A dose gradient analysis tool for IMRT QA

The use of intensity-modulated radiation therapy (IMRT) has led to an increase in the number of complex fields that require measurement and comparison to calculated dose distributions in 2D. Current dose-evaluation techniques, including isodose line comparisons, displays of the dose difference between calculated and measured distributions, and distance-to-agreement (DTA) comparisons, are useful for display of differences between two different dose distributions but are often of limited value for the assessment of the discrepancies in terms of significance and/or cause. In this paper, we present a new gradient compensation method for the evaluation of local dosimetric differences as a function of the dose gradient at each point in the dose distribution. To apply the method, the user specifies a distance parameter (typically 1 mm), which is the geometric tolerance the user is prepared to accept for the dose comparison. The expected geometric uncertainties in the comparison process can include finite calculation and measurement grids, small misalignments of measured and calculated results, and volume-averaging effects in the measurement detector. Since these uncertainties can obscure the interpretation of any of the analysis tools described above, removing dose differences related to the tolerable geometric uncertainty helps the gradient compensation method highlight algorithm and delivery-related differences. The remaining dose differences not explained by the geometric tolerance can then be evaluated graphically (dose difference display) or analytically (dose difference dose-volume histograms)over the entire comparison region.