Evaluation of a novel pelvic end-to-end (PETE) phantom in MR-only and MR-IGRT workflows.

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Conference Proceeding

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Med Phys


Purpose: MR-only treatment planning and MR-IGRT leverages MRI'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∼300 ms) and 7 mg/L (T1∼900 ms) Mn2+ (as MnCl24H2O) 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 (∼90 cc, 150 cc, 250 cc, and 350 cc) and rectal (∼30 cc, 60 cc, 90 cc, 120 cc, 150 cc) 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 True- FISP 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 30 cc and 60 cc rectal volumes differed by 10.7% (∼3.7 cc) and 2.4% (∼1.5 cc) with 1.3 ± 0.5 mm and 0.4 ± 0.5 mm centroid displacements, respectively. Duplicated bladder volumes varied by 1.7% (∼4.5 cc) with corresponding centroid displacements of 0.4 ± 0.3 mm. 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.





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