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When a Half Dozen Does Not Equal Six: Aligning Plan Dose to Shifted Measurements with a Commercial Cylindrical Phantom for IMRT QA

B Leong

B Leong*, L Padilla , J Evans , Virginia Commonwealth University, Richmond, VA


SU-I-GPD-T-268 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: For measurement-based patient specific IMRT QA, it is sometimes necessary to shift the device along the superior-inferior direction, e.g. to protect sensitive electronics. Two methods exist in SNC Patient (Sun Nuclear Corporation) to align plan dose to shifted ArcCHECK measurements, which we have found to produce differences in gamma analysis passing rates. We conducted an internal review of shifted IMRT QA cases over a six month period to evaluate the magnitude and prevalence of this effect.

Methods: SNC Patient extracts the plan dose map assuming a fixed relationship between the diode locations and the plan isocenter. For shifted QA measurements, the plan dose can be aligned by (a) recomputing TPS dose for the shifted isocenter or (b) entering the shifted coordinates into the SNC plan dose extraction routine. Twenty-seven QA measurements with SI shifts from 1.0cm to 8.5cm were re-analyzed (gamma analysis summarized by passing rate) comparing both methods for extracting shifted plan dose.

Results: Analysis showed a change of gamma passing rate in 15/27 cases [-0.5%:+4.8%], and no change in 12/27. Of the 15, 13 exhibited higher passing rates for method (b). Sub-component analysis showed that method (b) leads to an increased number of pixels [0%:47%] utilized in the gamma comparison, while the number of failing pixels remains largely the same. While the majority of differences (12/15) were marginal (≤±0.5%), 3 cases (11% of overall 27) saw passing rate changes >1.0%.

Conclusion: Our study demonstrates that two methods for aligning plan dose for shifted QA measurements may not produce equivalent results. It is conceivable that the variation in passing rate for the 2 methods can potentially make the difference between passing and failing a plan for clinics utilizing gamma analysis . This highlights the importance of establishing consistent QA procedures to avoid process-biased results.

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