S1651
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
Digital Poster Highlight 2376 Uterine Dose Estimation for Different LINACs and Entities during CBCT and Treatment Stefan Menger 1 , Stefan Weick 1 , Khaled Resho 1,2 , Julian Niedling 1 , André Toussaint 1 , Gary Razinskas 1 1 Department of Radiation Oncology, University Hospital Würzburg, Würzburg, Germany. 2 Fachbereich Ingenieurwissenschaften, Hochschule RheinMain, Wiesbaden, Germany Purpose/Objective: Although rare, radiotherapy of pregnant patients may be required and demands careful planning and rigorous dose assessment. This study aims to quantify the uterine dose during external beam radiotherapy for various anatomical entities. The analysis compares two linear accelerators - Halcyon (Varian) and VersaHD (Elekta) - for both megavoltage (MV) treatment plans and cone-beam CT (CBCT) imaging procedures. Material/Methods: Guideline conform treatment plans were generated in Eclipse (Varian) for five entities: meningioma, parotid gland, esophagus, breast, and thoracic vertebra. Uterine doses were measured using an anthropomorphic Alderson phantom extended with water-equivalent RW3 slabs to simulate the pregnant abdomen. OSL dosimeter (RadPro) were placed in four central cranio-caudal positions to represent different stages of pregnancy. Measurements were performed for both MV treatment fields and CBCT acquisitions on Halcyon (with and without HyperSight) and VersaHD
Figure 1: Relative Dose for VersaHD and Halcyon LINACs, over the distance to the treatment field edge for treatment fields (MV) and CBCT. Conclusion: This investigation demonstrates comparable fetal dose profiles with VersaHD and Halcyon LINACs, offering a rough practical approximation for estimating fetal exposure based on prescribed dose and proximity to the radiation field. Keywords: Out of field dose, Uterine dose, Pregnant 3D printed phantoms for verification of single isocentre multi-target stereotactic radiosurgery. Emily Searle 1 , Nicholas Arico 2 , Cameron Stanton 1 , Kankean Kandasamy 3 , Benjamin Zwan 1 1 Radiation Oncology, Central Coast Cancer Centre, Gosford, Australia. 2 Radiation Oncology, Sunshine Coast University Hospital, Sunshine Coast, Australia. 3 Radiation Oncology, GenesisCare, Sydney, Australia Purpose/Objective: Stereotactic radiosurgery (SRS) delivers high doses of targeted radiation to small, well-defined tumours. Linac-based single-isocentre multi-target (SIMT) SRS reduces treatment times, thereby minimizing intrafraction motion and improving dose conformity to target lesions [1,2]. While there are several commercially available phantoms for SIMT SRS QA, they are costly, usually not water-equivalent and utilize detectors that require correction factors [3,4]. In this work we present a cost-effective tissue-equivalent 3D- printed phantom for SIMT SRS. Material/Methods: The phantom was constructed with 3D-printed Polylactic Acid (PLA). Tissue equivalence was optimized by varying the shell thickness and infill density, matching depth-dose and output factor Digital Poster 2550 measurements in PLA to Plastic Water (PW). Photon energies of 6X, 6FFF and 10FFF with field sizes 4x4, 2x2 and 1x1cm2 were included in the optimization. The phantom was custom designed as a 20cm diameter sphere, comprising multiple 3D printed components,
systems. Results:
The measured uterine doses showed no substantial differences between the two LINAC platforms. Across all treatment scenarios, the CBCT dose contribution was negligible compared to the MV treatment dose. Figure 1 shows the uterine treatment- and CBCT-dose (D) for both LINACs as a function of distance from the treatment field edge (r), normalized to the prescribed treatment dose. A distance-dependent function D = a × e-b×r/(r-r0)2, motivated by photon scatter and inverse-square behavior, was fitted to the treatment data: VersaHD: a=0.78825cm2, b=0.0013cm-1, r0=1.28928cm; Halcyon: a=0.58368cm2, b=0.01845cm- 1, r0=1.09179cm. These parameters enable an estimation of the uterine dose as a function of treatment geometry and field proximity.
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