Monte Carlo techniques for scattering foil design and dosimetry in total skin electron irradiations.

Academic Article

Abstract

  • Total skin electron irradiation (TSEI) with single fields requires large electron beams having good dose uniformity, dmax at the skin surface, and low bremsstrahlung contamination. To satisfy these requirements, energy degraders and scattering foils have to be specially designed for the given accelerator and treatment room. We used Monte Carlo (MC) techniques based on EGS4 user codes (BEAM, DOSXYZ, and DOSRZ) as a guide in the beam modifier design of our TSEI system. The dosimetric characteristics at the treatment distance of 382cm source-to-surface distance (SSD) were verified experimentally using a linear array of 47 ion chambers, a parallel plate chamber, and radiochromic film. By matching MC simulations to standard beam measurements at 100cm SSD, the parameters of the electron beam incident on the vacuum window were determined. Best match was achieved assuming that electrons were monoenergetic at 6.72MeV, parallel, and distributed in a circular pattern having a Gaussian radial distribution with full width at half maximum=0.13cm. These parameters were then used to simulate our TSEI unit with various scattering foils. Two of the foils were fabricated and experimentally evaluated by measuring off-axis dose uniformity and depth doses. A scattering foil, consisting of a 12×12cm2 aluminum plate of 0.6cm thickness and placed at isocenter perpendicular to the beam direction, was considered optimal. It produced a beam that was flat within ±3% up to 60cm off-axis distance, dropped by not more than 8% at a distance of 90cm, and had an x-ray contamination of <3%. For stationary beams, MC-computed dmax, Rp, and R50 agreed with measurements within 0.5mm. The MC-predicted surface dose of the rotating phantom was 41% of the dose rate at dmax of the stationary phantom, whereas our calculations based on a semiempirical formula in the literature yielded a drop to 42%. The MC simulations provided the guideline of beam modifier design for TSEI and estimated the dosimetric performance for stationary and rotational irradiations.

    • Authors

    Published In

  • Medical Physics  Journal

    • Keywords

  • Ancillary equipment, Collimators, Diseases, Dosimetry, Dosimetry/exposure assessment, Electron beams, Electron radiation effects, Electron scattering, Gaussian distribution, Ionization chambers, Monte Carlo, Monte Carlo methods, Photons, Radiation therapy equipment, Skin, Wedges and compensators, X-ray scattering, aluminium, bremsstrahlung, diseases, dosimetry, electron beams, foils, phantoms, radiation therapy, scattering foil, skin, total skin electron irradiation

    • Digital Object Identifier (doi)

    Pubmed Id

  • 7649915

    • Author List

  • Ye S-J; Pareek PN; Spencer S; Duan J; Brezovich IA

    • Start Page

  • 1460

    • End Page

  • 1468

    • Volume

  • 32

    • Issue

  • 6Part1