Purpose: The dosimetric effect of leaf tip corner details in sliding window dose distributions has been studied, a solution to account for this corner leakage has been designed and implemented. Methods: The dosimetric effect of leaf tip corner deviations from perfect 90 degree borders between the front face and leaf side, was analyzed for sliding window IMRT beams. The study was carried out using film, EPID and diode arrays on Elekta Linacs fitted with MLCi and MLCi2 MLC models. Although the effect is general and does not depend on a particular MLC model, the magnitude and distribution is dependent on MLC model. The study consisted of simple fields designed to de‐couple the combined effects of the leaf properties (transmission, interleaf leakage, tongue and grove, corner leakage), such that corner leakage could be quantified. An enhancement to the transmission filter of a commercial Monte Carlo treatment planning system (Monaco, Elekta AB) was implemented to account for this effect, allowing the user to model the corner leakage for individual leaf tip corners. Results: The analysis of test beams indicates that corner leakage will introduce additional fluence in sliding window IMRT distributions. Of the two mlc models studied, this effect is more important for MLCi, but is also present for MLCi2. Generally, corner leakage will be present in every mlc in which the corner is not perfectly cut at 90 degrees. In this light, small manufacturing differences give rise to small but visible effects on the dose distribution. Correct modeling of these small effects is increasingly important as users seek improved patient specific QA results, in particular using local gamma. Conclusion: In this work we study the effect of mlc leaf corner leakage in sliding window distributions, presenting a software implementation to model this effect in a Monte Carlo‐based Treatment Planning system. This work was funded by Elekta Inc.

Purpose: Intensity Modulated Arc Therapy (IMAT) is gaining widespread attention from various radiation oncology vendors and several centers are implementing them clinically. The aim of this study is to compare Plan quality between Rapid Arc (Varian), Volumetric Modulated Arc Therapy (Elekta) and Smart Arc (Pinnacle) for the same dataset and plan objectives. Methods: 3 anonymized prostate cancer cases with the same DICOM RT structure set were used to compare between the 3 IMAT delivery schemes. The Rapid Arc plans were generated using Eclipse Treatment Planning System (TPS), the VMAT plans were generated using MONACO TPS, and the Smart Arc plans were generated using Pinnacle TPS. The Dose Volume Histograms (DVH) was exported for each plan and plotted together to compare the DVH between 3 different TPS for each patient. In each case the plan objectives were kept the same. Also, the RT dose files were compared from the 3 different delivery methods by importing them into a 3rd party software. Results: To achieve the same plan quality, the Rapid Arc plans were generated using 2 arcs. There was no substantial improvement in plan quality when 2 arcs were used in VMAT plans using MONACO, therefore plans were generated using only one 360 degree arc for both VMAT and Smart Arc plans. The VMAT plans have more dose heterogeneity across the target volume while organs at risk sparing varied depending on the case. The Smart Arc plans had the lowest total monitor units among the 3 delivery methods while the Rapid Arc plans had the largest because 2 arcs were needed to achieve same plan quality.Conclusion: All 3 IMAT methods produced acceptable plans for Prostate IMRT. The relative strengths of each system varied depending on the complexity of the case and there is a trade off between each delivery method.