Development of a CT/MRI-compatible anthropomorphic multimodal deformable male pelvic phantom for end-to-end quality assurance testing in MR-guided online adaptive radiotherapy.

Current quality assurance phantoms often lack MR signal, deformability, or real-time dosimetry capability, limiting comprehensive end-to-end evaluation for MR-guided online adaptive radiotherapy (MRgOART), including verification of deformable image registration (DIR) accuracy and the adaptive workflow within a single phantom. This study aimed to develop a CT/MRI-compatible anthropomorphic, deformable male pelvic phantom for a comprehensive end-to-end test of MRgOART. Reference CT value and MR signal intensity of the prostate, bone, and urine were obtained from a cohort of 30 prostate cancer patients who underwent treatment with a 0.35 T MR-Linac. MR images were acquired using a TRUFI (balanced steady-state free precession) sequence. Candidate materials were optimized to reproduce patient-like CT and MR signal characteristics, and their temporal stability was evaluated over 6 months. Relative electron density (rED) was validated using elemental analysis to ensure dose-calculation reliability. The phantom incorporated deformable prostate, rectum, and bladder models and allowed insertion of an ionization chamber for real-time internal dose measurement. The optimized materials reproduced CT value and MR signal intensity within the mean ± 1 standard deviation of patient reference data, providing clear image contrast without artifacts. CT values remained stable over time, while minor fluctuations in MR signal intensity remained within the range of physiological variability. Composition-based rED values showed small deviations from those derived by CT, which were deemed acceptable for the end-to-end test. The developed phantom enables realistic, multimodal, and deformable end-to-end evaluation of MRgOART, supporting accurate assessment of DIR, adaptive planning, and dose delivery with real-time dosimetry.