A multiple-source photon beam model and its commissioning process for VMC++ Monte Carlo code

The use of Monte Carlo methods in photon beam treatment planning is becoming feasible due to advances in hardware and algorithms. However, a major challenge is the modeling of the radiation produced by individual linear accelerators. Monte Carlo simulation through the accelerator head or a parameterized source model may be used for this purpose. In this work, the latter approach was chosen due to larger flexibility and smaller amount of required information about the accelerator composition. The source model used includes sub-sources for primary photons emerging from target, extra-focal photons, and electron contamination. The free model parameters were derived by minimizing an objective function measuring deviations between pencil-beam-kernel based dose calculations and measurements. The output of the source model was then used as input for the VMC++ code, which was used to transport the particles through the accessory modules and the patient. To verify the procedure, VMC++ calculations were compared to measurements for open, wedged, and irregular MLC-shaped fields for 6MV and 15MV beams. The observed discrepancies were mostly within 2%, 2 mm. This work demonstrates that the developed procedure could, in the future, be used to commission the VMC++ algorithm for clinical use in a hospital.