ESTRO 2025 - Abstract Book

S2779

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

ESTRO 2025

References: 1. Njeh CF. Tumor delineation: The weakest link in the search for accuracy in radiotherapy. J Med Phys. 2008;33(4):136-40. 2. Potter R, Tanderup K, Kirisits C, de Leeuw A, Kirchheiner K, Nout R, et al. The EMBRACE II study: The outcome and prospect of two decades of evolution within the GEC-ESTRO GYN working group and the EMBRACE studies. Clin Transl Radiat Oncol. 2018;9:48-60. 3. Smolders A, Bengtsson I, Forsgren A, Lomax A, Weber DC, Fredriksson A, et al. Robust optimization strategies for contour uncertainties in online adaptive radiation therapy. Phys Med Biol. 2024;69(16).

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Digital Poster External photon beam radiotherapy of pregnant patients with brain tumor: treatment planning strategy to reduce fetal dose Magali EDOUARD 1 , Catherine Jenny 2 , Mehdi Khalal 2 , Antoine Lecarpentier 1 , Adel Chaabane 1 , Philippe Maingon 2 , Aurélie Isambert 1 , Christelle Huet 3 , Michel Chea 2 1 PSE-Sante/ SER/UEM, IRSN, Fontenay-aux-roses, France. 2 Groupe hospitalier La Pitié-Salpêtrère, AP-HP Sorbonne University, Paris, France. 3 PSE-Sante/SDOS/LDRI, IRSN, Fontenay-aux-roses, France Purpose/Objective: Approximately 2% of pregnant women who develop cancer during pregnancy will be treated with radiotherapy [1]. When the primary tumor is sufficiently distant from the fetus, radiotherapy remains a safe therapeutic option. The ICRP considers that the termination of the pregnancy is not justified below 100 mGy delivered to the fetus [2]. Due to the limitations of algorithms implemented in TPS for out-of-field dose assessments and considering the influence of the linear accelerator design, treatment parameters, patient morphology, and its potential impact on medical decisions, fetal dose assessment should be performed through appropriate measurements [3]. The aim of this study is to provide general guidelines for fetal dose assessment and optimization in order to keep the fetal exposure below the 100 mGy threshold and to minimize it. The impact of ballistic treatment parameters on the fetal dose for a pregnant woman suffering from a brain tumor undergoing radiotherapy was evaluated to reduce fetal exposure. Material/Methods: Dose measurements were performed with a TrueBeam STx 6 MV using a Farmer ionization chamber. The ballistic effects were assessed using water-equivalent slabs (inline dose profiles) and the ATOM phantom, with homemade complementary bellies affixed to simulate the three pregnancy trimesters [4]. Eight treatment plans were evaluated, including 3D conformal radiotherapy (3D-CRT) and Volumetric Modulated Arc Therapy (VMAT) plans, with varying collimator angles, filter types and modulation levels. Results: Regarding out-of-field doses, using an optimized collimator angle reduces fetal exposure by a factor of 2 compared to the one typically used in clinical practice. The optimal collimator angle for a 6 MV beam, both FF and FFF, depends on the distance from isocenter and is not always achieved by placing the distal jaw in the cranio-caudal axis of the patient, as suggested in the literature. Fetal doses for FFF plans are consistently reduced by about 40% compared to FF plans. Conclusion: Adjusting the collimator angle and modulation level in VMAT plans can minimize the fetal dose. The out-of-field dose does not continuously decrease due to machine-specific leakage characteristics. Using VMAT instead of 3D CRT does not significantly increase fetal dose when ballistic parameters are optimized while it can provide better