Monitor unit


A monitor unit (MU) 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.[1]

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.[2][3]

The most common definitions are:[4]

  1. The monitor chamber reads 100 MU when an absorbed dose of 1 gray (100 rads) 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 (i.e. usually at 100 cm from the source) with a field size at the surface of 10 cm × 10 cm.
  2. The monitor chamber reads 100 MU when an absorbed dose of 1 Gy (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 10 cm × 10 cm 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 isocenter based on the source-to-axis distance (SAD).


  1. ^ Halperin, Edward C.; Perez, Carlos A.; Brady, Luther W. (2008). Perez and Brady's principles and practice of radiation oncology (5th ed.). Philadelphia: Lippincott Williams & Wilkins. p. 152. ISBN 9780781763691.
  2. ^ Lillicrap, S C; Owen, B; Williams, J R; Williams, P C (1 October 1990). "Code of Practice for high-energy photon therapy dosimetry based on the NPL absorbed dose calibration service". Physics in Medicine and Biology. 35 (10): 1355–1360. doi:10.1088/0031-9155/35/10/301.
  3. ^ Nath, Ravinder; Biggs, Peter J.; Bova, Frank J.; Ling, C. Clifton; Purdy, James A.; van de Geijn, Jan; Weinhous, Martin S. (July 1994). "AAPM code of practice for radiotherapy accelerators: Report of AAPM Radiation Therapy Task Group No. 45" (PDF). Medical Physics. 21 (7): 1093–1121. doi:10.1118/1.597398. PMID 7968843.
  4. ^ Mayles, Philip; Nahum, Alan; Rosenwald, Jean-Claude (2007). "Chapter 20: From Measurements to Calculations". Handbook of Radiotherapy Physics - Theory and Practice. ISBN 0-7503-0860-5.