A PENELOPE‐based system for the automated Monte Carlo simulation of clinacs and voxelized geometries—application to far‐from‐axis fields

Abstract

Purpose: Two new codes,PEN E ASY and PEN E ASY L INAC, which automate the Monte Carlo simulation of Varian Clinacs of the 600, 1800, 2100, and 2300 series, together with their electron applicators and multileaf collimators, are introduced. The challenging case of a relatively small and far‐from‐axis field has been studied with these tools. Methods: PEN E ASY is a modular, general‐purpose main program for the PENELOPE Monte Carlo system that includes various source models, tallies and variance‐reduction techniques (VRT). The code includes a new geometry model that allows the superposition of voxels and objects limited by quadric surfaces. A variant of the VRT known as particle splitting, called fan splitting, is also introduced.PEN E ASYLINAC, in turn, automatically generates detailed geometry and configuration files to simulate linacs with PEN E ASY. These tools are applied to the generation of phase‐space files, and of the corresponding absorbed dose distributions in water, for two 6 MV photon beams from a Varian Clinac 2100 C/D: a 40 × 40 cm 2 centered field; and a 3 × 5 cm 2 field centered at (4.5, −11.5) cm from the beam central axis. This latter configuration implies the largest possible over‐traveling values of two of the jaws. Simulation results for the depth dose and lateral profiles at various depths are compared, by using the gamma index, with experimental values obtained with a PTW 31002 ionization chamber. The contribution of several VRTs to the computing speed of the more demanding off‐axis case is analyzed. Results: For the 40 × 40 cm 2 field, the percentages γ 1 and γ 1.2 of voxels with gamma indices (using 0.2 cm and 2% criteria) larger than unity and larger than 1.2 are 0.2% and 0%, respectively. For the 3 × 5 cm 2 field, γ 1 = 0%. These figures indicate an excellent agreement between simulation and experiment. The dose distribution for the off‐axis case with voxels of 2.5 × 2.5 × 2.5 mm 3 and an average standard statistical uncertainty of 2% (1σ) is computed in 3.1 h on a single core of a 2.8 GHz Intel Core 2 Duo processor. This result is obtained with the optimal combination of the tested VRTs. In particular, fan splitting for the off‐axis case accelerates execution by a factor of 240 with respect to standard particle splitting. Conclusions: PEN E ASY and PEN E ASY L INAC can simulate the considered Varian Clinacs both in an accurate and efficient manner. Fan splitting is crucial to achieve simulation results for the off‐axis field in an affordable amount of CPU time. Work to include Elekta linacs and to develop a graphical interface that will facilitate user input is underway.