Dose calibration uncertainty and plan-specific dose calibration for IMRT QA

Abstract

Purpose: This study was to investigate IMRT QA dose calibration uncertainty that has not been realized before, and develop plan-specific calibration methods to reduce such uncertainty. Methods: IMRT plans were created on the Varian Eclipse treatment planning system. IMRT QA used a PTW Octavius phantom with an Octavius 2D array ionization chamber detector and was also tested for a solid water cube with a Farmer chamber. Dose calibration for a solid phantom was analyzed using the AAPM TG-51/TG-21 formulism. The reference fields used for IMRT dose calibration included squared fields and IMRT plan-specific fields that included geometric equivalent fields, and dose-equivalent fields defined by the 50% isodose distributions. Results: Thirteen IMRT QA cases including three VMAT cases were used for this study. For the single field calibration on the PTW Octavius device, the calibration factors varied with field size by up to 5.6%, and varied with gantry angle by up to 4.9%. Those variations changed IMRT QA passing rates by up to 16.2%. Using the plan-specific calibration, the Gamma passing rate was increased by up to 27.7%. The VMAT passing rates varied by up to 30%, and the equivalent field dose calibration gave the highest passing rate. For absolute point dose measurement using a Farmer chamber, calibration factors varied by up to 3.7% with different field sizes and different gantry angles. The dose equivalent field calibration achieved the best IMRT QA results. Conclusion: Different fields can have different dose calibration factors. Conventional dose calibration methods for IMRT QA using simple reference fields include significant uncertainty that would affect IMRT QA results. Dose calibration uncertainty is caused by the complex composition of phantom materials and complex design and characteristics of the detectors. The plan-specific dose calibration can reduce dose calibration uncertainty and improve IMRT QA accuracy.