SU‐E‐T‐455: Evaluation of the Influence of the Dosimetric Leaf Gap on RapidArc Plans Using Gafchromic Film and Monte Carlo Simulations

Purpose: Correct setting the dosimetric leaf gap (DLG) in the planning system is essential for proper calculation of the dose distribution for RapidArc plans. We evaluated the influence of the DLG by using Gafchromic EBT2 film, as well as running Monte Carlo(MC) simulations. Methods: Gafchromic EBT2 film was sandwiched between acrylic slabs, and irradiated during delivery of RapidArc plans. The films were scanned with Epson V700 scanner, corrected for scanner nonuniformity, and converted to dose. The agreement between the dose distribution measured with film (for DLG values ranging from 1.3mm to 5.0mm) was compared to the corresponding dose calculated in Eclipse using the AAA8615 algorithm. After registering the film images to the calculated dose, agreement was quantified by comparing line profiles and examining gamma maps. No normalization or spatial shifting was employed. MC simulations were carried out on a phantom; gamma maps were calculated between the simulated and planned doses. Results: Comparing line profiles from film to the corresponding profiles from AAA revealed the best agreement at DLG of 1.6mm. For larger values of the DLG the AAA calculations overestimated the film profiles, while smaller values underestimated the film data. The gamma maps in the region conformal to the PTV had nearly 100% passing rate for gamma (3%, 3mm). MC simulations yielded an optimum DLG value of 2.2mm Conclusions: EBT2 films proved to be sensitive to detect variations in dose distributions calculated for various values of the DLG. Such sensitivity was only possible in the absence of normalization of the data sets, as the DLG influences the average calculated dose in the PTV. There is a small disagreement of the optimum value of DLG established using these two methods, which may be attributed to the film data being a subset of the MC calculated 3D data set.