Open AccessA Monte Carlo study of different LET definitions and calculation parameters for proton beam therapy
Author(s) -
Edward Smith,
Carla Winterhalter,
Tracy Underwood,
A. Aitkenhead,
Jenna Richardson,
Michael J. Merchant,
N.F. Kirkby,
Karen J Kirkby,
Ranald I Mackay
Publication year - 2021
Publication title -
biomedical physics and engineering express
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.333
H-Index - 16
ISSN - 2057-1976
DOI - 10.1088/2057-1976/ac3f50
Subject(s) - sobp , linear energy transfer , monte carlo method , proton , particle therapy , bragg peak , range (aeronautics) , proton therapy , nuclear medicine , ion , physics , relative biological effectiveness , mathematics , radiation treatment planning , computational physics , beam (structure) , statistics , irradiation , nuclear physics , materials science , radiation therapy , medicine , optics , quantum mechanics , composite material
The strong in vitro evidence that proton Relative Biological Effectiveness (RBE) varies with Linear Energy Transfer (LET) has led to an interest in applying LET within treatment planning. However, there is a lack of consensus on LET definition, Monte Carlo (MC) parameters or clinical methodology. This work aims to investigate how common variations of LET definition may affect potential clinical applications. MC simulations (GATE/GEANT4) were used to calculate absorbed dose and different types of LET for a simple Spread Out Bragg Peak (SOBP) and for four clinical PBT plans covering a range of tumour sites. Variations in the following LET calculation methods were considered: (i) averaging (dose-averaged LET (LET d ) & track-averaged LET); (ii) scoring (LET d to water, to medium and to mass density); (iii) particle inclusion (LET d to all protons, to primary protons and to particles); (iv) MC settings (hit type and Maximum Step Size (MSS)). LET distributions were compared using: qualitative comparison, LET Volume Histograms (LVHs), single value criteria (maximum and mean values) and optimised LET-weighted dose models. Substantial differences were found between LET values in averaging, scoring and particle type. These differences depended on the methodology, but for one patient a difference of ∼100% was observed between the maximum LET d for all particles and maximum LET d for all protons within the brainstem in the high isodose region (4 keV μ m −1 and 8 keV μ m −1 respectively). An RBE model using LET d including heavier ions was found to predict substantially different LET-weighted dose compared to those using other LET definitions. In conclusion, the selection of LET definition may affect the results of clinical metrics considered in treatment planning and the results of an RBE model. The authors’ advocate for the scoring of dose-averaged LET to water for primary and secondary protons using a random hit type and automated MSS.