Methods: Lung patients undergoing MRIgART were prospectively enrolled and treated with a biologically effective dose (BED) higher than 100 Gy. The gross tumor volume (GTV) was delineated on a simulation MRI co-registered with the staging CT, and the planning target volume (PTV) was generated by isotropically expanding the GTV by 3  mm. Gross tumor volume (GTV) was delineated on simulation MRI co-registered with staging CT, and planning target volume (PTV) was created by isotropically expanding GTV by 3  mm. ASRT involves a rapid online workflow where only body contour and target are recontoured, minimizing changes to organs at risk (OARs). Physicists update the electron density map to reflect anatomical changes, recalculating the predicted.plan using original beam fluence and re-optimizing it to meet clinical goals. Plans followed ICRU91 guidelines with an isodose line near 80%. Dose differences between predicted, adapted, and original plans were analyzed for target coverage and OARs. Treatment duration and toxicity were recorded. Survival outcomes were assessed using Kaplan-Meier analysis. Results: A total of 41 patients were enrolled, receiving 232 fractions, of which 178 (76.7%) were treated with ASRT. Online adaptation significantly improved target coverage (median V100% change: +0.1%±2.5%) compared to predicted plans (-2.6%±7.3%, p < 0.001), without significant differences in OAR doses. Median total treatment time was 28 min, with an average 5-minute ASRT workflow. No grade ≥ 3 toxicities were reported; grade 2 events occurred in 10.3% of patients. At a median follow-up of 19.8 months, 1-year local control, disease-free survival, and overall survival for the whole cohort were 94%, 80%, and 84%, respectively. Conclusion: Daily changes in the body contour and ED heterogeneity can result in major effects on plan dosimetry which can be quickly corrected with ASRT procedure. ASRT is a safe, effective, and time-efficient alternative for online MRIgRT adaptation in thoracic tumours, with promising clinical outcomes and toxicity profiles.