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

TL;DR

This work tackles the challenge of measuring LET in proton therapy by coupling a high-resolution MiniPIX-Timepix detector with a comprehensive TOPAS CCC beamline simulation for a 60 MeV ocular beam. An end-to-end TOPAS model reproduces the clinical beamline geometry, while PMMA modulators probe depth around the Bragg Peak, enabling LET_d reconstruction from detector cluster data. The empirical LET spectra in silicon broadly agree with TOPAS predictions, though discrepancies reflect experimental artefacts, contamination, and voxel-scoring limitations. Together, the approach demonstrates the MiniPIX detector’s capability for LET quantification and validates the TOPAS CCC framework for ocular proton therapy, with implications for radiobiology studies and enhanced treatment planning.

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.

Figures (10)

• Figure 1: Image of the CCC treatment line from the wall of the treatment room to the patient nozzle (top). Delivery components downstream of the wall are enclosed but exposed to air. 3D rendering of the treatment line in CAD (bottom), components and direction of beam labelled.
• Figure 2: CCC treatment beamline model visualised in TOPAS (top), wireframe view with global axes (beam direction, +$z$). Bird's-eye view with major components labelled (bottom).
• Figure 3: Graph showing the simulated CCC dose profile with points of interest denoted and corresponding thickness (mm) of PMMA absorbers required.
• Figure 4: The MiniPIX detector secured to a motorised, rotating base. A retractable 1 mm aluminium sheath covers the chip sensitive area and the centre of the sensor is positioned along the origin of the rotational axis.
• Figure 5: Experimental setup of the MiniPIX detector angled at 45$^{\circ}$ to the beam, positioned 18 cm from the collimator. Different thickness blocks of PMMA (Lucite) were placed upstream to change the WET, shifting the depth of the BP (24.40 mm block is pictured).