ESTRO 2025 - Abstract Book

S3313

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

ESTRO 2025

The least CTV movement occurred in the left-right (LR) direction (95% range: -0.23 to 0.23 cm, full range: -0.24 to 0.37 cm) (Figure 1b). The CTV moved more in the anterior-posterior (AP) and cranial-caudal (CC) directions. CC motion showed a 95% range of -0.38 to 0.66 cm, while AP motion had a broader range (-0.80 to 0.21 cm). On average there was a -0.25 cm motion posteriorly, likely due to gravity. 100% of bladders over 500cc were associated with less motion, whilst 50% of the bladders of volumes between 300-400cc showed limited motion (Figure 1a). Conclusion: We found that very high-volume bladders were associated with reduced CC and AP motion. Systematic motion in the posterior direction warrants exploring the use of anisotropic margins for intrafraction motion. Patients were imaged with a bladder filling protocol of 300ml 15-minutes before irradiation. Despite this, bladder volumes varied greatly, highlighting insufficient patient compliance. Overall, bladder volumes under 500cc are an unreliable indicator for CTV motion. Large variation of intra-fractional motion of the CTV per patient per fraction was observed (Figure 2), making derivation of patient -specific margins challenging

Keywords: Cervical Cancer, CTV motion, MR-Linac

References: [1] Taylor, A. and Powell, M.E.B. (2008). An assessment of interfractional uterine and cervical motion: Implications for radiotherapy target volume definition in gynaecological cancer. Radiotherapy and Oncology, 88(2), pp.250–257. doi:https://doi.org/10.1016/j.radonc.2008.04.016.

3572

Poster Discussion Delivery of online adaptive palliative radiotherapy without pre-treatment CT imaging Koen J. Nelissen 1,2 , Angelique R.W. van Vlaenderen 1,2 , Ashaya T. Jaglal 1,2 , Sandy Loopeker 1 , Jorrit Visser 1,2 , Jan Wiersma 1,2 , Wilko F.A.R. Verbakel 1,2,3 , Suresh Senan 1,2 , Eva Versteijne 1,2 1 Radiation Oncology, Amsterdam UMC, location Vrije Universiteit Amsterdam, Amsterdam, Netherlands. 2 Cancer Treatment and Quality of Life, Cancer Center Amsterdam, Amsterdam, Netherlands. 3 Radiotherapy solutions, Varian, a Siemens Healthineers Company, Palo Alto, USA