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

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

Estimation of backscatter from internal shielding in electron beam radiotherapy using Monte Carlo simulations (EGSnrc) and Gafchromic film measurements

“…In addition, it should be noted that Weaver et al performed their measurements in a polystyrene phantom, whereas ours were performed in water. Various authors[4111415] have measured and MC simulated the EBF at various energies ranging from 4 to 33 MeV and demonstrated the large variation in the estimated EBF values. However, they all have noted the same trend of decreasing EBF with increasing energy of the electron beam.…”

“…De Vries and Marsh have simulated Siemens Artiste electron beam and calculated EBF values. [14] They also derived an empirical equation to calculate EBF from lead shielding for their local linac.…”

Internal lead shielding for clinical electron treatments

“…deVries and Marsh have performed MC calculations of the EBF as a function of the electron energy at the surface of the lead 3 . The mean energy of the electrons at the lead surface ranged from 0.2 to 14.0 MeV.…”

“…This is based on more recent Monte Carlo (MC) calculations (see below). 3 Electron backscatter at a tissue/lead interface has been extensively studied both by measurement and MC simulation (see Perez-Calatayud et al and references therein). 4 What has been missing is an easy, accurate, self -consistent method for putting this information to use in clinical applications.…”

“…In addition to this, the formula for the enhancement due to the backscatter (Equation ) is claimed to be in significant error for energies below 5 MeV at the lead surface and invalid for energies less than 3 MeV. This is based on more recent Monte Carlo (MC) calculations (see below) 3 …”