Development of novel pelvic end-to-end (PETE) phantom for mronly radiotherapy: Patient-informed design considerations.

Purpose: Implementation of MR-only treatment planning and MR-IGRT has enabled the integration of MRI\u27s powerful soft tissue contrast while reducing MR/CT co-registration uncertainty. However, MR-compatible phantoms are currently limited to benchmark these new workflows. This work describes the development of a novel pelvic end-to-end (PETE) MR-compatible phantom using patient-informed design considerations. Methods: For geometrical considerations, T2-weighted MR-SIM data of 23 prostate cancer patients acquired in treatment position were evaluated for phantom habitus, sacrum to external spacing, and bladder volume. To determine bladder filling kinematics, temporal T2-weighted MRIs acquired for 10 healthy male volunteers after bladder void, consumption of 20 oz of water, and 10 oz consumed mid-session were analyzed. Water-filled medical balloons (polyisoprene and silicone) were imaged with increasing volume (150-400 mL) to assess appropriateness for bladder shape and volume. Finally, 3D printed pelvises derived from average adult males were considered. Results: On average, the ideal pelvis phantom has a width of 36.5 ± 1.9 cm, height = 23.0 ± 1.4 cm, and sacrum to external spacing of 1.6 ± 0.4 cm. The mean bladder volume was 237 cc (range = 80.4-441 cc). The bladder filling experiment revealed an average bladder volume difference of 62 ± 15% between empty and full bladders. Furthermore, the bladder showed a large (\u3e10 mm) longitudinal displacement when filled. The silicone balloon was limited to 250 mL and assumed a pear shape. The polyisoprene balloon reached a volume of 400 mL but assumed a spherical shape. 3D printed pelvises had excellent spatial representation in CT but did not generate MR signal. Conclusion: Design considerations for the PETE phantom have identified suitable materials and properties, particularly for bladder filling conditions. Future work will include further assessment of materials that exhibit MR relaxation times comparable to human tissue, detectable on MRI, and incorporate 3D-printed deformable organs derived from patient imaging data to emulate pelvic anatomy