Experimental comparison of clinically used ion beams for imaging applications using a range telescope

Benedikt Kopp,Chiara Gianoli,Stephan Brons,Lorena Magallanes,Bernd Voss,Sebastian Meyer,Katia Parodi

doi:10.1088/1361-6560/ab87f6

Abstract

In particle therapy, the x-ray based treatment planning converting photon attenuation values to relative stopping power ratio (RSP) introduces clinically relevant range uncertainties. Recently, novel imaging technologies using transmission ion beams have been investigated to directly assess the water equivalent thickness (WET) of tissue, showing improved accuracy in RSP reconstruction, while potentially reducing the imaging dose. Due to their greater availability, protons have been mostly used for ion imaging. To this end, in this work, the influence of three ion species (protons, helium and carbon ions) on the image quality of radiographic WET retrieval has been explored with a dedicated experimental setup and compared to Monte Carlo (MC) simulations. Three phantom setups with different tissue interfaces and features have been irradiated with clinically validated proton, helium and carbon ion pencil beams under comparable imaging dose and beam settings at the Heidelberg Ion-Beam Therapy Center. Ion radiographies (iRADs) were acquired with an integration mode detector, that functions as a range telescope with 61 parallel plate ionization chambers. For comparison, experiments were reproduced in-silico with FLUKA MC simulations. Carbon ions provide iRADs with highest image quality in terms of normalized root mean square error, followed by helium ions and protons. All ions show similar capabilities of resolving WET for the considered phantoms, as shown by the similar average relative error < 3%. Besides for the slab phantom, MC simulations yielded better results than the experiment, indicating potential improvement of the experimental setup. Our results showed that the ability to resolve the WET is similar for all particles, intrinsically limited by the granularity of the detector system. While carbon ions are best suited for acquiring iRADs with the investigated integration mode detector, helium ions are put forward as a less technical challenging alternative.

Highlights

Ion beam therapy offers various advantages compared to conventional radiotherapy based on x-rays
The qualitative difference between the heRAD and cRAD is hardly appreciable in comparison to the pRAD. This observation is reflected in the normalized root mean square error (NRMSE) of the Ion radiographies (iRADs) being 7.8%, 10.5%, and 16.3% for the experimentally acquired cRAD, heRAD, and pRAD, respectively
We have investigated the imaging capabilities of clinically validated protons, helium and carbon ions under comparable dose and beam setting with an integration mode detector that functions as a range telescope

Summary

Introduction

Ion beam therapy offers various advantages compared to conventional radiotherapy based on x-rays. Range uncertainties mainly arise from inter- and intra-fractional anatomical variations, patient setup errors and the determination of the relative (to water) ion stopping power (RSP) map of the patient. The latter uncertainty is mostly due to the current practice of relating the ion RSP to the Hounsfield unit (HU) information of x-ray imaging (i.e. treatment planning CT image). Thereby, a semi-empirical calibration is deployed to convert HU into RSP (Schaffner and Pedroni 1998, Rietzel et al 2007, Yang et al 2012) This procedure can cause range uncertainties of 1% to 5% (Yang et al 2012, Paganetti 2012). The refinement of such RSP maps and the application of range verification techniques either prior to, during, or after the therapy (Knopf and Lomax 2013, Parodi 2014) are desirable in order to ensure an accurate and precise dose application

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Discussion

Conclusion