Evaluation of backscatter dose from internal lead shielding in clinical electron beams using EGSnrc Monte Carlo simulations

Rowen J De Vries,Steven Marsh

doi:10.1120/jacmp.v16i6.5563

Abstract

Internal lead shielding is utilized during superficial electron beam treatments of the head and neck, such as lip carcinoma. Methods for predicting backscattered dose include the use of empirical equations or performing physical measurements. The accuracy of these empirical equations required verification for the local electron beams. In this study, a Monte Carlo model of a Siemens Artiste linac was developed for 6, 9, 12, and 15 MeV electron beams using the EGSnrc MC package. The model was verified against physical measurements to an accuracy of better than 2% and 2 mm. Multiple MC simulations of lead interfaces at different depths, corresponding to mean electron energies in the range of 0.2–14 MeV at the interfaces, were performed to calculate electron backscatter values. The simulated electron backscatter was compared with current empirical equations to ascertain their accuracy. The major finding was that the current set of backscatter equations does not accurately predict electron backscatter, particularly in the lower energies region. A new equation was derived which enables estimation of electron backscatter factor at any depth upstream from the interface for the local treatment machines. The derived equation agreed to within 1.5% of the MC simulated electron backscatter at the lead interface and upstream positions. Verification of the equation was performed by comparing to measurements of the electron backscatter factor using Gafchromic EBT2 film. These results show a mean value of 0.997±0.022 to 1σ of the predicted values of electron backscatter. The new empirical equation presented can accurately estimate electron backscatter factor from lead shielding in the range of 0.2 to 14 MeV for the local linacs.PACS numbers: 87.53.Bn, 87.55.K‐, 87.56.bd

Highlights

Electron beam therapy (EBT) can be the preferred method of treatment for several superficial treatment sites of the head and neck
The aim of this work is to use Monte Carlo (MC) method to evaluate the accuracy of the empirical electron backscatter factor (EBF) and Electron backscatter intensity (EBI) equation for the local electron beams
The MC percentage depth dose (PDD) curves agreed with the measured PPD curves to within 2% from the surface to depths of Rp

Summary

Introduction

Electron beam therapy (EBT) can be the preferred method of treatment for several superficial treatment sites of the head and neck. One of the benefits of EBT is the sharp dose gradient beyond the therapeutic depth sparing the deeper healthy tissue. Dose to deep tissues can be further reduced in certain situations by introducing shielding. The thickness of this Pb layer is optimized to the mean electron energy at the Pb/tissue interface to achieve the required dose reduction.[1]. 140 de Vries et al.: Electron backscatter from lead shielding electrons are known as backscattered electrons. The backscattered electrons are absorbed into the upstream tissue and this impact on the dose distribution must be considered. Inaccuracies in this upstream dose enhancement can cause unaccounted intermediate or late complications for the patient.[3]

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