Tissue-to-air ratio (TAR) is a term used in
radiotherapy
Radiation therapy or radiotherapy, often abbreviated RT, RTx, or XRT, is a therapy using ionizing radiation, generally provided as part of cancer treatment to control or kill malignant cells and normally delivered by a linear accelerator. Radia ...
treatment planning
In radiotherapy, radiation treatment planning (RTP) is the process in which a team consisting of radiation oncologists, radiation therapist, medical physicists and medical dosimetrists plan the appropriate external beam radiotherapy or internal b ...
to help calculate
absorbed dose
Absorbed dose is a dose quantity which is the measure of the energy deposited in matter by ionizing radiation per unit mass. Absorbed dose is used in the calculation of dose uptake in living tissue in both radiation protection (reduction of harmf ...
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.
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:
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
The ionization chamber is the simplest type of gas-filled radiation detector, and is widely used for the detection and measurement of certain types of ionizing radiation, including X-rays, gamma rays, and beta particles. Conventionally, the term ...
for identical source to detector distances and field sizes.
[Faiz M. Khan. (2010) "The Physics of Radiation Therapy " Lippencott, Wilkins and Williams Publ.]
See also
*
Dosimetry Radiation dosimetry in the fields of health physics and radiation protection is the measurement, calculation and assessment of the ionizing radiation dose absorbed by an object, usually the human body. This applies both internally, due to ingested o ...
*
Percentage depth dose curve In radiotherapy, a percentage depth dose curve (PDD) (sometimes ''percent'' depth dose curve) relates the absorbed dose deposited by a radiation beam into a medium as it varies with depth along the axis of the beam. The dose values are divided by t ...
References
{{DEFAULTSORT:Tissue To Air Ratio
Radiation therapy
Medical physics