Impact of phantom design on fetal radiation exposure assessment in pregnant women undergoing head and neck 6MV photon radiotherapy.

PURPOSE: This study investigates phantom-based methods for fetal radiation exposure assessment in photon external beam radiotherapy for a pregnant patient, using representative uterine locations as a conservative surrogate rather than direct measurements of fetal position or organs. METHODS: Fetal exposure was assessed with in vivo dosimetry at the uterine fundus level for a head and neck VMAT treatment. This estimate was compared to phantom-based measurements using four phantoms, ranging from water-equivalent slabs to an anthropomorphic phantom adjusted to match patient geometry. Additional measurements were performed at different locations within the region representing the uterus to assess spatial dose variability. Measurements were performed on a TrueBeam 6 MV linear accelerator using a Farmer ionization chamber and radiophotoluminescent dosimeters. RESULTS: Using the anthropomorphic ATOM phantom, adjusting the distance between the treatment isocenter and the measurement point to match the patient-specific geometry reduced the difference between phantom-based and in vivo dose estimates from 21 % to 4 %. In the present case, after distance scaling, differences in material thickness and composition influenced the dose measured at the uterine reference location. Phantom lateral positioning also influenced significantly dose estimation, with a 4 cm lateral offset resulting in dose differences up to 22 %. CONCLUSIONS: Accurate reproduction of patient-specific irradiation geometry and scattering and attenuation volumes is essential to minimize uncertainties in phantom-based fetal exposure assessment. A conservative multipoint measurement approach at representative uterine locations provides a robust strategy for estimating fetal exposure. Nevertheless, in vivo verification measurements remain recommended to confirm phantom-based estimates under actual treatment conditions.