Monte Carlo simulation and measurement of radiation leakage from applicators used in external electron radiotherapy

Abstract

External electron radiotherapy is performed using a cone or applicator to collimate the beam. However, because of a trade-off between collimation and scattering/bremsstrahlung X-ray production, applicators generate a small amount of secondary radiation (leakage). We investigate the peripheral dose outside the radiation field of a Varian-type applicator. The dose and fluence outside the radiation field were analyzed in a detailed Monte Carlo simulation. The differences between the calculation results and data measured in a water phantom in an ionization chamber were less than ±1% in regions more than 3 mm below the surface of the phantom and at the depth of dose maximum. The calculated fluence was analyzed inside and outside the radiation field on a plane just above the water phantom surface. Changing the electron energy affected the off-axis fluence distribution outside the radiation field; however, the size of the applicator had little effect on this distribution. For each energy, the distributions outside the radiation field were similar to the dose distribution at shallow depths in the water phantom. The effect of secondary electrons generation by photon transmission through the alloy making up the lowest scraper was largest in the region from the field edge to directly below the cutout and at higher beam energies. The results of the Monte Carlo simulation confirm that the peripheral dose outside the field is significantly affected by radiation scattered or transmitted from the applicator, and the effect increases with the electron energy.

Experimental assessment of out‐of‐field dose components in high energy electron beams used in external beam radiotherapy

“…A recent investigation with Monte Carlo has been reported by Shimozato et al (13) They calculated the fluence distributions and the dose distributions outside the applicator for a Varian‐type applicator, Clinac 2100CD. They found that the out‐of‐field dose was about 1% of maximal central axis dose for 16 MeV electron beam.…”

“…However, Shimozato et al (13) have reported that the bremsstrahlung spectrum outside Varian applicators have a peak around 250 KeV, 350 KeV, 500 KeV, 750 KeV, and 1 MeV for 4 MeV, 6 MeV, 9 MeV, 12 MeV, and 16 MeV electron beams, respectively. So we used three different dosimeter types to compare their responses.…”

“…The two main components of out‐of‐field doses outside the applicator are bremsstrahlung photon contamination and scattered electrons 11 , 13 . The bremsstrahlung photons can be produced in different structures of the accelerator head and in the patient irradiated volume (14) .…”

“…Another detailed experimental study has been performed on an applicator type EA200 on the Siemens Primus (12) . A recent investigation, comparing experimental data and Monte Carlo simulations, has been reported for a Varian‐type applicator (13) . In spite of a fair number of studies published on the out‐of‐field dose from high‐energy electron beams, the out‐of‐field doses from electron beams from the Digital Electron Variable Applicator (DEVA) on the Siemens Primus have not yet been studied.…”

Out‐of‐field doses and neutron dose equivalents for electron beams from modern Varian and Elekta linear accelerators

“…Dose leakage from electron therapy has been the subject of limited investigation 1 , 2 , 3 , 4 , 5 , 6 A better understanding of the magnitude of out‐of‐field dose associated with this treatment modality is needed, particularly for newer models of linear accelerator. The importance of studying this issue was made clear by Yeboah et al, (7) who showed very high out‐of‐field doses when utilizing electron therapy, often largely exceeding out‐of‐field doses from comparable photon treatment fields.…”