2.3 - Properties Of Photon Beams

A photon beam consists of numerous photons which pass from the target, through beam modifying devices, and into the patient or phantom. Their usefulness depends on the energy and the volume to be treated.

Properties of Megavoltage Beams

Megavoltage beams are much more commonly used due to their broad applications. In general, they show:

  • A low surface dose
  • A build up region
  • A depth of maximum dose, either zmax or dmax depending on the textbook
  • A gradual loss of dose with depth beyond zmax
  • A sharp physical penumbra at the beam edge
  • Inhomogeneity causes increased or decreased attenuation based on the density of the material, rather than the atomic number

Higher energy photon beams (in the clinically used range) show reduced attenuation compared with lower energy photon beams; however the electrons liberated by these photon beams have increased range, leading to a slight increase in the width of the penumbra. This is particularly important in the lung, where the penumbra is up to three times wider, and with small field sizes where the penumbra contributes a significant portion of the beam.

Properties of Kilovoltage Beams

  • Maximum dose occurs at the surface
  • A variable rate of dose fall off, depending on beam energy (sharper for lower energy beams)
  • Very sharp penumbra at the surface
  • High atomic number inhomogeneity causes markedly increased attenuation

Kilovoltage beams are typically only useful for superficial lesions, as they deposit 100% of their dose on the skin surface; this limits their application for deeper treatments. For energies under 150 keV, treatments are limited to lesions of < 0.5 cm thickness due to rapid dose fall off. Orthovoltage treatments are usually limited to lesions of < 2 cm in thickness.

Additional Topics

There are some special properties of photon beams that should be discussed separately due to their importance: