SU-E-T-61: IMRT Patient QA with Dosimetry Check: The Effect of Detector Selection in Volumetric Dose Reconstruction

Abstract

Purpose: to evaluate the effect of detector selection in conjunction with the Dosimetry Check system for CT based dose reconstruction from measurement of the fluence maps of IMRT beams. Methods: Twenty step and shoot IMRT patients were used to evaluate the performance of the Dosimetry Check software (Math Resolution, LLC, Maryland). Integrated images for each patient were taken with an aSi1000 EPID (Varian Medical Systems), Seven29 (PTW Freiburg, Germany), and an I'mRT MatriXX (IBA dosimetry, Germany). The treatments were delivered using a Varian Novalis Tx with an HD 120 MLC system. Each plan was delivered and measured by all detectors. An integrated image was taken for each individual field and imported into the Dosimetry Check software. Using the Pinnacle treatment planningsoftware, the CT scans, structure, plan, and dose files were imported in to the Dosimetry Check software. The software then used the imported fluence fields to calculate the absolute dose and dose volume histogram to compare with the reference data exported from the treatment planning system. Gamma analysis was used to compare the measured data with the reference data. A 3% ‐ 3mm criteria with a 5% dose threshold was used. Results: Good agreement was found between the calculated doses and the reference doses for all patients. For all detectors the gamma index calculations ranged from 90% to 100% pass rate. Similar results were seen for each patient using one of the ionization chamber arrays, while a 1–2% increase in the gamma index was seen when using the EPID. Conclusions: A slight increase in performance is observed with increasing detector resolution. It is desirable to use the aSi1000 EPID to get the best agreement between calculated and reference doses. However, the ionization chamber arrays both performed adequately and can be used for IMRT QA with the Dosimetry Check software.