A Monte Carlo study of different LET definitions and calculation parameters for proton beam therapy

Edward A K Smith; Carla Winterhalter; Tracy S A Underwood; Adam H Aitkenhead; Jenny C Richardson; Michael J Merchant; Norman F Kirkby; Karen J Kirby; Ranald I Mackay

doi:10.1088/2057-1976/ac3f50

A Monte Carlo study of different LET definitions and calculation parameters for proton beam therapy

Biomed Phys Eng Express. 2021 Dec 17;8(1). doi: 10.1088/2057-1976/ac3f50.

Authors

Edward A K Smith^{1

2}, Carla Winterhalter^{1

3}, Tracy S A Underwood^{1

3}, Adam H Aitkenhead^{1

2}, Jenny C Richardson^{1

2}, Michael J Merchant^{1

3}, Norman F Kirkby^{1

3}, Karen J Kirby^{1

3}, Ranald I Mackay^{1

2}

Affiliations

¹ Division of Cancer Sciences, School of Medical Sciences, Faculty of Biology, Medicine and Health, The University of Manchester, Manchester, United Kingdom.
² Christie Medical Physics and Engineering, The Christie NHS Foundation Trust, Manchester, United Kingdom.
³ The Christie NHS Foundation Trust, Manchester Academic Health Science Centre, Manchester, United Kingdom.

PMID: 34874308
DOI: 10.1088/2057-1976/ac3f50

Abstract

The strongin vitroevidence 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_dto water, to medium and to mass density); (iii) particle inclusion (LET_dto 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_dfor all particles and maximum LET_dfor all protons within the brainstem in the high isodose region (4 keVμm^-1and 8 keVμm^-1respectively). An RBE model using LET_dincluding 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.

Keywords: LET; clinical methodology; gate; monte carlo; proton beam therapy; treatment planning.

Creative Commons Attribution license.

MeSH terms

Humans
Linear Energy Transfer*
Monte Carlo Method
Proton Therapy* / methods
Protons
Relative Biological Effectiveness

Substances

Protons