LET measurements and simulation modelling of the charged particle field for the Clatterbridge ocular proton therapy beamline

Abstract: Proton therapy can achieve a highly targeted treatment by utilising the advantageous dosimetric characteristics of the Bragg Peak. Protons traversing through a material will deposit their maximum energy at the Bragg Peak through ionisation and other interactions, transferring minimal excess dose to surrounding tissue and organs. This rate of energy loss is also quantified by the linear energy transfer (LET), which is indicative of radiation quality and radiobiological effects. However it is a challenging physical quantity to measure, as characterisation of radiation fields and the impact of LET on treatment requires advanced tools and technology. The MiniPIX-Timepix is a miniaturised, hybrid semiconductor pixel detector capable of high resolution spectrometric tracking, enabling wide-range detection of the deposited energy, position and direction of single particles. Experimental measurements were performed at a clinical facility, the Clatterbridge Cancer Centre which houses a 60 MeV ocular proton therapy beamline. A realistic end-to-end model of the facility was developed in the Monte Carlo code TOPAS (TOol for PArticle Simulation) and was used to simulate the experimental conditions. The detector was held at 45$^{\circ}$ and 60$^{\circ}$ perpendicular to the beam, and placed downstream of various thickness Polymethyl methacrylate (PMMA) blocks to acquire data along the dose deposition depth. Empirical cluster data providing track length and the energy deposition distributions were used to obtain the LET spectra. The determined values for the LET in silicon and dose averaged LET across the BP show general agreement with simulated results, supporting the applicability of the TOPAS CCC model. This work explores the capability of the MiniPIX detector to measure physical quantities to resolve the LET, and discusses experimental considerations and further possibilities.