2.3.2 - Filtration Of Photon Beams

Filtration of Kilovoltage Beams

This is the topic of a question in the old CA05

Filtration has two major roles in kilovoltage radiotherapy:

  • Filtration hardens the beam, attenuating low energy photons and shifting the spectrum towards higher energy photons. The low energy photons are not needed, as they would simply excess dose to the most superficial parts of the skin.
  • Filtration smooths the beam spectrum, particularly with relation to the characteristic radiation produced in the target. This prevents excessive photons with unwanted energies from contributing to the dose.

Thoraeus Filter

A Thoraeus filter contains three layers designed to harden and smooth the spectrum of higher energy kilovoltage beams generated by electrons incident on a tungsten target. Tungsten, in addition to providing a broad spectrum of bremsstrahlung radiation, produces characteristic radiation in the range of 58 - 69 keV.

This is due to electrons colliding with orbital electrons in the tungsten atoms. The K-shell binding energies of tungsten vary from 58 - 69 keV, and when an electron is removed from this shell a higher orbital electron will 'fall' in to replace it, giving rise to x-rays with that specific energy - characteristic radiation. L-shell and M-shell electrons are also ionised through electron collisions, but as their binding energies are much lower they are effectively filtered out by x-ray tube itself.

An example of an unfiltered 200 keV beam is below.

This beam is not ideal, as it has a large number of low energy photons with peaks between 58-69 keV. These photons will contribute significantly to surface dose at the expense of dose to deeper structures.

Tin, which has a K-shell binding energy of 29.2 keV, readily absorbs photons of energies 30 - 70 keV through photoelectric interactions. This includes the characteristic radiation produced by tungsten. By adding a tin filter, the spectrum changes:

Unfortunately, the tin filter generates characteristic radiation of its own, due to the K-shell binding energies of 29.2 keV. This leads to a peak of energy at this level.

To compensate for the characteristic radiation of tin, a metal with a lower atomic number, copper is added to the filter. The K-edge of copper is 9 keV, and therefore radiations in the range of 9 - 30 keV are strongly absorbed through photoelectric interactions in this layer.

The addition of a thin aluminium filter beyond the copper filter absorbs the very low energy characteristic photons generated by electron interactions in the copper.

The thickness of a thoraeus filter depends on the voltage of the x-ray tube and the desired energy spectrum. Of the three filters, the copper and aluminium filters remain an identical thickness. The tin filter has the greatest effect on overall beam attenuation and varies from 0.2 - 0.6 mm. Thicker filters mean the beam is hardened to a greater extent, but at the expense of a lower dose rate.

Filtration of Megavoltage Photon Beams

Photons produced in a linear accelerator also have a broad energy spectrum. However, these photons pass through the target (instead of deflected laterally), providing significant attenuation of low energy photons. The beam also passes through a flattening filter to change the beam profile at depth. This filter also attenuates a significant amount of the undesired characteristic and lower energy photons from the beam.