[1] 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] 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.
Nearly 60% of the reported errors involved a lack of an appropriate independent secondary check of the treatment plan or dose calculation [5] With the development and technological advances, radiotherapy requires that high doses of radiation are delivered to the tumor with increasing precision.
More recently, the new ICRU recommendations in Publication 62[7] Commercially available computerized treatment planning systems are often used in radiotherapy services to perform monitoring unit (MU) calculations to deliver the prescribed dose to the patient.