Editing Monitor unit (section)

==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.<ref>{{cite journal|last1=Lillicrap|first1=S C|last2=Owen|first2=B|last3=Williams|first3=J R|last4=Williams|first4=P C|title=Code of Practice for high-energy photon therapy dosimetry based on the NPL absorbed dose calibration service|journal=Physics in Medicine and Biology|date=1 October 1990|volume=35|issue=10|pages=1355–1360|doi=10.1088/0031-9155/35/10/301|bibcode=1990PMB....35.1355L }}</ref><ref>{{cite journal|last1=Nath|first1=Ravinder|last2=Biggs|first2=Peter J.|last3=Bova|first3=Frank J.|last4=Ling|first4=C. Clifton|last5=Purdy|first5=James A.|last6=van de Geijn|first6=Jan|last7=Weinhous|first7=Martin S.|title=AAPM code of practice for radiotherapy accelerators: Report of AAPM Radiation Therapy Task Group No. 45|journal=Medical Physics|date=July 1994|volume=21|issue=7|pages=1093–1121|doi=10.1118/1.597398|pmid=7968843|bibcode=1994MedPh..21.1093N |url=http://www.aapm.org/pubs/reports/rpt_47.PDF}}</ref>

The most common definitions are:<ref>{{cite book |last1=Mayles |first1=Philip |last2=Nahum |first2=Alan |last3=Rosenwald |first3=Jean-Claude |year=2007 |title=Handbook of Radiotherapy Physics - Theory and Practice |chapter=Chapter 20: From Measurements to Calculations |publisher=Taylor & Francis |isbn=978-0-7503-0860-1}}</ref>

# The monitor chamber reads 100&nbsp;MU when an [[absorbed dose]] of 1 [[Gray (unit)|gray]] (100 [[rad (unit)|rads]]) is delivered to a point at the depth of maximum dose in a water-equivalent [[Imaging phantom|phantom]] whose surface is at the [[isocenter]] of the machine (i.e. usually at 100&nbsp;cm from the source) with a field size at the surface of 10&nbsp;cm&nbsp;×&nbsp;10&nbsp;cm.
# The monitor chamber reads 100&nbsp;MU when an absorbed dose of {{convert|1|Gy|rad|abbr=on}} 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&nbsp;cm&nbsp;×&nbsp;10&nbsp;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 technique|isocentric]] treatment [[treatment planning|plans]], so newer definitions of the monitor unit are based on geometry at the [[isocenter]] based on the source-to-axis distance (SAD).