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Evaluation of Trends and Performance Indicators for IROC Houston's SRS Head Phantom

H Mehrens*, T Nguyen, M Glenn, D Branco, S Edward, S Hartzell, N Hernandez, P Alvarez, A Molineu, P Taylor, D Followill, S Kry, MD Anderson, Houston, TX


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: To evaluate trends and performance indicators on IROC Houston’s SRS head phantom.

Methods: The SRS head phantom contains a 1.9 cm spherical PTV where two TLDs and two planes of GAFchromic film are located. Institutions are instructed to deliver a treatment consistent with their clinical practice. Current passing criteria are: TLD/TPS within 0.95 – 1.05 and = 85% of pixels passing a 5%/3mm gamma analysis. SRS phantom data was collected from 2012-2018 (with gamma analysis only fully implemented and available in 2013). The Pearson chi-squared test was utilized to determine statistical significance.

Results: The overall pass rate for the SRS head phantom (N = 793) was 84% with 11% failing TLD (N = 87) and 9% (N = 54) failing gamma analysis where 17 sites failed both. The average PTV TLD ratio was 0.98 and the average gamma showed 95.2% pixels passing. The passing rates were higher for GammaKnife (91%), CyberKnife (91%) and TomoTherapy (90%) as compared to C-arm linacs, for example, TrueBeam (83%) and Trilogy (81%), however, they were not statistically significantly different from one another. Through the years, the overall average TLD value has increased steadily towards 1.00 starting with an average value of 0.972 in 2012 to 0.989 in 2017 but with a slight dip to 0.983 in 2018.

Conclusion: The SRS head phantom has become an increasingly useful tool for QA both for credentialing purposes for NCI-sponsored trials as well as clinical quality assurance. While improvement has been made throughout the years, the SRS head phantom lags behind other phantoms in pass rates and illustrates areas to further improve the practice of radiation oncology.

Funding Support, Disclosures, and Conflict of Interest: This work was supported by NCI grant CA214526 and IROC-Houston NCI grant #CA180803.


Stereotactic Radiosurgery, Phantoms, Quality Assurance


TH- External Beam- Photons: Quality Assurance - Linear accelerator

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