Design, development and characterization of pregnant anthropomorphic phantoms for fetal dose measurements in proton therapy

Abstract Objective. Radiotherapy during pregnancy remains clinically challenging because of concerns about fetal radiation exposure. Proton therapy (PT) with pencil beam scanning, offers substantial reductions in out-of-field (OOF) dose compared to photon therapy, but secondary neutron production creates a need for accurate fetal dosimetry. Approach. This study describes the design, development, material characterization, and computational modeling of two anatomically realistic pregnant anthropomorphic phantoms. MaTORI10 and MaTORI30, representing 10 and 30 weeks of gestation, are designed and characterized specifically for fetal dose assessment in PT. Candidate materials for lung, soft tissue, and bone—including existing phantom materials, quality assurance materials, and novel 3D-printed or castable polymers—were evaluated using MCNP 6.2 Monte Carlo (MC) radiation transport simulations and compared against reference pregnant-tissue compositions. Depth-dose distributions, neutron yields, and OOF neutron spectra were simulated for each material to characterize their radiological behavior. Materials, deemed suitable based on preliminary evaluations, were implemented in the physical phantoms, which were constructed by integrating 3D-printed components and casted bone into the ATOM® female phantom. Pregnancy geometries, from the University of Florida pregnant phantom library International Commission on Radiological Protection (2000 Ann. ICRP 30(1)), were merged with ATOM computed tomography (CT) and surface scans to create anatomically realistic models with dedicated detector inserts. A voxelized computational version of each phantom was generated using CT scans in combination with a surface scan to allow MC radiation transport simulations in TOPAS 3.8. A virtual PT brain irradiation plan (3 cm radius, 78–116 MeV, spread‐out Bragg peak (SOBP) 10 cm range, 5 cm modulation in water) was used, with and without a range shifter (RS), to quantify in-field and fetal doses. Main results. Material testing showed substantial variations across commercial and 3D-printed substitutes, primarily driven by differences in hydrogen content and density. Both MaTORI10 and MaTORI30 were successfully constructed and validated through CT scanning. The computational phantoms reproduced the full internal geometry using eight segmented materials and, in the virtual PT plan, enabled the scoring of depth-dose curves and fetal neutron dose equivalents, supporting their suitability for PT dosimetry studies. The modeled fetal dose calculated for MaTORI30 was on average 29% higher without RS and 21% higher with RS when comparing the MaTORI materials with reference materials. For MaTORI10, the modeled fetal dose was 10% higher without RS and 18% higher with RS relative to reference materials. Significance. The MaTORI10 and MaTORI30 phantoms provide the first anatomically detailed, pregnant anthropomorphic phantoms whose tissue-equivalent properties were computationally characterized for fetal dose assessment in PT. These phantoms provide a platform for experimental and MC-based assessment of fetal dose, facilitating safer radiotherapy and optimal planning for pregnant patients.