R8.4: Functional Subunits (FSU)

Functional sub-units are a concept used to model the radiation response of normal tissues. A functional sub-unit is a compartment of an organ that performs part of the organs function. FSUs may be arranged in serial, parallel, or a combination of both.
Serial FSU arrangement involves each FSU in a line with the others. If a single FSU fails, then the organ experiences significant deleterious effects. A typical serial FSU would be the spinal cord or gastrointestinal tract. Organs with predominately serial FSU arrangement are susceptible to high point doses.
Parallel FSU arrangement occurs when each FSU is able to function independently of the others. Loss of a single FSU leads to a slight decrease in function of the organ. Parallel FSU organs include the liver or kidneys. Organs with parallel arrangement of FSUs are more sensitive to volume effects.
Some organs present a combination of serial and parallel FSU arrangement. The lung airways are arranged in serial, but the smaller airways have a parallel FSU setup. Although the brain is arranged in a parallel FSU structure, loss of a critical brain region (eg. optic chiasm) can lead to serious complications.