ESTRO 2025 - Abstract Book

S2873

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

ESTRO 2025

modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) in comparison to standalone IMRT and VMAT in preoperative radiotherapy for rectal cancer.

Material/Methods: CT images of 30 male rectal cancer patients were used to design 4 different treatment plans including 7-fields IMRT, 2-full arcs VMAT, hybrid technique which combined the 7-fields IMRT and 1-full arc VMAT (H-1Arc), and hybrid technique composed of 7-fields IMRT and 1-full arc VMAT (H-2Arc). All the treatment plans were generated by RayStation (Ver.8A, RaySearch, Sweden). Radiation dose fractionation was 45 Gy/25 fractions for mesorectum and at-risk lymph nodes (PTV45) and 50 Gy/25 fractions for the primary tumor and involved lymph nodes (PTV50). Plans were analyzed using dose-volume histogram (DVH) parameters, conformity index (CI), and homogeneity index (HI). Equivalent uniform dose (EUD) and monitor units (MUs) were also evaluated and compared among the plans. The repeated measure analysis of variance test was used to compare the four techniques. P values < 0.05 were considered statistically significant. Results: The H-2Arc plan provided significantly improved bladder, small bowel and femoral heads DVH parameters compared to VMAT (V35, V40 of small bowel and V40 of bladder) and IMRT (V40 of small bowel, V40 of bladder, Dmax and V40 of femural heads). This hybrid plan improved the CI, HI compared to IMRT and VMAT. The H-1Arc plan was associated with a significant reduction in DVH parameters compared to VMAT (V35, V40 of small bowel and V40 of bladder) and IMRT (V40 of small bowel, Dmax and V40 of femural heads). It showed improvements in CI and HI for the PTV45 region. Both hybrid plans achieved a lower EUD for the small bowel and bladder compared to VMAT and for the femoral heads compared to IMRT. Although VMAT had the lowest MU, H-2Arc required fewer MUs than H-1Arc (P = 0.001) and showed no significant difference compared to IMRT (P = 0.069). No significant differences in dosimetric or radiobiological outcomes were observed between the two hybrid techniques, except for the MU. Conclusion: Hybrid radiotherapy techniques improve dose distribution in target tissues while sparing organs at risk by combining the advantages of IMRT and VMAT. Among the evaluated approaches, H-2Arc emerges as the preferred treatment plan for neoadjuvant radiotherapy in rectal cancer, offering both dosimetric benefits and acceptable MU efficiency.

Keywords: hybrid radiotherapy, IMRT, VMAT

3408

Digital Poster Assessment of the dosimetry robustness of multi-isocenters Volumetric Modulated Arc Therapy technique for Total Body Irradiation Aicha Traore-Diallo, Vithun Anthonipillai, Bader El Mollen, Valentine Martin Radiotherapy, Gustave Roussy, Villejuif, France Purpose/Objective: Total Body Irradiation is increasingly performed using Volumetric Modulated Arc Therapy with multiple isocenters, which allows an optimized dose distribution, improving whole-body coverage while reducing dose to critical structures. This study evaluates the dosimetric robustness of this by examining the effect of the distance between consecutive isocenters and patient positioning offsets, in order to identify tolerance.

Material/Methods: For our first five patients, two scans were acquired: one head first supine (HFS) and one feet first supine (FFS). These