Multi-centric analysis of linear energy transfer distribution from clinical proton beam based on TOPAS

Abstract

Linear energy transfer (LET), a key parameter of relative biological effectiveness (RBE) in proton therapy, is not easy to measure or simulate in a standardized protocol. A multi-centric analysis of dose-averaged LET (LETd) and track-averaged LET (LETt) including individual particle contributions, scoring slab thickness, inhomogeneous medium and clinical beam configurations are investigated using the Monte Carlo code TOPAS. Primary and secondary protons are scored separately within the simulation. It is found that the primary protons are the major contribution to LET at the dose plateau and Bragg Peak (BP) regions, whilst the secondary protons have complete contribution to the tail-LET beyond the BP. The calculated LETt is always lower than LETd, nearly a factor of two lower in the region distal to the BP. Varying the scoring slab thickness shows a negligible influence on LETt, whereas, there is up to 26% discrepancy in LETd at 5-cm depth with thicknesses varying from 0.1 mm to 2 mm for 153.3 MeV proton beam. Moreover, smaller scoring thickness of 0.1 mm results in large fluctuations for tail LETd. LETd distributions with 3-cm thick bone- and lung-slab at the entrance of water phantom against primary and secondary protons' contribution are derived. A small trough appears at the position of inserted inhomogeneous material and large spikes which could account for the changes of cross section in nuclear interactions by secondary protons display on the tail of LETd curve. Regarding the modulated beams, substantial-high LET values are obtained at both-sides along the beam path which should gain more attention when calculating LET in clinical applications. In conclusion, the selection of Monte Carlo simulation schemes indeed affects the calculated LET values, more procedures should be considered when building bridges between LET and radiological effectiveness.