Monitor unit

A monitor unit (MU) is a measure of machine output of a linear accelerator in radiation therapy.

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Linear accelerators are calibrated for a specific energy such that 1 MU gives an absorbed dose of 1 centigray (or rad) at a depth of Dmax (or maximum dose) for a field size of 10x10 cm for a source-to-axis distance (SAD) of 100 cm. Some linear accelerators are calibrated using source-to-skin distance (SSD) instead of SAD. Monitor unit can be different for different sites.