A monitor unit is a measure of machine output of a linear accelerator in radiation therapy

Radiation therapy

Radiation therapy , radiation oncology, or radiotherapy , sometimes abbreviated to XRT or DXT, is the medical use of ionizing radiation, generally as part of cancer treatment to control malignant cells.Radiation therapy is commonly applied to the cancerous tumor because of its ability to control...

.

Linear accelerators are calibrated to give a particular absorbed dose

Absorbed dose

Absorbed dose is a measure of the energy deposited in a medium by ionizing radiation per unit mass...

 under particular conditions. One standard calibration is such that 100 MU gives an absorbed dose of 1 Gray

Gray (unit)

The gray is the SI unit of absorbed radiation dose of ionizing radiation , and is defined as the absorption of one joule of ionizing radiation by one kilogram of matter ....

 (or 100 rad

Rad (unit)

The rad is a unit of absorbed radiation dose. The rad was first proposed in 1918 as "that quantity of X rays which when absorbed will cause the destruction of the malignant mammalian cells in question..." It was defined in CGS units in 1953 as the dose causing 100 ergs of energy to be absorbed by...

) at a depth of 5 cm in tissue-equivalent phantom

Imaging phantom

Imaging phantoms, or simply "phantoms", are specially designed objects that are scanned or imaged in the field of medical imaging to evaluate, analyze, and tune the performance of various imaging devices. These objects are more readily available and provide more consistent results than the use of...

, for a 10x10 cm² field at 95 cm source-to-surface distance (SSD).

Some linear accelerators are calibrated using source-to-axis distance (SAD) instead of SSD, and calibration (monitor unit definition) may vary depending on hospital custom.