SU-E-T-54: A Methodology for Monte Carlo Simulation Time Reduction in Dose Calculation for Reference Dosimetry

Abstract

Purpose: To develop and optimize the simulation methodology to calculate the response of ionization chambers over a range of beam qualities commonly used in radiotherapy. The methodology can be used to determine the perturbation and beam quality correction factors for thimble ionizationchambers in high energy photons in a simple way. Methods: The Monte Carlo simulation code PENELOPE was used to simulate the dose in the sensitive volume of the NE2571 ionization chamber placed in a cubic water phantom, at the reference depth, for four different photon beams: 60Co, 4 MV, 6 MV and 10 MV. On a first step simulation, the radiation spectrum that reaches the geometric region of a 6 cm diameter sphere involving the chamber is sampled. Then, in a second simulation, the response of the ionization chamber is simulated using the sampled spectrum and without the cubic water phantom. The results of dose to the sensitive volume of the chamber in the two steps simulation was compared to the dose simulated in a one step simulation with the primary beam incident on the cubic phantom. Results: The dose values obtained without the water phantom, using the pre-sampled spectrum, are similar to the values obtained with those obtained in chamber positioned inside the phantom, with a maximum relative difference of 2.5% for the 10 MV beam. With the pre-sampled spectrum the time required for the simulation was about 30 times lower than for the one step simulation. The best results showed a relative difference of 0.4% for the 6 MV beam. Conclusions: The pre-sampling spectrum methodology proposed presented potential for significant simulation time reduction in dose calculations for reference dosimetry, studies of perturbation factors for thimble ionization chambers and beam quality correction determinations.