Medicine:Tissue-to-air ratio

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Tissue-to-air ratio (TAR) is a term used in radiotherapy treatment planning to help calculate absorbed dose to water in conditions other than those directly measured.

Definition

The TAR at a point in a water phantom irradiated by a photon beam is taken to be the ratio of the total absorbed dose at that point to the absorbed dose at the same point in a minimal-scatter phantom with just-sufficient build-up.[1][2]

Tissue-air ratio is defined as the ratio of the dose to water at a given depth to the dose in air measured with a buildup cap:

[math]\displaystyle{ TAR={{D(f,z)} \over {D(f,0)}} }[/math]

where D(f,z) is the dose at a given depth z and distance focus-detector f; and D(f,0) is the dose in air (z=0).

  • TAR increases with increasing beam energy because higher energy radiation is more penetrating
  • TAR decreases with depth because of attenuation
  • TAR increases with field size due to increased scatter contribution


Measurements for each are taken using an ion chamber for identical source to detector distances and field sizes.[3]

See also

References

  1. ↑ Johns H. E. and Cunningham J. R. (1983). The Physics of Radiology. Charles C. Thomas Publ.
  2. ↑ Hendee W., Ibbott G. and Hendee E. (2005). Radiation Therapy Physics. Wiley-Liss Publ. ISBN:0-471-39493-9.
  3. ↑ Faiz M. Khan. (2010) "The Physics of Radiation Therapy " Lippencott, Wilkins and Williams Publ.



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