The volume effect is seen in normal tissues, where the tolerance dose of a tissue is related to the volume of that tissue irradiated. Tissues may be considered to have functional subunits (FSU), where each subunit performs some of the function of that organ. The arrangement of these subunits, and their relationship to each other, is what leads to the volume effect.
- In some tissues, each FSU performs its function relatively independently of the others. Such tissues are considered parallel, and examples include the lung, liver and kidney. In these tissues, the total volume irradiated is very important in determining the outcome. For example, the liver is capable of sustaining life even if half its volume is made non-functional by radiation.
- In some tissue, each FSU is critical for the function of other FSUs. These organs are considered serial, and include the spinal cord and gastrointestinal tract. Loss of one FSU of the spinal cord will lead to loss of all FSUs caudal to that point. In these tissues, it is vital that radiation dosage dose not exceed tolerance at any point.
- The concept of serial and parallel arrangement of FSUs is not entirely correct, as many tissues have both serial and parallel components. For example, the lungs rely on the trachea and airways (serial arrangement) to function. The FSUs of the brain have both serial and parallel components. It is possible to lose part of the occipital cortex and still have vision, but if that part is the fovea then function will be critically impaired.