11.08 - Calculation Of Monitor Units

A monitor unit is a measurement of ionisation occurring in a treatment beam within the treatment head. One monitor unit is typically equal to a specific dose of radiation, at a specific depth in a water phantom, for a beam of a particular energy, with a particular field size and at a certain distance from the target. Monitor units are used to measure the output of the machine to deliver accurate dose.
For teletherapy machines, treatment time is used instead of monitor units. The treatment time is based on similar measurements to monitor units, but uses the known output of the radioactive source to calculate the time required to leave the source in the ‘on’ position.

Factors influencing calculation of Monitor Units

A monitor unit is based on a certain set of beam setup parameters, and beams different to this setup need to be corrected to ensure correct dose is delivered. The factors influencing this correction are:

• Beam Energy. Typically each beam energy on a linac will have its own monitor unit calculation for a standard field.
• Source Surface Distance. An increased SSD will mean that increased monitor units will be required to deliver a dose at depth due to the effects of the inverse square law. This must be taken into account, and is usually only a problem for extended SSD treatments (using a fixed SSD technique)
• Tissue-Phantom Ratio / Tissue-Maximum Ratio (TPR). Used for fixed source-axial distance calculations only. The TPR/TMR describes the dose rate relative to a dose rate for a similar beam at a different depth within the target. This allows a monitor unit correction to take into account different depths of a target.
• Percentage Depth Dose (PDD). Used for fixed source-surface distance calculations only. The PDD describes the dose rate at different depths within a target for an equal source-surface distance. If the target is located at a different depth to the standard field, monitor units will need to be adjusted.
• Output Factor (OF). The output factor is change in dose rate that occurs with different field sizes. Large fields will usually have a higher output factor, leading to increased dose rates for the same number of monitor units.
• Wedge Factor (WF). Takes into account the effect of the wedge on the attenuation of the radiation beam. If a wedge is present, increased monitor units will be required to reach the same dose.
• Calibration Factor (CF). Not commonly used, typically 1 in most scenarios.