Monitor unit


A monitor unit is a measure of machine output from a clinical accelerator for radiation therapy such as a linear accelerator or an orthovoltage unit. Monitor units are measured by monitor chambers, which are ionization chambers that measure the dose delivered by a beam and are built into the treatment head of radiotherapy linear accelerators.

Calibration and dose quantities

Linear accelerators are calibrated to give a particular absorbed dose under particular conditions, although the definition and measurement configuration may vary among medical clinics.
The most common definitions are:
  1. The monitor chamber reads 100 MU when an absorbed dose of 1 gray is delivered to a point at the depth of maximum dose in a water-equivalent phantom whose surface is at the isocenter of the machine with a field size at the surface of 10 cm × 10 cm.
  2. The monitor chamber reads 100 MU when an absorbed dose of is delivered to a point at a given depth in the phantom with the surface of the phantom positioned so that the specified point is at the isocentre of the machine and the field size is 10 cm × 10 cm at the isocentre.
Some linear accelerators are calibrated using source-to-axis distance instead of source-to-surface distance, and calibration may vary depending on hospital custom.
Early radiotherapy was performed using "constant SSD" treatments, and so the definition of monitor unit was adopted to reflect this calibration geometry.
Modern radiotherapy is performed using isocentric treatment plans, so newer definitions of the monitor unit are based on geometry at the isocenter based on the source-to-axis distance.