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SU‐E‐T‐171: Quantitative Evaluation and Correction of the Gantry Angular Dependence of MapCheck2 for Patient QA Measurement
Author(s) -
Zhou H,
Wu C
Publication year - 2011
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3612121
Subject(s) - imaging phantom , physics , linear particle accelerator , optics , diode , beam (structure) , detector , nuclear medicine , photon , medicine , optoelectronics
Purpose: To quantitatively evaluate and correct the gantry angular dependence of MapCheck2 for patient QA measurements Methods: A MapCheck2 device (SunNuclear Corp.) is used for the study. The phantom setup uses 5cm solid water slabs both above and below the device. A set of 10 × 10 cm2 fields were measured at different gantry angles from 0 to 360 degree at 20 degree interval on a Varian 21iX LINAC. To the first order approximation, the center diode detector measurement values are extracted and compared against the corresponding TPS calculated values. The ratio of the two is assumed to represent the angular correction factors for the whole 2‐D diode array. MapCheck2 phantom QA for a typical prostate 7‐fields 6 MV IMRT case was conducted with the beams delivered at actual planned gantry angles. Measurement data corrected with the above factors were compared with TPS computed values to evaluate the effectiveness of the correction. Results: Significant diode angular dependence is found for both 6 MV and 15 MV photons. The worst angular dependence (about 13% over‐response for 6 MV and 8% for 15 MV) is found at gantry angles of 90 (or 270 degree), where the beam irradiates parallel to the 2‐D diode array plane. On the representative 7‐fields IMRT plan QA, when absolute dose comparison is made with a Gamma criteria of 3%/3mm DTA and 10% low dose threshold, the QA pass rate is significantly improved when the derived correction factors for the 2‐D diode array are applied. The corrected MapCheck2 measurement gives a 98.7% pass rate, compared to the un‐ corrected raw measurementˈs pass rate of 73.1%. Conclusions: MapCheck2 gantry angular dependence is quantitatively measured and a set of approximate correction factors derived. When applied, MapCheck2 measurement gives very good agreement with the TPS computed planar dose.