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Impact of Verification Planning Phantom Geometry to the Pre-Treatment IMRT QA Gamma Passing Rate

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M Su

M Su, L Tena, S Sura, Icahn School of Medicine at Mount Sinai , New York, NY


SU-I-GPD-T-298 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To study the correlation between the IMRT plan field position in verification planning phantom (VPP) and MapCheck (SunNuclear Corp. FL) QA gamma (γ) passing rate.

Methods: In this study, gamma passing rates from 9 IMRT plans, which created with Eclipse planning system (V11, Varian, Palo Alto, CA), were considered. All MUs were calculated in 0.25cm grid. Two VPPs (30x30x12cm³, 40x40x12cm³) were manually created with 0.3cm slice interval and CT number set to zero. Each IMRT plan was converted into two VPP plans with all field gantry angles projected to zero. One phantom plan was set at the default VPP iso-center (default setting) and other was carefully set to match that at least 5cm phantom margin around the every planning fields by either made the plan iso-center shift or selected a larger VPP (margin setting). IMRT plans were delivered by Varian 21Ex with 120 MLC Linac. 2D Doses were measured by MapCheck. MapCheck combine feature was used for the larger field plan dose formed. Gamma passing rates were computed by MapCheck with absolute dose between measured and calculated from two VPP plans at selected γ/absolute dose (AD) tolerance levels. The threshold was set to 10.

Results: 9 IMRT plans had total MUs between 1064 and 2557, number of QA points between 239 and 588 for γ passing rate calculation. Gamma passing rates were computed at four γ/AD tolerance levels: γ/AD = 2mm/2%, 2mm/3%, 3mm/2% and 3mm/3%. The results showed that an average percentage γ passing rate improved with the field margin setting plan by: 2.37±2.37, 1.40±2.11, 1.40±1.46 and 0.62±0.93, respectively.

Conclusion: Phantom geometry was a factor of IMRT QA γ passing rate. To improve the γ passing rate for dose comparison, a VPP plan should also be calculated with at least 5cm phantom margin around all treatment fields.

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