ESTRO 2025 - Abstract Book

S3266

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

ESTRO 2025

Conclusion: Enabling stereoscopic imaging between couch rotations is crucial to increase patient position accuracy. Strong correlation between SGRT and IGRT positioning suggest marked benefit with online SGRT. However, the correlation is not strong enough for sole use of SGRT for the whole SRS treatment but may obviate the need of intra-beam IGRT.

Keywords: SGRT, IGRT

2558

Digital Poster Potential of margin individualization for dose escalated radiotherapy of rectal cancer Chavelli M Kensen 1 , Lisa Wiersema 1 , Anja Betgen 1 , Doenja MJ Lambregts 2 , Corrie AM Marijnen 1 , Uulke A van der Heide 1 , Tomas M Janssen 1 1 Radiation Oncology, Netherlands Cancer Institute, Amsterdam, Netherlands. 2 Radiology, Netherlands Cancer Institute, Amsterdam, Netherlands Purpose/Objective: Escalating the dose on the gross tumor volume (GTV) in rectal cancer has the potential to increase clinical complete response rates. Accurate boosting of the GTV has become feasible using MRI guided online adaptive radiotherapy. However, motion of the (meso-)rectum is substantial and the position and shape of the GTV within the rectum might impact intrafraction motion and required planning target volume (PTV) margins. The aim of this work was to determine the impact of tumor volume, shape and location in the rectum on intrafraction motion during dose-escalated MRI-guided radiotherapy of rectal cancer and to explore the potential of PTV margin individualization. Material/Methods: 77 rectal cancer patients, treated with short course radiotherapy (SCRT) on a 1.5T MR-Linac, were included in the study. For all five treatment fractions per patient, the GTV of the primary tumor was manually delineated on T2w weighted images acquired for online plan adaptation (MRI adapt ). GTV delineations on the MRI acquired after irradiation (MRI post ) were obtained by patient-specific finetuning using the delineation on MRI adapt , of a deep-learning GTV auto-segmentation model. Intrafraction motion was calculated as ¾ of the center of gravity (COG) displacement of the GTV between MRI adapt and MRI post . PTV margins were calculated using the van Herk recipe. The effect of tumor volume, shape and distance from the anal verge (lower: <5 cm; mid-upper > 5 cm) on intrafraction motion was studied using linear mixed effect modeling and individualized margins were calculated for each group. Results: Mixed effect modelling showed that intrafraction motion was correlated with distance from the anal verge with larger displacement in in Anterior-Posterior (AP; p=0.001) and Cranial-Caudal (CC; p=0.043) direction for lower tumors compared to mid-upper tumors (figure 1). Tumor volume and tumor shape had no significant impact. Group mean was significantly different from zero (T-test, p<0.05) in CC direction for the full cohort and low rectal tumors, resulting in asymmetric margins. PTV margins required for the full cohort were 2.8 mm LR, 6.3 mm AP, 2.2 mm cranial and 5.6 mm caudal. Individualizing on tumor location resulted in reduced margins of 3.5 mm AP and 3.2 mm cranial-caudal for mid upper rectal tumors.