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Prescription Level Distance-To-Agreement Criteria for IROC-Houston's SRS Head Phantom

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


(Sunday, 7/14/2019)  

Room: ePoster Forums

Purpose: To evaluate the addition of a prescription level distance-to-agreement (DTA) acceptance criteria for IROC-Houston’s SRS head phantom.

Methods: The SRS head phantom contains one insert that includes a 1.9 cm spherical PTV where two TLDs and two planes of GAFchromic film are located. Institutions are instructed to deliver approximately 30 Gy to the center of the PTV. The prescription dose covering the PTV varies depending on the delivery method: GammaKnife - 15 Gy, CyberKnife - 20 Gy, C-arm linac and TomoTherapy - 25 Gy. Current passing criteria are: TLD/TPS within 0.95 – 1.05 and ≥ 85% of pixels passing a 5%/3mm gamma analysis. Since gamma analyses are not sensitive in small area targets, we performed an additional DTA analysis on a subset of phantom results (n=66). DTAs were obtained at the prescription level from 3 profiles taken through the center of the PTV.

Results: The pass rate for the SRS phantom, since 2016, is 84% while our subset of data had a pass rate of 73% using current acceptance criteria. Our subset of data from 2018 included 66 phantoms: 59 TomoTherapy/C-arm linacs, 5 GammaKnifes and 2 CyberKnifes. The pass rate dropped from 73% to 50% or 71% with the additional maximum DTA ≤2 mm or ≤3 mm criteria, respectively. The average DTA values for all orientations were 0.94 ± 0.45 (range of 1 – 4mm) and 1.48 ± 0.56 (range of 1-6mm) for phantoms passing and failing our current passing criteria, respectively.

Conclusion: The addition of a DTA acceptance criteria of 2 mm at the prescription dose level for SRS head phantoms significantly lowered the passing rates. This tighter criteria would ensure better target coverage and be more consistent with the goals of SRS treatment.

Funding Support, Disclosures, and Conflict of Interest: This work received funding from the NIH/NCI grant #CA180803.


Not Applicable / None Entered.


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