Room: Karl Dean Ballroom B1
Purpose: MR-only treatment planning and MR-IGRT leverages MRIâ€™s powerful soft tissue contrast for high precision radiation therapy. However, anthropomorphic MR-compatible phantoms are currently limited. This work evaluates a custom-designed modular pelvic end-to-end (PETE) MR-compatible phantom to benchmark MR-only and MR-IGRT workflows.
Methods: PETE simulates average male pelvis anatomy, made of an acrylic body oval and a cast mold urethane skeleton, with silicone balloons simulating bladder and rectum, a silicone sponge prostate, and hydrophilic poly(vinyl alcohol) foam to simulate fat/tissue separation between organs and spine. Access ports enabled retrofitting the phantom with other inserts. The habitus and bladder were filled with 15 mg/L (T1~300ms) and 7 mg/L (T1~900ms) MnÂ²â?º (as MnClâ‚‚Â·4Hâ‚‚O) to simulate fat/muscle and urine, respectively. A silicone balloon enables air to be inputted to mimic an air-filled rectum. PETE underwent CT-SIM, true fast and steady precession (TrueFISP) MR-SIM in an MR-LINAC, and T1-weighted (T1W) and T2-weighted (T2W) imaging at 1.0T MR-SIM. Scans were acquired with various bladder (~90cc, 150cc, 250cc, and 350cc) and rectal (~30cc, 60cc, 90cc, 120cc, 150cc) volumes to assess filling capabilities and component interactions. Bladder and rectum filling reproducibility was assessed via volume and centroid differences.
Results: Acceptable contrast was achievable in CT-SIM and TrueFISP images, however bladder was challenging to distinguish from background. The desired contrast for T1W and T2W MR-SIM (dark and bright bladders, respectively) was achieved. Rectum and bone exhibited no MR signal. Increasing bladder and rectal volumes induced organ shape variations. Reproduced 30cc and 60cc rectal volumes differed by 10.7% (~3.7cc) and 2.4% (~1.5cc) with 1.3Â±0.5mm and 0.4Â±0.5mm centroid displacements, respectively. Duplicated bladder volumes varied by 1.7% (~4.5cc) with corresponding centroid displacements of 0.4Â±0.3mm.
Conclusion: A novel modular phantom was developed that accurately and reproducibly simulates status changes. Future work includes the insertion of ion chambers/film for dose verification and end-to-end testing.
Funding Support, Disclosures, and Conflict of Interest: The submitting institution holds research agreements with Philips Healthcare, ViewRay, Inc., and Modus Medical. Research partially supported by the National Cancer Institute of the National Institutes of Health under Award Number R01CA204189.