S1989
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
apertures (i.e., high complexity), which can reduce the robustness to delivery variations and pose challenges for accurate dose calculations. The aim of this study was to compare dose uncertainties in 3D related to delivery variations and the use of different dose calculation methods for VMAT plans of various complexity. Material/Methods: Twelve treatment plans of previously treated patients have been selected to represent different treatment sites (prostate, head & neck, lung, and gynecological cancer). The clinical plan used for patient treatment was reoptimized in Eclipse (Varian Medical Systems, a Siemens Healthineer company), to create one plan with reduced complexity (“simple plan”) and one of higher complexity (“complex plan”) but with dose distributions maintained similar. The anisotropic analytical algorithm (AAA, version 16.1.0) was used for intermediate and final dose calculation in the planning procedure. Dose uncertainty related to delivery variations were simulated by repeated random simulations of realistic offsets in certain treatment planning parameters (e.g. in jaw or multi-leaf collimator position) and estimated by calculating voxel-wise 2 standard deviations (2SD) in dose between a large number of simulations. Dose uncertainty related to the dose calculation was assessed by looking into dose distribution differences, voxel-wise 2SD, across six various dose calculation methods. Each of the total 36 plans was recalculated with AAA (v.18.1.0), AcurosXB (v.16.1.0 and v.18.1.0) in Eclipse and with the Collapsed cone and Monte Carlo algorithms incorporated in RayStation (RaySearch Laboratories, v.14.0.0.3338). Results: Visual evaluation of 3D distributions of the 2SD values revealed that the 2SD values were generally larger outside the PTV (example in Figure 1). For the clinical plans, the maximum 2SDs to a volume of 2 cm2 (2SD2cc) inside the planning target volume (PTV) were on average 1.8% and 3% due to delivery variations and differences between calculation methods respectively (Table 1). In the region outside and within 1 cm from the PTV, the corresponding values were larger (5.8% and 7.2%). The aforementioned values were generally larger for the more complex plans.
Conclusion: Uncertainties in dose were observed mainly at the edge of the high dose region, both regarding uncertainties related to delivery variations and use of different calculation methods. Generally, the uncertainties related to dose calculation methods were larger than those related to delivery variations. Keywords: Delivery errors, calculation methods, complexity Digital Poster 4274 filling the brachytherapy gap : a stereotactic boost alternative for cervical cancer Mouna Ben Rejeb, Aouatef Hamdoun, Syrine Lahiouel, Ghada Abdessatar, Lilia Ghorbal, Lotfi Kochbati Radiation Oncology, Abderrahman Mami Hospital, Ariana, Tunisia Purpose/Objective: Locally advanced cervical cancer (LACC) are usally treated with concurrent chemoradiation and brachytherapy (BT) boost. However, in centers where BT is not available, an external beam alternative is needed. This study aimed to evaluate whether a stereotactic body radiation therapy (SBRT) boost could reproduce BT dose objectives without exceeding recommended organs at risk (OARs) constraints. Material/Methods: A dosimetric study was performed on 20 patients with LACC who had received pelvic external beam radiotherapy (EBRT) (45 Gy in 25 fractions). For each patient, SBRT boost (25 Gy in 5 fractions, 5 Gy/fraction) was generated retrospectively using Varian Eclipse TPS
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