Poster - Thurs Eve-33: Initial implementation of a novel, measurement-based IMRT QA method

Current measurement-based QA for IMRT typically involves a composite dose delivery to a phantom. However, this approach does not allow a direct dosimetric evaluation of the delivered treatment with respect to the patient anatomy. In this work we implement a novel, measurement-based IMRT QA method which provides an accurate reconstruction of the 3D-dose distribution in the patient model. The RPC Head&Neck phantom and two clinical prostate cases have been examined to date. Step & shoot plans were developed satisfying required dose metrics. A 2D-array of dose chambers (MatriXX, IBA Dosimetry) was mounted on a linear accelerator to capture delivered fluence. The measurement data were read directly by the control software (COMPASS, IBA Dosimetry), which also provides the ability to import patient plan data from the TPS. The COMPASS software also includes a dose calculation engine and head fluence model and requires beam commissioning procedures analogous to those of a TPS. Reconstructed doses and DVHs were compared to those calculated by the TPS. The beam model in the COMPASS software was able to predict percentage depth dose and X and Y profiles for MLC-defined apertures ranging from 1×1-20×20 cm∧2 to within 1.5% (depth-dose), 2.0% (in-field profiles), and 2.5% (out-of-field profiles). Reconstructed doses in the test plans were mostly within 2% of those in the TPS. DVHs compared to <1.2%. Reconstructed doses were overlaid on CT data and contoured structures, to enable a clinically useful understanding of discrepancies as compared to the TPS plan. Research partially sponsored by IBA Dosimetry.