TH‐C‐BRB‐10: Clinical Implication of the New Dosimetry Formalism in IMRT Quality Assurance

Abstract

Purpose: To apply the new dosimetry formalism [Med. Phys. 35, 5179 (2008)] to clinical IMRT quality assurance (QA). Methods: 20 different linear accelerator (Varian Clinac 21 EX)‐based clinical IMRT fields were transferred to the CT images of a 30×30×17 cm3 Solid Water phantom to create IMRT QA fields. The phantom position was adjusted for each QA field to place the detector or chamber at the lowest dose gradient region in a virtual PTV. The reference doses in the IMRT QA and 10×10 cm2 fields were measured using a PTW micro liquid ion chamber (microLion). Based on the new dosimetry formalism, the clinical correction factor of each IMRT QA field was measured for a calibrated Exradin A12 Farmer‐type chamber in a fully‐rotated delivery and a delivery at a single gantry angle, a collapsed delivery. For each QA field, the measured dose with the correction factor was compared with a calculated dose using Analytical Anisotropic Algorithm (AAA) or Monte Carlo (MC) methods. Results: The clinical correction factor deviated from unity by up to 2.4% and 3.7% in the fully‐rotated and collapsed deliveries, respectively, depending on the dose homogeneity at the Exradin A12 collecting volume. In the fully‐rotated delivery, the measured dose with the correction factor is different from the calculated dose to within 5% and 3% for the AAA and MC, respectively. In the collapsed delivery, the discrepancy between the measured and AAA‐calculated doses was to within 8%, while it was improved to within 3.5% compared with the MC‐calculated dose. When applying the clinical correction factor, the decrease of the measured and calculated dose discrepancy is more significant for an IMRT QA field having higher dose heterogeneity. Conclusions: This work proves that the suggested dosimetry technique is effective to improve the dosimetric consistency of clinical IMRT QA.