ESTRO 2025 - Abstract Book

S3204

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

ESTRO 2025

589

Digital Poster Dosimetric impact of intra- and interfraction motion on VMAT-TBI OAR dose distribution Enrica Seravalli, Hidde Buiting, Mirjam Bosman, Peter Stijnman, Bianca Hoeben Radiotherapy, University Medical Center Utrecht, Utrecht, Netherlands Purpose/Objective: Conformal TBI techniques purpose treatment optimization and CT-based dose planning incorporation [1]. Multi isocenter VMAT allows better dose homogeneity and allows individualized OAR sparing compared to 2D conventional TBI. However, VMAT steep dose gradients may lead to OAR dose uncertainties due to patient positioning and organ motion [2]. The impact of respiration (intrafraction motion) and interfraction motion on the lung- and kidney-dose during VMAT-TBI was evaluated. Motion amplitudes of lungs and kidneys were determined on radiotherapy planning 4D CT scans of 20 patients of similar ages as the VMAT-TBI patients in our clinic (average 16 y/o, range 10 – 23 y/o), since only 3D CT scans were acquired for VMAT-TBI patients. A mid-position image was reconstructed from the 4D CT 10 respiratory phases [3,4], and motion amplitudes of lungs and kidneys were computed. The average motion amplitude was used to expand and reduce the lung and kidney volumes on the planning CT for 26 VMAT-TBI patients (average 15 y/o, range 5 – 32 y/o). The expanded and reduced volumes Dmean was calculated and compared to the planned Dmean of the VMAT-TBI dose distribution. Interfraction motion The planning CT and delineations of 26 VMAT-TBI patients were deformed according to the anatomy imaged on daily acquired CBCT scans (6 per patient, 1 per fraction) [5]. The dose distribution was recalculated on the deformed CT per fraction. Recomputed Dmean of lungs and kidneys was compared to the planned one. Material/Methods: Intrafraction motion

Results: Intrafraction motion

The maximum Dmean absolute difference between expanded/reduced contour compared to the original contour for both kidneys, left and right lung was 0.5 Gy, 0.7 Gy and 1.0 Gy, respectively. For the expanded kidneys and lungs the 10 Gy constraint was exceeded for 3/26 and 1/26 patients when it was not already exceeded in the planned Dmean. For the expanded left and right lung an 8 Gy constraint was exceeded for 1/26 and 5/26 patients.