Optimum field size and choice of isodose lines in electron beam treatment

Indra J. Das, Chee W. Cheng, Glenn A. Healey

Research output: Contribution to journalArticle

17 Scopus citations


Purpose: A method is provided for the optimum filed size and the choice of isodose line for the dose prescription in electron beam therapy. Methods and Materials: Electron beam dose uniformity was defined in terms of target coverage factor (TCF) which is an index of dose coverage of a given treatment volume. The TCF was studied with respect to the field size, the beam energy, and the isodose level for prescription from the measured data for various accelerators. The effects of the TCF on air gap between electron applicator/cone and the surface was investigated. Electron beams from scattering foil and scaned beam units were analyzed for the target coverage. Results: A mathematical method is provided to optimize a filed size for target coverage by a given isodose line in terms of TCF which is strongly dependent on the type of accelerator and the design of the collimator. For a given type of collimating system, the TCF does not depend on the type of electron beam production (scattering foil or swept scanned beam). Selection of isodose line for dose prescription is very critical for the value of the TCF and the dose coverage. The TCF is inversely proportional to the isodose value selected for the treatment and nearly linear with field size and beam energy. Air gap applicator and the surface reduces the dose uniformity. Tertiary collimator moderately improves the lateral coverage for high energy beams. Conclusions: To adequately cover the target volume in electron beam treatment, lateral and depth coverage should be considered. The coverage at depth is strongly dependent on the choice of isodose line or beam normalization. If the dose prescription is at dmax (i.e., the 100% isodose line is selected), the choice of beam energy is not critical for depth coverage since dmax is nearly independent of energy for smaller fields. The 100% isodose line should not be chosen for treatment because of the significant constriction of this isodose line and inadequate coverage at depth. For a higher TCF, a minimum air gap between the cone to the surface of the patient is desired. If such is not possible, then a tertiary collimator at the skin is required. Whenever, a tertiary collimator is used, it is advised to icrease the collimator field size by a factor of 1.4.

Original languageEnglish (US)
Pages (from-to)157-163
Number of pages7
JournalInternational journal of radiation oncology, biology, physics
Issue number1
StatePublished - Jan 1 1995


  • Dose prescription
  • Electron beam
  • Normalization
  • Target coverage
  • Uniformity index

ASJC Scopus subject areas

  • Radiation
  • Oncology
  • Radiology Nuclear Medicine and imaging
  • Cancer Research

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