MRIgRT Dynamic Lung Motion Thorax Anthropomorphic QA Phantom: Design, Development, Reproducibility, and Feasibility Study

Purpose: To design, manufacture, and evaluate a dynamic magnetic resonance imaging/computed tomography (MRI/CT)-compatible anthropomorphic thorax phantom used to credential MR image-guided radiotherapy (MRIgRT) systems participating in NCI-sponsored clinical trials.

Method: The dynamic anthropomorphic thorax phantom was constructed from a water-fillable acrylic shell that contained several internal structures representing radiation-sensitive organs within the thoracic region. A custom MR/CT visible cylindrical insert was designed to simulate the left lung with a centrally located tumor target. The surrounding lung tissue was constructed from a heterogeneous in-house mixture using petroleum jelly and miniature (2-4 mm diameter) styrofoam balls and the tumor structure was manufactured from liquid PVC plastic. An MR conditional pneumatic system was developed to allow the MRIgRT insert to move in similar inhale/exhale motions. TLDs and radiochromic EBT3 film were inserted into the phantom to measure absolute point doses and dose distributions, respectively. The dynamic MRIgRT thorax phantom was evaluated through a reproducibility study and a feasibility study. Comprehensive end-to-end examinations were done where the phantom was imaged on a CT, an IMRT treatment plan was created and an MR image was captured to verify treatment setup. Then, the phantom was treated on an MRIgRT system. The reproducibility study evaluated how well the phantom could be reproduced in an MRIgRT system by irradiating three times on an Elekta's 1.5 T Unity system. The phantom was shipped to three independent institutions and was irradiated on either an MRIdian cobalt-60 ( Co) or an MRIdian linear accelerator system. Treatment evaluations used TLDs and radiochromic film to compare the planned treatment reported on the treatment planning software against the measured dose on the dosimeters.

Results: The phantom on the Unity system had reproducible TLD doses measurements (SD < 1.5%). The measured TLD to calculated dose ratios from the reproducibility and feasibility studies ranged from 0.93 to 1.01 and 0.96 to 1.03, respectively. Using a 7%/5 mm gamma analysis criteria, the reproducibility and feasibility studies resulted in an average passing rate of 93.3% and 96.8%, respectively. No difference was noted in the results between the MRIdian Co and MRIdian 6 MV linac delivery to the phantom and all treatment evaluations were within IROC-Houston's acceptable criterion.

Conclusions: A dosimetrically tissue equivalent, CT/MR visible, motion-enabled anthropomorphic MRIgRT thorax phantom was constructed to simulate a lung cancer patient and was evaluated as an appropriate NIH credentialing tool used for MRIgRT systems.