S2332
Physics - Quality assurance and auditing
ESTRO 2026
Digital Poster 708
Design and development of an anthropomorphic brain phantom for quality assurance of multi- lesion SRS Michael Yang 1,2 , Paola Alvarez 1 , Tina Briere 1 , Ruitao Lin 3 , Stephen Kry 1 , Mallory Glenn 1 1 Radiation Physics, UT MD Anderson Cancer Center, Houston, USA. 2 Medical Physics, UTHealth Houston Graduate School of Biomedical Sciences, Houston, USA. 3 Biostatistics, UT MD Anderson Cancer Center, Houston, USA
Purpose/Objective: Stereotactic radiosurgery (SRS) for multiple
intercranial lesions presents unique challenges in quality assurance. Despite the growing adoption of single- and multi-isocenter, multi-target radiotherapy techniques, there remains a critical need for tools to provide an independent assessment of treatment delivery. To address this gap, we designed an anthropomorphic phantom that closely mimics a clinical brain metastases patient and can assess the end-to-end performance of multi-lesion SRS treatments. This phantom is designed as a postal audit for dosimetry evaluations by the Radiation Quality Assurance Lab (RQALab). Material/Methods: The anthropomorphic phantom resembles a human head and was fabricated using three materials to simulate human tissue, bone, and solid tumor targets: high-impact polystyrene, polybutylene terephthalate (PBT), and solid water, respectively. Using a database of internationally treated clinical multi-met patients, key clinical features were chosen, including tumor size and spacing. Additional phantom features allow for the use of multiple image guidance strategies (e.g. anatomy-based alignment). Thermoluminescent dosimeters (TLD) and MD-V3 radiochromic films are used to measure target dosimetry. After fabrication and construction of the phantom was completed, CT scans were performed to confirm phantom design and feature visualization. Results: The multi-lesion SRS phantom was constructed and contains five total targets with three targets being dosimetrically monitored using TLD and film. Using these dosimeters in combination with the created treatment plan, positioning and absolute dosimetric differences can be analyzed and reported. The two remaining targets are not measured and provide additional challenge for treatment planning. The solid tumor targets in the phantom are spheres of 0.5, 1.0, and 1.9 cm diameters representing the median, mean, and relative maximum from a set of clinical patient plans. The dosimetric targets were spaced with an average distance of 5 cm between the tumor and the
End-to-end dose deviations were comparable between MC and TPS (+1.8% vs +1.7%, p=0.72). RC-CCCS significantly overestimated dose (+6.2%, p<0.001), particularly for off-axis measurements (+7.5%). Conclusion: Our results demonstrate excellent agreement between Pinnacle ³ , RadCalc-MC, and end-to-end tests, confirming the robustness of MC beam modelingfor small field. In contrast, RadCalc-CCCS revealed clinically relevant discrepancies, highlighting the critical importance of recognizing algorithm-specific limitations when performing secondary dose verification in radiosurgery PSQA. References: [1] Zhu TC, et al. Report of AAPM Task Group 219 on independent calculation - based dose/MU verification for IMRT. Medical Physics 2021;48. https://doi.org/10.1002/mp.15069.[2] Mastella E, et al. Clinical implementation of a secondary dose calculation system for patient-specific quality assurance of complex VMAT and SBRT treatments. Physica Medica 2025;135.
https://doi.org/10.1016/j.ejmp.2025.105025. Keywords: small fields, beam modeling, plan complexity
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