Electron Therapy Overview

 

Resources:

 

  1. The Physics of Radiation Therapy (Khan)
  2. AAPM TG-25: Clinical Electron Dosimetry (1991)
  3. AAPM TG-25: Supplement (2009)

 

Electron Interactions:

 

  1. Electrons can undergo the following interactions:
  1. Elastic collisions with atomic electrons and nuclei.
  2. Inelastic collisions with atomic electrons (ionization and excitation).
  3. Inelastic collisions with atomic nuclei (bremsstrahlung).
  1. In low Z absorbers, electrons primarily lose energy through ionization and excitation but in high Z materials bremsstrahlung production becomes significant.
  1. Bremsstrahlung production is approximately proportional to Z* E.
  1. Stopping power (S) ratios are used to relate the energy loss of electrons per unit path length.
  1. They are frequently used normalized to the density of the material (image001) to become (image003).
  2. Restricted mass stopping powers (image005) are used to calculate doses to a medium with the restriction that only interactions below a certain cutoff energy are used in calculating dose.  The reasoning is that electrons created with higher energy travel away from the site and do not contribute to dose locally.

 

LINACs and Electron Energy:

 

  1. When exiting the bend magnet, electrons have a very narrow energy window with a most probable energy (average) equal to that specified by the selected energy (e.g. 6, 9, 12 MeV).
  2. By the time the electrons reach the surface of a patient, they have undergone scattering in the foils, collimation system, and air.  This broadens the energy window.
  3. At depth in the patient, due to the large amount of scattering occurring, the energy window is significantly broadened.
  4. The energy of an electron beam is usually determined through the use of the practical range, RP, and the Markus Range equation (ICRU 21):

image007

  1. It has also been shown that the mean energy at the patient’s surface can be related to R50 by:

image009

  1. The average energy at depth (z) can then be approximated as:

image011

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