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Design and Implementation of a Head and Neck Anthropomorphic Phantom Used in MRI Guided Radiotherapy Systems

A Steinmann*, P Alvarez , H Lee , Z Wen , G Sawakuchi , L Court , R Stafford , C Fuller , D Followill , UT MD Anderson Cancer Center, Houston, TX


(Sunday, 7/29/2018) 5:05 PM - 6:00 PM

Room: Karl Dean Ballroom B1

Purpose: Design, manufacture, and evaluate a tissue equivalent MRI/CT visible anthropomorphic end-to-end QA Head and Neck (HN) phantom to credential MRI-guided-radiotherapy systems (MRgRT) wishing to participate in NCI-sponsor clinical trials.

Methods: The phantom is equipped with a water-fillable acrylic shell and a custom insert that mimics organ at risk (OAR) and target structures. The insert consists of a primary and secondary PTV constructed out of liquid PVC plastic, an acrylic OAR and surrounding tissue fabricated out of Superflab. EBT3 film and TLDs were used to measure the dose distribution and absolute dose, respectively. Commissioning of the phantom was first performed in-house by conducting an end-to-end test that included: imaging on a GE Lightspeed CT simulator, planning on Monaco TPS, registering with MRI and irradiating it on the Elekta’s 1.5T Unity system. The phantom was irradiated three times using the same plan to determine reproducibility. Three institutions, equipped with either ViewRay MRIdian Co-60 or Linac systems, conducted a feasibility study by preforming independent end-to-end studies. TLD were evaluated in both commissioning and feasibility studies by comparing ratios of measured TLD to reported TPS calculated values.

Results: The commissioning of the phantom on the Unity linac resulted in reproducible TLD doses (SD < 1.5%). The measured TLD to calculated dose ratios for the Unity system ranged from 0.94 to 0.98. The Viewray dose result comparisons had a larger range (1.02-0.94) but these depended on the TPS dose calculations from each site. Using a 7%/4mm (85% pixels passing) gamma analysis, Viewray institutions had an average passing rates of 97.1%. All of the results were within IROC’s 7%/4mm criterion. There was no difference in the results noted between the ViewRay Co-60 and 6MV delivery to the phantom.

Conclusion: MRgRT HN phantom can be used as a credentialing tool for NCI-clinical trials using MRgRT systems.

Funding Support, Disclosures, and Conflict of Interest: IROC Houston grant # CA180803


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