MO-D-108-02: A Secondary Monitor Unit Calculation Algorithm Using Superposition of Symmetric, Open Fields for IMRT and VMAT Treatment Plans

Abstract

Purpose: To perform a secondary dose calculation for an IMRT or VMAT plan to a point on or off axis within 2% using open field data. Methods: The calculation is performed by subdividing dose into the contributions from each opposing leaf pair for a given MLC configuration. Leaf pair dose is determined by drawing four rectangular fields based on leaf positions that are symmetric about the point of calculation. Field dose is determined using isocentric monitor unit calculation formalism with open field data. Superposition of these fields yields the dose from the leaf pair to the point. VMAT plans are approximated by a static MLC configuration at four degree intervals. The algorithm requires standard open field data (e.g., Scps, TPRs), and the MLC control point information. These calculations were done with additional measured small field output factors down to a 1.2 cm2. Algorithm doses were compared with homogeneous and heterogeneous Pinnacle calculations of a series of prostate, head and neck, and chest wall treatment plans. For each treatment plan the calculation point was taken as the isocenter and/or the center of the PTV. Delivery techniques included SMLC‐IMRT and VMAT. Results: Good agreement was obtained between doses calculated by the algorithm and the Pinnacle Treatment Planning System. For homogeneous cases, the total calculated doses were within 2% for all patient plans. Individual beam errors were as high as 7.8% with an average of 2.8%. Larger errors were associated for beam segments with leaf positions very near the calculation point. Larger errors were also found for heterogeneous calculations, where a simple ratio of TPR correction factor was utilized in the algorithm. Conclusion: Results demonstrate that clinically acceptable agreement is obtained using this method. Further improvement could be made with more accurate heterogeneity correction factors and/or incorporation of MLC leaf end effects. This work was supported in part by a research agreement with Oncology Data Systems, Inc.