Poster — Thur Eve — 45: Comparison of different Monte Carlo methods of scoring linear energy transfer in modulated proton therapy beams

Abstract

In this work, we demonstrate inconsistencies in commonly used Monte Carlo methods of scoring linear energy transfer (LET) in proton therapy beams. In particle therapy beams, the LET is an important parameter because the relative biological effectiveness (RBE) depends on it. LET is often determined using Monte Carlo techniques. We used a realistic Monte Carlo model of a proton therapy nozzle to score proton LET in spread‐out Bragg peak (SOBP) depth‐dose distributions. We used three different scoring and calculation techniques to determine average LET at varying depths within a 140 MeV beam with a 4 cm SOBP and a 250 MeV beam with a 10 cm SOBP. These techniques included fluence‐weighted (Φ‐LET) and dose‐weighted average (D‐LET) LET calculations from: 1) scored energy spectra converted to LET spectra through a lookup table, 2) directly scored LET spectra and 3) accumulated LET scored ‘on‐the‐fly’ during simulations. All protons (primary and secondary) were included in the scoring. Φ‐LET was found to be less sensitive to changes in scoring technique than D‐LET. In addition, the spectral scoring methods were sensitive to low‐energy (high‐LET) cutoff values in the averaging. Using cutoff parameters chosen carefully for consistency between techniques, we found variations in Φ‐LET values of up to 1.6% and variations in D‐LET values of up to 11.2% for the same irradiation conditions, depending on the method used to score LET. Variations were largest near the end of the SOBP, where the LET and energy spectra are broader.