MCNPX Monte Carlo Simulation Research Articles

Properties of unflattened photon beams shaped by a multileaf collimator

Several studies have shown that removal of the flattening filter from the treatment head of a clinical accelerator increases the dose rate and changes the lateral profile in radiation therapy with photons. However, the multileaf collimator (MLC) used to shape the field was not taken into consideration in these studies. We therefore investigated the effect of the MLC on flattened and unflattened beams. To do this, we performed measurements on a Varian Clinac 21EX and MCNPX Monte Carlo simulations to analyze the physical properties of the photon beam. We compared lateral profiles, depth dose curves, MLC leakages, and total scatter factors for two energies (6 and 18 MV) of MLC-shaped fields and jaw-shaped fields. Our study showed that flattening filter-free beams shaped by a MLC differ from the jaw-shaped beams. Similar differences were also observed for flattened beams. Although both collimating methods produced identical depth dose curves, the penumbra size and the MLC leakage were reduced in the softer, unflattened beam and the total scatter factors showed a smaller field size dependence.

TH-C-T-617-05: Monte Carlo Simulations of a Nozzle for the Treatment of Ocular Tumors with High-Energy Proton Beams

Purpose: To develop a Monte Carlo simulation model for ocular proton beam therapy, validate its predictions with measurements, and commission an ocular treatment planning system using simulated proton beam data. Method and Materials: We commissioned the EYEPLAN ocular treatment planning system for protonradiotherapy using only dosimetric data from Monte Carlo simulations. The commissioning comprised two main tasks: generating nozzle‐specific parameters and dose profiles and entering them into the treatment planning system, and testing the accuracy of the planning system's dose predictions under various beam conditions that are representative of ocular melanoma treatments. The MCNPXMonte Carlo simulation code was used with a detailed, 3‐dimensional model of an ocular beamline. Simulations were carried out to generate both input dose distributions for the treatment planning system as well as validation data to test the accuracy of the TPS predictions. The simulation model was benchmarked against measured dose distribution from Harvard Cyclotron Laboratory (Cambridge) and the Northeast Proton Therapy Center (Boston). Measurements were made with ionization chambers, diodes, and radiographic film. Results: Benchmark comparisons revealed maximum differences between absorbed dose profiles from simulations and measurements of 6% and 0.6 mm, while typical differences were less than 2% and 0.2 mm. The computation time for the entire virtual commissioning process is less than one day. Conclusion: The study revealed that, after a significant development effort, a Monte Carlo model of a proton therapy apparatus is sufficiently accurate and fast for commissioning a treatment planning system. With relatively little additional effort, additional capability can be added to the model, such as the prediction of output factors.