TAR (tissue air ratio)

TAR concept is usually not used with the accelerator photon beams because the dose in free air is not well defined due to conceptual difficulties.

TAR concept involves equilibrium dose in air ("free space dose") as defined in Khan (2003), which becomes more and more ambiguous and also difficult to evaluate.

For further study (see Attix 1999) Note look for where the difficulty in the evaluation of this quantity for Co-60 beams is discussed. So the TMR concept replaces this quantity by a phantom defined quantity that is less ambiguous.

TAR can be measured directly or derived from the PDD data.

TAR at dmax and PSF (peak scatter factor) are the same quantity.

In general TAR decreases with an increase in depth.

At shallow depths TAR is greater for low beam qualities (kV x-ray region) because of increased side scatter.

At larger depths TAR is larger for high energy photons compared to low beam energies, due to increased penetration.

TAR increases with an increase in field size.

TAR decreases with increasing field asymmetry. This occurrs due to decrease in side scatter.