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IROC Houston QA Center End-To-End QA Phantom Program Key Findings Over the Past 15 Years

D Followill

D Followill*, A Molineu , P Alvarez , J Lowenstein , P Taylor , N Hernandez , T Nguyen , H Mehrens , B Lewis , S Kry , UT MD Anderson Cancer Center, Houston, TX


SU-E-108-4 (Sunday, July 30, 2017) 1:00 PM - 1:55 PM Room: 108

Purpose: IROC-H’s phantom program has been in existence for over 15 years. This work will describe the phantom program and its key findings to improve the accuracy of dose delivery.

Methods: IROC-H has anthropomorphic lung, liver, head and neck, prostate head SRS and spine phantoms that are used for credentialing and QA purposes for photon and proton therapy. The phantoms include tissue equivalent structures that mimic targets and organs at risk. The breathing motion platforms can be included for some lung and liver phantoms. Absolute dose and dose distributions are measured with TLD and radiochromic film in various planes within the target of the phantoms, respectively.

Results: Since 2001, 4,364 phantoms have been irradiated with 746 in 2016 alone. The historical phantom pass rate ranges between 73% to 86% for photons and 50% to 96% for protons. The IMRT H&N phantom identified the need for MLC positioning QA and to model the rounded MLC leaf-ends. The photon lung phantom identified dose calculation differences in the lung between pencil beam algorithms vs. superposition convolution algorithms vs. Monte Carlo algorithms for both target coverage and dose calculation accuracy. The lung phantom also identified a 14% discrepancy in the dose calculation in lung tissue with Accuray’s pencil beam algorithm. The dual target moving liver phantom high failure rate brought to light the need for the use of multiple isocenters. The proton phantoms have shown the need for different plastics for tissue equivalency and the inability of the proton pencil beam dose calculation algorithms to model the distal end of the proton beam in lung tissue resulting in an under-dosing of the distal target margin.

Conclusion: The IROC-H end-to-end QA phantom program has and continues to improve dose delivery to radiotherapy patients by identifying systematic deficiencies and errors at radiotherapy facilities.

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