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Intercomparison of Patient-Specific IMRT QA Devices for Small-Field, Stereotactic VMAT Treatments Using Gamma Index Analysis

D Alvarez

D Alvarez*, D Wieczorek , V Mishra , A Gutierrez , Miami Cancer Institute, Miami, FL 33176


SU-I-GPD-T-557 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Multiple devices are currently commercially available for the purpose of patient-specific IMRT QA. This study evaluates the dosimetric delivery accuracy of stereotactic treatment fields when using three different dosimeters: EPID portal dosimetry, 2D diode array, and radiochromic film.

Methods: Both SBRT or SRS treatment plans in Eclipse were used in this study with field sizes restricted between 1.4 x 1.4 cm² and 5 x 5 cm². Nine treatment fields from three different patients (brain, lung, and mediastinum) were measured on a TrueBeam STX using the EPID, MapCheck2 array in a MapPhan, and Gafchromic EBT2 film. Measurements were analyzed using each vendor’s proprietary software. Gamma Index analysis was ran on each field using 2% (of the maximum planned dose)/2 mm DTA criteria with a 90% passing tolerance.

Results: Evaluating the mean gamma passing rates, all three dosimetric methods were viable for field sizes in the 5 x 5 cm² range (means of 97.0%, 97.0%, and 100.0% for Mapcheck2, EBT2 film, and EPID, respectively). For field sizes between 1.4 x 1.4 cm² and 2 x 3 cm², the MapCheck2 failed (88.4% and 70.4% for mediastinum and brain, respectively) while EBT2 film and EPID had comparable results (100.0% and 99.6%, respectively for mediastinum and 96.0% and 97.7% for brain, respectively). A significant reason for the results is due to the device resolutions—7.07 mm for MapCheck2, 0.353 mm for EBT2 film, and 0.358 mm for our EPID.

Conclusion: For field sizes less than 5 x 5 cm², it is not advisable to use a 2D diode array. EBT2 film and EPID yielded comparable passing results for field sizes between 1.4 x 1.4 cm² and 5 x 5 cm². Given the time-consuming task of using EBT2 film compared to the efficiency of portal dosimetry, EPID should be highly considered.

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