The elimination of an artifact in the FLURZnrc-derived electron-fluence spectrum close to the Spencer-Attix-Nahum cut-off Δ

Abstract

Objective. An artifact in the electron fluence, differential in energy, Φ E , computed by the EGSnrc Monte-Carlo user-code FLURZnrc, was identified and a methodology has been developed to eliminate it. This artifact manifests itself as an ‘unphysical’ increase in Φ E at energies close to the production threshold for knock-on electrons, AE; this in turn causes an over-estimation of the Spencer-Attix-Nahum (SAN) ‘track-end’ dose by a factor ∼1.5, thereby inflating the dose derived from the SAN cavity integral. For SAN cut-off Δ SAN = 1 keV for 1 MeV and 10 MeV photons in water, aluminium and copper, with maximum fractional energy loss per step ESTEPE = 0.25 (default value), this anomalous increase in the SAN cavity-integral dose is of the order of 0.5%–0.7%. Approach. The dependence of Φ E on the value of AE (the maximum energy loss involved in the restricted electronic stopping power (dE/ds) AE ) at or close to Δ SAN was investigated; this was done for different values of ESTEPE. Main results. The error in the electron-fluence spectrum occurs when Δ SAN is set close to or equal to AE; this error disappears (at the 0.1% level or better) if AE is set ≤ 0.5 × Δ SAN. However, if ESTEPE ≤ 0.04 the error in the electron-fluence spectrum is negligible even when Δ SAN = AE. Significance. An artifact in the FLURZnrc-derived electron fluence, differential in energy, at or close to electron energy AE has been identified. It is shown how this artifact can be avoided, thereby ensuring the accurate evaluation of the SAN cavity integral.