Monitor unit

Monitor unit (MU) is a term used in the field of radiation therapy to quantify the amount of radiation delivered to a patient. The monitor unit is a measure of the amount of radiation produced by a linear accelerator, which is a machine used to deliver radiation therapy. The number of monitor units determines the duration of the radiation beam-on time.

Definition

A monitor unit is defined as the amount of charge, in Coulombs, collected in the ionization chamber of a linear accelerator when it is operated at its reference conditions. The reference conditions are typically a specified dose rate, a specified energy, and a specified distance from the source of radiation.

Calculation

The calculation of monitor units is a complex process that involves several factors. These factors include the prescribed dose, the depth of the target, the field size, the source-to-surface distance, and the output factor of the linear accelerator. The calculation is typically performed by a medical physicist or a dosimetrist using specialized software.

Clinical significance

The accurate calculation and delivery of the correct number of monitor units is crucial in radiation therapy. It ensures that the patient receives the prescribed dose of radiation, which is necessary to kill the cancer cells while sparing the surrounding healthy tissue. Errors in the calculation or delivery of monitor units can lead to underdosing or overdosing, which can have serious consequences for the patient.

See also

• Radiation therapy
• Linear accelerator
• Medical physicist
• Dosimetrist