title: Electron wedges for radiation therapy.
abstract: OBJECTIVE
  Brain tumors can be advantageously treated with electron over photon radiation, by exploiting the rapid fall-off in dose with depth. This advantage could be further enhanced by utilizing multiple electron beams. However, in some beam configurations, wedged dose profiles would be necessary for the dose uniformity. Unlike photons, shaped pieces of material placed in electron beam severely degrade the energy, give additional scattering and, therefore, are suboptimal. The purpose of this study was to create wedged electron fields, using intensity modulation. The combination of electron wedges enables a more uniform coverage of brain tumors with a reduced dose to normal tissue.
  
  
  METHODS
  Intensity modulation was performed for 10 to 50 MeV electrons using a narrow scanning elementary beam of a racetrack Microtron accelerator, delivering radiation pulses with coordinates and intensities prescribed by a custom scan matrix. Dispensing more pulses (or longer pulses) within the field to increase the local dose, one can sharpen the penumbra at depth and generate wedged dose distributions of arbitrary angle as well as many other desired profiles. We modulated the electron beams, measured dose distributions using film in an anthropomorphic phantom, and compared the results with conventional techniques.
  
  
  RESULTS
  Intensity modulation of electron beams decreases the 50-90% penumbra at depth by 40% and increases the flatness by 80%. Wedged profiles at depth can be created for any angle up to about 70 degrees, depending on the beam energy. Multiple modulated electron beams give smaller 20-70% but larger 70-100% isodose regions than photon beams.
  
  
  CONCLUSIONS
  Electron beams can improve dose distributions in brain compared to the same number of photon beams, reducing the 20-70% isodoses region in normal tissue by 30%. Intensity modulation significantly improves the dose distribution from combined electron beams providing a sharper penumbra, better conformity, and reduced margin.
authors.affiliation: Department of Medical Physics, Memorial Sloan-Kettering Cancer Center, New York, NY 10021, USA.
authors.forename: E P
authors.initials: EP
authors.lastname: Lief
authors.forename: Y C
authors.initials: YC
authors.lastname: Lo
authors.forename: J L
authors.initials: JL
authors.lastname: Humm
mesh_terms: D001932:Brain Neoplasms / Q000532:radiotherapy*; D004583:Electrons / Q000627:therapeutic use*; D006801:Humans; D010315:Particle Accelerators; D019047:Phantoms, Imaging; D017785:Photons / Q000627:therapeutic use; D055585:Physical Phenomena; D010825:Physics; D012542:Scattering, Radiation