Fluence correction factor for graphite calorimetry in a clinical high-energy carbon-ion beam

A Lourenço,R Thomas,G Royle,H Bouchard,M Homer,H Palmans,S Rossomme,J Renaud,T Kanai

doi:10.1088/1361-6560/aa6147

Abstract

The aim of this work is to develop and adapt a formalism to determine absorbed dose to water from graphite calorimetry measurements in carbon-ion beams. Fluence correction factors, , needed when using a graphite calorimeter to derive dose to water, were determined in a clinical high-energy carbon-ion beam. Measurements were performed in a 290 MeV/n carbon-ion beam with a field size of 11 × 11 cm2, without modulation. In order to sample the beam, a plane-parallel Roos ionization chamber was chosen for its small collecting volume in comparison with the field size. Experimental information on fluence corrections was obtained from depth-dose measurements in water. This procedure was repeated with graphite plates in front of the water phantom. Fluence corrections were also obtained with Monte Carlo simulations through the implementation of three methods based on (i) the fluence distributions differential in energy, (ii) a ratio of calculated doses in water and graphite at equivalent depths and (iii) simulations of the experimental setup. The term increased in depth from 1.00 at the entrance toward 1.02 at a depth near the Bragg peak, and the average difference between experimental and numerical simulations was about 0.13%. Compared to proton beams, there was no reduction of the due to alpha particles because the secondary particle spectrum is dominated by projectile fragmentation. By developing a practical dose conversion technique, this work contributes to improving the determination of absolute dose to water from graphite calorimetry in carbon-ion beams.

Highlights

The quantity of interest in radiation therapy dosimetry is absorbed dose to water
A formalism was developed and adapted to derive absorbed dose to water, using a graphite calorimeter in carbon-ion beams. This procedure has the advantage of involving measurements being done independently from ionization chamber perturbation factors caused by the use of different phantom materials
Fluence corrections, needed for the conversion of dose to graphite from a graphite calorimeter to dose to water, were measured experimentally in a high-energy carbon-ion beam and compared with numerical simulations

Summary

Introduction

The quantity of interest in radiation therapy dosimetry is absorbed dose to water. The determination of this quantity must be accurate, reproducible and traceable in order to assure tumour control and mitigate normal tissue complications. Calorimeters determine absorbed dose by measuring the temperature rise in the medium as a result of radiation. A conversion procedure is required to determine absorbed dose to water. The conversion requires (i) the stopping-power ratio between water and graphite and (ii) the fluence correction factor, , that corrects for the difference between the fluence distributions at equivalent depths in the two materials [Lühr et al., 2011, Palmans et al, 2013, Rossomme et al, 2013]. The necessity of stems from the differences between the nonelastic nuclear interactions cross sections in oxygen and other nuclei.

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