ESTRO 2025 - Abstract Book

S2831

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2025

2683

Digital Poster Fraction-variant VMAT treatments for gynecological cancer patients Nathan Torelli, Madalyne Day, Jan Unkelbach Department of Radiation Oncology, University Hospital Zurich and University of Zurich, Zurich, Switzerland Purpose/Objective: Increasing the number of arcs in volumetric modulated arc therapy (VMAT) allows for better intensity modulation and may improve the dosimetric plan quality. However, this leads to a longer delivery time, which may cause patient discomfort and increase the risk of intra-fractional motion. In this study, we investigated whether the delivery of different VMAT plans in distinct fractions may improve the dosimetric quality and delivery efficiency for the treatment of gynecological cancer patients. Material/Methods: We developed a direct aperture optimization algorithm, which allows for the simultaneous optimization of different VMAT plans to be delivered in different fractions, based on their cumulative physical dose. Each VMAT plan (defined by different aperture shapes and MU weights) is constrained to deliver a uniform dose within the target volume, such that the entire treatment does not alter the fractionation scheme and is robust against inter-fractional setup errors. This approach was retrospectively evaluated for five gynecological cancer patients, who were all originally treated with 3-arc VMAT plans at our institution. For each patient, different 1-arc and 2-arc fraction-variant VMAT treatments were generated and benchmarked against fraction-invariant treatments, which deliver the same VMAT plan in every fraction. Results: Figure 1 illustrates a fraction-variant treatment that delivers 4 different 2-arc VMAT plans in 6 fractions each. While each individual plan is sub-optimal in terms of target coverage and dose conformity, the cumulative treatment achieves a more homogeneous target dose and maintains an excellent organ-at-risk (OAR) sparing. Compared to a treatment that delivers the same 2-arc VMAT plan in every fraction, the mean bladder dose is reduced from 15.9 Gy to 11.0 Gy (-30.8%), the mean rectum dose is reduced from 20.9 Gy to 19.2 Gy (-8.1%) and the homogeneity index (D 90% /D 10% ) within the PTV increases from 0.73 to 0.78 (+6.8%).