A monitor unit (MU) is a measure of machine output of a linear accelerator in radiation therapy. Monitor units are measured by monitor chambers, which are ionization chambers that measure the dose delivered by a beam and built into the treatment head of radiotherapy linear accelerators.
Linear accelerators are calibrated to give a particular absorbed dose under particular conditions, although the definition and measurement configuration will vary between centres.
The most common definitions are:
- The monitor chamber reads 100 MU when an absorbed dose of 1 Gray (or 100 rad) is delivered to a point at the depth of maximum dose in a water-equivalent phantom whose surface is at the isocentre of the machine (i.e. usually at 100 cm from the source) with a field size at the surface of 10x10 cm2.
- The monitor chamber reads 100 MU when an absorbed dose of 1 Gray (or 100 rad) 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 10x10 cm2 at the isocentre.
Some linear accelerators are calibrated using source-to-axis distance (SAD) instead of source-to-surface distance (SSD), and calibration (monitor unit definition) 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 isocentre based on the source-to-axis distance (SAD).
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