A semi-empirical model of in-field and out-of-field bremsstrahlung dose distribution in high energy electrons beams used in external radiotherapy

Abstract

s / Physica Medica 30 (2014) e123ee145 e132 A SEMI-EMPIRICAL MODEL OF IN-FIELD AND OUT-OF-FIELD BREMSSTRAHLUNG DOSE DISTRIBUTION IN HIGH ENERGY ELECTRONS BEAMS USED IN EXTERNAL RADIOTHERAPY M. Mohamad Alabdoaburas , J. Vu Bezin , J.P. Mege , A. Veres , F. Devathaire , D. Lefkopoulos , I. Diallo b,c University Paris-Sud, Orsay, France; b Inserm, CESP Centre for Research in Epidemiology and Population Health, U1018, Radiation Epidemiology Team, Orsay, France; Gustave Roussy, Paris, France; University Paris-Sud, Orsay, France; e equal-estro, Paris, France Introduction: A better understanding of the dose distribution effects in normal tissues becomes essential to know how best to exploit modern technologies in radiotherapy. Bremsstrahlung dose component became important when scattering dual-foils and scrapers applicator are used compared to scanned electron beam and cone applicator. Several works have been reported for analyzing bremmstrahlung dose component inside irradiation field. The aim of this work to develop a semi-empirical model for estimating the bremsstrahlung dose distribution at any point in the patient in-field and out-of-field. Materials and methods: Bremsstrahlung dose component in electrons beams arises from four main sources: scattering dual-foils, jaws, applicator's scrapers and insert of cerrobend, we neglected bremsstrahlung generating in the patient. Our model consists in twomain parts: in the first part, we calculated the angular energetic fluence distribution for every bremsstrahlung photon source, by using an existing model based onmultiscattering theorem. In the second part, we calculated the relative number of irradiated pixel by primary electrons beam in every bremsstrahlung photon source. All programmings were executed by Matlab. All detailed information about geometry, material composition, and dimensions of each component have been provided by manufacturer. Most of data about the electrons interactions with matters based on ICRU-1984 data. Measurements of bremsstrahlung dose component are achieved via thermoluminescent dosimeters powder TLD-700, from 0cm to 70cm from the beam central axis, at depth of 10cm in water phantom for Varian 2300C/D operated at 6, 12, 9 and 18MeV. Results: We evidenced that out-of-field bremsstrahlung dose issuing essentially from applicator's scrapers, while out-of-field bremsstrahlung dose generating in scattering dual-foils is neglected. In-field bremsstrahlung dose comes essentially from scattering dual-foils which represents 70%-80% of total in-field bremsstrahlung dose. Our modelling results have shown very good agreement between calculated and measured values in-field and out-of-field. The difference between calculated and measured values is less than 10%, which stills small in absolute term. Conclusion/perspective: Our approach can be used to provide good representation of bremsstrahlung dose distributions in all healthy tissues for any applicator size and type and for any electron beam energy used in high-energy electron beam therapy. The calculating time is relatively short. To take account total peripheral dose out-of-field, an additional model is under developing to estimate out-of-field scattered electrons dose component.