Backscattering in electron beam therapy for energies between 3 and 35 MeV

Abstract

Whenever a heterogeneity is present in an electron beam treatment field during radiotherapy, there is the possibility of tissue overdosage at the tissue-heterogeneity interface due to electrons backscattered from the heterogeneity. Measurements of this effect were made in a polystyrene phantom using a purpose-built thin-window parallel-plane ionisation chamber. Materials of various atomic numbers were used as scatterers and the investigations were made over a wide range of electron energies. Electron backscatter factor (EBF), defined as the ratio fo dose at the interface surface with and without the scatterer present, was found to increase with increasing atomic number and decrease with increasing beam energy. Both of these relationships were found to be non-linear. The EBF dependence on the scatterer thickness was also investigated. All data in this work were expressed in relation to the beam energy incident on the scatterer in preference to the nominal beam energy set on the accelerator. This approach enables the dose enhancement at an interface to be predicted from a knowledge of the heterogeneity (atomic number and thickness,), its depth in tissue and the beam energy being used for treatment. The results of this work were compared with the published data and an explanation is offered to account for the difference.

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

“…The EBF is defined as the ratio of dose at the Pb interface to the dose at the same point in the absence of Pb (4) . It is usually quantified on the central axis of the electron beam.…”

“…Of particular significance is the work of Klevenhagen et al (4) who comprehensively investigated EBF values for several materials and depths using a purpose built ionization chamber. Measurements were made using a range of energies from 3 to 35 MeV produced by a 20 MeV linac (SL 75/20, Philips, UK) and a 35 MeV betatron (BBC betatron Asklepitron 35; Brown Boveri Corp., Zurich, Switzerland).…”

“…However, evidence in the literature 4 , 5 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 suggests that there may be a different relationship for EBF and EBI in the energy region below 4 MeV. This corresponds to the energy of electrons often present at the Pb interface during treatments utilizing 6 and 9 MeV electron beams.…”

Backscattered Dose Perturbation Effects at Metallic Interfaces Irradiated by High-Energy X- and Gamma-Ray Therapeutic Beams

“…The significant increase in dose observed is mainly due to the scatter of secondary electrons across the interface, where a nonequilibrium zone of secondary electron fluence exists during photon irradiation. The dose perturbation effects due to forward scattering of photons have been thoroughly investigated by many authors in past [5, 26-28, 31, 38-40].The presence of metallic inhomogeneity in the electron beam results in an enhancement of dose at the tissue-metal interface due to the backscattered electrons [12,18,19,25,29,32,37]. The dose enhancement at orthovoltage X-rays and cobalt-60 ( 60 Co) gamma rays due to the backscattered secondary electrons in cell irradiation procedures was reported by Murthy & Lakshmanan [23].…”

Dosimetric Impact of Surgical Clips in Electron Beam Treatment of Breast Cancer

“…1,2 The surgical clips are made of steel or titanium, which could produce significant backscatter dose in electron beam therapy (EBT) as reported in various investigations. [3][4][5][6][7][8][9][10][11] Because electron beams have limited range, it is expected that these clips could produce dose reduction based on materials and electron energies. 5,12,13 To see the dosimetric effect of the surgical clips, a single Johnson & Johnson (Langhorne, PA) Ethicon Endo-Surgery, Inc. Ligaclip model LT100 cartridge was used.…”