Tissue tolerance describes the dose of radiation an organ can receive before it fails. It has expanded to include concepts such as serial/parallel functional sub units, volume effects and is one reason for using dose volume histograms. Tolerance values are usually quoted for conventional fractionation using photons or electrons.
The original method of providing tolerance doses was to quote the 5% risk of a particular outcome at 5 years - the TD5/5 value. The dose leading to a 50% complication rate at 5 years is TD50/5. other figures may be quoted but these two are the most common. It is also important to note that TD5/5 values are dependent on radiation quality and fractionation of dose.
The identification of volume as an important concept in organs with parallel arrangement of FSUs allowed tolerance doses to be specified for partial irradiation of organs. In general, organs of this type (liver, lung, kidney) tolerate a higher dose if they are only partially irradiated. Another way of giving a tolerance is the 'mean dose' to the entire organ.
In organs with serial arrangement of FSUs, point doses are mote important than volume irradiated. This is because loss of a single FSU leads to significant loss of function.