6.2 - Monitor Units

A single monitor unit is the amount of charge recorded in the ionisation chamber mounted in the head of the linear accelerator which correlates with a dose of 1 cGy delivered to a water phantom under reference conditions. This is an important measurement, as the output of the linear accelerator can only be read in charge passing through the ionisation chamber.
Reference conditions differ between departments, and the amount of charge in the linac ionisation chamber that represents 1 cGy under reference conditions varies between machines. Therefore 1 MU is used to standardise treatment machines within a single radiotherapy centre.

## Calculation of Monitor Units

Because reference conditions are only rarely used in modern radiotherapy, and the output of the beam can change depending on the SSD, the field size and the desired dose at depth, monitor units must be adjusted for each beam used in a treatment. To make things more interesting, the calculation of monitor units is dependent on whether a fixed SSD or fixed SAD technique is being used.

### Calculation of MU for Fixed SSD treatment

In this circumstance, monitor units are affected by:

• Dose - The desired dose (in cGy)
• SSD - The source surface distance (if different to reference conditions)
• OF - The field size (usually referred to as the output factor)
• PDD - The percent depth dose of the point in question
• WF (wedge factor) The presence of any beam modifying devices in the beam (such as a wedge)
• CF - The calibration factor (only important if 1 MU is not equal to 1 cGy under reference conditions)

The formula for calculation of MU for a fixed SSD treatment is:

(1)
\begin{align} MU=\frac{\text{Dose (in cGy)}}{\text{CF}.(\text{PDD.OF.WF})} \end{align}

### Calculation of MU for Fixed SAD (Isocentric) treatment

In this circumstance, monitor units are affected by:

• Dose - The desired dose (in cGy)
• OF - The field size
• TPR - The tissue phantom ratio for the point where dose is to be delivered
• WF - The wedge factor
• CF - The calibration factor (only important if 1 MU is not equal to 1 cGy under reference conditions)

The formula for fixed SAD is:

(2)
\begin{align} MU=\frac{\text{Dose (in cGy)}}{\text{CF}.(\text{TPR.OF.WF})} \end{align}