A study of Type B uncertainties associated with the photoelectric effect in low-energy Monte Carlo simulations

TL;DR

This study quantifies Type B uncertainties in low-energy photon dose calculations due to different Monte Carlo code implementations, highlighting their significance below 200 keV and implications for dosimetry accuracy.

Contribution

It provides the first comprehensive estimation of Type B uncertainties from cross-section variations in low-energy Monte Carlo simulations for dosimetry.

Findings

Type B uncertainties for absorbed dose range from 0.3% to 1.7% depending on energy.

Photon-fluence spectrum uncertainties are negligible above 25 keV.

Uncertainties are significant below 30 keV, affecting dose accuracy.

Abstract

The goal of this manuscript is to estimate Type B uncertainties in absorbed-dose calculations arising from the different implementations in current state-of-the-art Monte Carlo codes of low-energy photon cross-sections (<200 keV). Monte Carlo simulations are carried out using three codes widely used in the low-energy domain: PENELOPE-2018, EGSnrc, and MCNP. Mass energy-absorption coefficients for water, air, graphite, and their respective ratios; absorbed dose; and photon-fluence spectra are considered. Benchmark simulations using similar cross-sections have been performed. The differences observed between these quantities when different cross-sections are considered are taken to be a good estimator for the corresponding Type B uncertainties. A conservative Type B uncertainty for the absorbed dose (k=2) of 1.2%-1.7% (<50 keV), 0.6%-1.2% (50-100 keV), and 0.3% (100-200 keV) is estimated. The photon-fluence spectrum does not present clinically relevant differences that merit considering additional Type B uncertainties except for energies below 25 keV, where a Type B uncertainty of 0.5% is obtained. Below 30 keV, mass energy-absorption coefficients show Type B uncertainties (k=2) of about 1.5% (water and air), and 2% (graphite), reaching values about 1% (40-50 keV) and 0.5% (50-75 keV). Type B uncertainties for the water-to-graphite ratios are observed for energies below 30 keV, about 0.7% (k=2). In contrast with the intermediate (about 500 keV) or high (about 1 MeV) energy domains, Type B uncertainties due to the different cross-sections implementation cannot be considered subdominant with respect to Type A uncertainties or even to other sources of Type B uncertainties. Therefore, the values reported here should be accommodated within the uncertainty budget in low-energy photon dosimetry studies.