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Modified Dose Calibration Methods for IMRT QA

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W Luo

W Luo1*, S Westlund1 , (1) University of Kentucky, Lexington, Kentucky


SU-F-T-274 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To investigate IMRT QA uncertainties caused by dose calibration and modify widely used dose calibration procedures to improve IMRT QA accuracy and passing rate.
Methods: IMRT QA dose measurement is calibrated using a calibration factor (CF) that is the ratio between measured value and expected value corresponding to the reference fields delivered on a phantom. Two IMRT QA phantoms were used for this study: a 30x30x30 cm³ solid water cube phantom (Cube), and the PTW Octavius phantom. CF was obtained by delivering 100 MUs to the phantoms with different reference fields ranging from 3x3 cm² to 20x20 cm². For Cube, CFs were obtained using the following beam arrangements: 2-AP Field – chamber at dmax, 2-AP Field – chamber at isocenter, 4-beam box – chamber at isocenter, and 8 equally spaced fields and chamber at isocenter. The same plans were delivered on Octavius and CFs were derived for the dose at the isocenter using the above beam arrangements. The Octavius plans were evaluated with PTW-VeriSoft (Gamma criteria of 3%/3mm).
Results: Four head and neck IMRT plans were included in this study. For point dose measurement with Cube, the CFs with 4-Field gave the best agreement between measurement and calculation within 4% for large field plans. All the measurement results agreed within 2% for a small field plan. Compared with calibration field sizes, 5x5 to 15x15 were more accurate than other field sizes. For Octavius, 4-Field calibration increased passing rate by up to 10% compared to AP calibration. Passing rate also increased by up to 4% with the increase of field size from 3x3 to 20x20.
Conclusion: IMRT QA results are correlated with calibration methods used. The dose calibration using 4-beam box with field sizes from 5x5 to 20x20 can improve IMRT QA accuracy and passing rate.

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