Monte Carlo method to study the proton fluence for treatment planning

The proton beam at the Hahn Meitner Institute (HMI) in Berlin will be used for proton therapy of eye melanoma in the near future. As part of the pre-therapeutic studies, Monte Carlo calculations have been performed to investigate the primary fluence distribution of the proton beam including the influence of scattering foils, range shifters, modulator wheels, and collimators. Any material in the beam path will modify the therapeutic beam because of energy loss, multiple scattering, range straggling, and nuclear reactions. The primary fluence information is a pre-requisite for most pencil-beam treatment planning algorithms. The measured beam penumbra has been used as one of the parameters to characterize a proton beam for further calculations in a treatment planning algorithm. However, this phenomenological quantity represents only indirect information about the properties of the proton beam. In this work, an alternative parameterization of the beam exiting the vacuum window of the accelerator, as well as the beam right in front of the patient collimator, is introduced. A beam is fully characterized if one knows (for instance from Monte Carlo simulations) the particle distribution in energy, position, and angle, i.e., the phase space distribution. Therefore, parameters derived from this distribution can provide an alternative input in treatment planning algorithms. In addition, the method of calculation is introduced as a tool to investigate the influence of modifications in the beam delivery system on the behavior of the therapeutic proton beam.     

A patient-specific Monte Carlo dose-calculation method for photon beams

A patient-specific, CT-based, Monte Carlo dose-calculation method for photon beams has been developed to correctly account for inhomogeneity in the patient. The method employs the EGS4 system to sample the interaction of radiation in the medium. CT images are used to describe the patient geometry and to determine the density and atomic number in each voxel. The user code (MCPAT) provides the data describing the incident beams, and performs geometry checking and energy scoring in patient CT images. Several variance reduction techniques have been implemented to improve the computation efficiency. The method was verified with measured data and other calculations, both in homogeneous and inhomogeneous media. The method was also applied to a lung treatment, where significant differences in dose distributions, especially in the low-density region, were observed when compared with the results using an equivalent pathlength method. Comparison of the DVHs showed that the Monte Carlo calculated plan predicted an underdose of nearly 20% to the target, while the maximum doses to the cord and the heart were increased by 25% and 33%, respectively. These results suggested that the Monte Carlo method may have an impact on treatment designs, and also that it can be used as a benchmark to assess the accuracy of other dose calculation algorithms. The computation time for the lung case employing five 15-MV wedged beams, with an approximate field size of 13 X 13 cm and the dose grid size of 0.375 cm, was less than 14 h on a 175-MHz computer with a standard deviation of 1.5% in the high-dose region.