Development and implementation in the Monte Carlo code PENELOPE of a new virtual source model for radiotherapy photon beams and portal image calculation

Abstract

This work aims at developing a generic virtual source model (VSM) preserving all existing correlations between variables stored in a Monte Carlo pre-computed phase space (PS) file, for dose calculation and high-resolution portal image prediction. The reference PS file was calculated using the PENELOPE code, after the flattening filter (FF) of an Elekta Synergy 6 MV photon beam. Each particle was represented in a mobile coordinate system by its radial position (r s ) in the PS plane, its energy (E), and its polar and azimuthal angles (φ d and θ d ), describing the particle deviation compared to its initial direction after bremsstrahlung, and the deviation orientation. Three sub-sources were created by sorting out particles according to their last interaction location (target, primary collimator or FF). For each sub-source, 4D correlated-histograms were built by storing E, r s , φ d and θ d values. Five different adaptive binning schemes were studied to construct 4D histograms of the VSMs, to ensure histogram efficient handling as well as an accurate reproduction of E, r s , φ d and θ d distribution details. The five resulting VSMs were then implemented in PENELOPE. Their accuracy was first assessed in the PS plane, by comparing E, r s , φ d and θ d distributions with those obtained from the reference PS file. Second, dose distributions computed in water, using the VSMs and the reference PS file located below the FF, and also after collimation in both water and heterogeneous phantom, were compared using a 1.5%-0 mm and a 2%-0 mm global gamma index, respectively. Finally, portal images were calculated without and with phantoms in the beam. The model was then evaluated using a 1%-0 mm global gamma index. Performance of a mono-source VSM was also investigated and led, as with the multi-source model, to excellent results when combined with an adaptive binning scheme.

Monte Carlo and analytic modeling of an Elekta Infinity linac with Agility MLC: Investigating the significance of accurate model parameters for small radiation fields

“…These typically consist of the photons from the target, primary collimator and flattening filter as well as electron contamination. [16][17][18][19][20][21] The data for particles (e.g., position and direction) generated by each source are derived from the phase space file calculated by MC simulations and scored in a specific plane. Tuning of parameters (e.g., virtual source size, energy fluence, weight of each source) of the VSMs can be achieved by comparison against MC simulations and/or measurements.…”

“…Tuning of parameters (e.g., virtual source size, energy fluence, weight of each source) of the VSMs can be achieved by comparison against MC simulations and/or measurements. Chabert et al 20 created a virtual source model of the Elekta Synergy 6 MV photon beam using phase space data file calculated by the PENELOPE 22 MC code and scored below the flattening filter. Their VSM model included three virtual sources including a primary source (photons from the target) and two scattered sources (photons from the primary collimator and flattening filter).…”

Validation of histogram‐based virtual source models for different IGRT kV‐imaging systems

“…The two main technics are full MC calculations or MC calculations based on virtual source model (VSM). 17 Full MC calculations start at the electron emission stage of the x-ray tube and end at the in-patient dose deposition. On the contrary, VSM-based MC calculations rely on a simplified description of the photon source 10,15,18 that are mainly based on measurements.…”

A photon source model based on particle transport in a parameterized accelerator structure for Monte Carlo dose calculations

“…In recent years, virtual source models have been proposed based on the correlated histogram or direct curve fitting for beam characteristics obtained from PSD . Especially, Gonzalez et al .…”

“…In recent years, virtual source models have been proposed based on the correlated histogram or direct curve fitting for beam characteristics obtained from PSD. [11][12][13] Especially, Gonzalez et al 13 proposed the model function suitable for describing beam characteristics by comparing and verifying several forms of the model function with PSD. These approaches solve the problems related to PSD such as a requirement of huge storage or a limited number of particles.…”