ESTRO 2025 - Abstract Book

S3331

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2025

Conclusion: The proposed personalized anisotropic margin strategy for cervical cancer online ART demonstrates the potential to significantly reduce the irradiation of healthy tissues while maintaining adequate CTV coverage. This approach may facilitate the clinical implementation of online ART and maximize the benefits for cervical cancer patients.

Keywords: Anisotropic Margin, Cervical Cancer, ART

References: 1. Berger, T., et al., Dosimetric Impact of Intrafraction Motion in Online-Adaptive Intensity Modulated Proton Therapy for Cervical Cancer. Int J Radiat Oncol Biol Phys, 2021. 109(5): p. 1580-1587. 2. Zhang, Y., et al., Prospects for daily online adaptive radiotherapy for cervical cancer: Auto-contouring evaluation and dosimetric outcomes. Radiat Oncol, 2024. 19(1): p. 6. 3. Yen, A., et al., Spare the Bowel, Don't Spoil the Target: Optimal Margin Assessment for Online Cone Beam Adaptive Radiation Therapy (OnC-ART) of the Cervix. Pract Radiat Oncol, 2023. 13(2): p. e176-e183.

3790

Digital Poster Implementation of a Novel Adaptive Strategy for Thoracic Tumors Using MRI-Linac

Francesco Catucci 1 , Francesco Preziosi 1 , Althea Boschetti 1 , Alessia Re 1 , Martina Iezzi 1 , Antonio Piras 1 , Sebastiano Menna 2 , Luca Vellini 2 , Flaviovincenzo Quaranta 2 , Elisa Pilloni 2 , Carmela Di Dio 1 , Flora Anna Mauro 1 , Andrea D'Aviero 1 , Danila Piccari 1 , Davide Cusumano 2 , Gian Carlo Mattiucci 1 1 Radiotherapy, Mater Olbia, Olbia, Italy. 2 Physics, Mater Olbia, Olbia, Italy Purpose/Objective: This study assesses the dosimetric impact of a new technique termed "Adaptive Skin Radiotherapy" (ASRT), designed for treating thoracic lesions with MRI-guided radiotherapy. A retrospective analysis was conducted on 35 patients treated with ASRT using a breath-hold protocol across five fractions, with total doses ranging from 35 to 55 Gy. Material/Methods: GTV was delineated on simulation MRI co-registered with the staging CT, while PTV was generated by isotropically expanding the GTV by 3 mm. ASRT employs a fast online process where only patient body and target are recontoured, avoiding significant OARs changes. This procedure, completed in less than 2 minutes, enables physicists to update the electron density map reflecting changes in patient body contour and lesion displacement. Based on these adjustments, the predicted plan is recalculated using the original beam fluence and then re optimized to align with original clinical objectives. Treatment plans were prescribed following ICRU 91, with isodose line close to 80%. Dose variation of predicted and reoptimized plans with respect original plan was calculated for target coverage parameters (including volume covered by prescription isodose line (V80%), maximum dose (Dmax), and percentage of volume receiving 100% of the dose) and OARs (D98%, D2% and D50% of chest wall and heart). PTV volumes were also collected from simulation MRIs to examine possible correlations with observed dose variations. Results: No significant variations were observed for OARs while substantial discrepancies were noted in PTV target parameters when comparing predicted and original plans (p<0.01 was observed V80% and Dmax). These discrepancies were significantly mitigated by implementing ASRT, as reported in Figure 1. Notably, a strong correlation was identified between PTV volume and dose variation in terms of target coverage by isodose line. Defining a reduction of ≥5% in target coverage as an adverse event, a clear threshold of 15 cc in PTV volume was