Tissue Air Ratio:
Tissue-air ratio (TAR) was first introduced by Johns in 1953 and was originally called the “tumor-air ratio.” At that time, this quantity was intended specifically for rotation therapy calculations. In rotation therapy, the radiation source moves in a circle around the axis of rotation, which is usually placed in the tumor. Although the SSD may vary depending on the shape of the surface contour, the source-axis distance remains constant.
Tissue Air Ratio (TAR) is the ratio of the absorbed dose at a given depth in tissue to the absorbed dose at the same point in air.
TAR = Dose in Tissue/Dose in Air
TAR is dependent on the following factors,
1. TAR increases with the Beam energy
2. TAR increases with the Field size
3. TAR decreases with the Depth
But independent of source to surface distance.
Figure (a) shows the illustration of TAR setup
Tissue Phantom Ratio:
The TPR is defined as the ratio of the dose at a given point in phantom to the dose at the same point at a fixed reference depth, usually 5 cm.
Tissue maximum ratio:
The TAR was defined for low X-ray energies at a time when high energy beams were rare. The TMR is a variation of the TAR that makes it suitable for use at high energies. TAR and TMR are alike, but not identical, concepts. The TAR uses the dose to tissue in air under conditions of minimum tissue absorption and scatter as the reference denominator. Because, for high energy photons, the secondary electron ranges becomes large, the tissue mass in air satisfying the condition of electronic equilibrium at its center also becomes large, measuring several centimeters in diameter. Such a large mass can also contribute a considerable amount of scattered photon fluence.
TMR is a special case of TPR and may be defined as the ratio of the dose at a given point in phantom to the dose at the same point at the reference depth of maximum dose.