CSnrc: Correlated sampling Monte Carlo calculations using EGSnrc

Abstract

CSnrc, a new user-code for the EGSnrc Monte Carlo system is described. This user-code improves the efficiency when calculating ratios of doses from similar geometries. It uses a correlated sampling variance reduction technique. CSnrc is developed from an existing EGSnrc user-code CAVRZnrc and improves upon the correlated sampling algorithm used in an earlier version of the code written for the EGS4 Monte Carlo system. Improvements over the EGS4 version of the algorithm avoid repetition of sections of particle tracks. The new code includes a rectangular phantom geometry not available in other EGSnrc cylindrical codes. Comparison to CAVRZnrc shows gains in efficiency of up to a factor of 64 for a variety of test geometries when computing the ratio of doses to the cavity for two geometries. CSnrc is well suited to in-phantom calculations and is used to calculate the central electrode correction factor Pcel in high-energy photon and electron beams. Current dosimetry protocols base the value of Pcel on earlier Monte Carlo calculations. The current CSnrc calculations achieve 0.02% statistical uncertainties on Pcel, much lower than those previously published. The current values of Pcel compare well with the values used in dosimetry protocols for photon beams. For electrons beams, CSnrc calculations are reported at the reference depth used in recent protocols and show up to a 0.2% correction for a graphite electrode, a correction currently ignored by dosimetry protocols. The calculations show that for a 1 mm diameter aluminum central electrode, the correction factor differs somewhat from the values used in both the IAEA TRS-398 code of practice and the AAPM's TG-51 protocol.

Monte Carlo calculations of , the beam quality conversion factor

“…The chamber used for this representative plot is inherently waterproof so this effect is not observed. Buckley et al 29 showed that there is very little difference between their Monte Carlo calculated P cel values and those in TG-51.…”

Monte Carlo study of Si diode response in electron beams

“…In addition to calculating the two dose values in one run, the water/silicon dose ratio D water / D Si is calculated directly by CSnrc. 16 This dose ratio represents the Si diode response in a water phantom if we assume the charge released is directly proportional to the energy deposited as described in Sec. II A.…”

“…In addition, we also study the energy dependence of the response of the diode model, which is an important characteristic of any radiation detector, and the field-size dependence of the response of the diode model, which is important for output factor measurements in clinical practice. The code used in this work is CSnrc, 16 a new EGSnrc user-code that implements a correlated sampling technique. CSnrc is very efficient in calculating dose ratios for similar geometries that are commonly encountered in radiation dosimetry.…”

Perturbation correction factors for cylindrical ionization chambers in high-energy electron beams

“…Recently, Buckley and Rogers [4] calculated P wall for Farmer-type chambers with various wall materials using the EGSnrc/CSnrc-code [5]. The P wall values vary by 0.8% with the wall material.…”