Backscatter towards the monitor ion chamber in high-energy photon and electron beams: charge integration versus Monte Carlo simulation

Abstract

In some linear accelerators, the charge collected by the monitor ion chamber is partly caused by backscattered particles from accelerator components downstream from the chamber. This influences the output of the accelerator and also has to be taken into account when output factors are derived from Monte Carlo simulations. In this work, the contribution of backscattered particles to the monitor ion chamber response of a Varian 2100C linac was determined for photon beams (6, 10 MV) and for electron beams (6, 12, 20 MeV). The experimental procedure consisted of charge integration from the target in a photon beam or from the monitor ion chamber in electron beams. The Monte Carlo code EGS4/BEAM was used to study the contribution of backscattered particles to the dose deposited in the monitor ion chamber. Both measurements and simulations showed a linear increase in backscatter fraction with decreasing field size for photon and electron beams. For 6 MV and 10 MV photon beams, a 2-3% increase in backscatter was obtained for a 0.5 x 0.5 cm2 field compared to a 40 x 40 cm2 field. The results for the 6 MV beam were slightly higher than for the 10 MV beam. For electron beams (6, 12, 20 MeV), an increase of similar magnitude was obtained from measurements and simulations for 6 MeV electrons. For higher energy electron beams a smaller increase in backscatter fraction was found. The problem is of less importance for electron beams since large variations of field size for a single electron energy usually do not occur.

Jaw position uncertainty and adjacent fields in breast cancer radiotherapy

“…A linear dependence is considered between the backscattered dose to the monitor chamber and the field size as suggested by Verhaegen et al (A2) It is assumed that the effect of the components located below the upper Y jaw, namely the lower X jaw and the MLC, is negligible. This assumption is consistent with the results reported on the dominating effect of the upper Y jaw on the backscatter compared to that of the lower X jaw 16 , 17 . The BSCF is therefore only dependent on the field length in the Y direction (FSy) and is given by: $BSCFfalse(FSyfalse)=\frac{a+b*10}{a+b*FSy}$ …”

“…A backscatter correction factor (BSCF) is used that relates the amount of backscattered dose for a certain field to the calibration field size. A linear dependence is considered between the backscattered dose to the monitor chamber and the field size as suggested by Verhaegen et al (A2) It is assumed that the effect of the components located below the upper Y jaw, namely the lower X jaw and the MLC, is negligible. This assumption is consistent with the results reported on the dominating effect of the upper Y jaw on the backscatter compared to that of the lower X jaw 16 , 17 .…”

MMCTP: a radiotherapy research environment for Monte Carlo and patient-specific treatment planning

“…The code, originally developed by Nelson and Jenkins in 1978 for applications in high-energy physics, was modified in 1985 to EGS4 [34] for improved accuracy in low energy radiation transport down to 1 ke V. Radiotherapy treatments use low energy radiation (0.5 keV-20 MeV). Thus, EGS4 is ideal for radiotherapy.…”

A Study on Effective Source-Skin Distance using Phantom in Electron Beam Therapy

“…*Corresponding author: Tel: +82-70-8680-5900 Fax: +82-2-3453-6618, e-mail: 8452404@hanmail.net ISSN (Print) 1226-1750 ISSN (Online) 2233-6656 depending on the scattering foil, the opening of the collimator, the structure of the electron cone, the size and shape of the irradiated field, and the density and thickness of the cut-out block or organizational composition material. These physical properties can occur due to the multiple scattering of the incident electrons, the lost energy in leading, or the displacement of the original motion direction while interacting with the atomic nucleus of the material or orbital electron when the electron beam passes through the material [5,6]. In addition, the incident angle of radiation can be affected by whether or not the filter is used [7,8].…”

“…The equipment used included a 6-20 MeV electron beam from a linear accelerator, and the distance was measured by a ionization chamber targeting the solid phantom. The measurement method for the effective source-skin distance according to the size of the radiation field changes the source-skin distance (100, 105, 110, 115 cm) for the electron beam energy (6,9,12, 16, 20 MeV). The effective source-skin distance was measured using the method proposed by Faiz Khan, measuring the dose according to each radiation field (6 × 6, 10 × 10, 15 × 15, 20 × 20 cm 2 ) at the maximum dose depth (1.3, 2.05, 2.7, 2.45, 1.8 cm, respectively) of each energy.…”