Quality assurance of monitor unit calculations

Monitor unit, MU, calculations are performed by a computerized treatment planning system, TPS, or by a MU calculation program run on a separate computer applying tables stored in that system, or a hand calculation applying tables with beam data. Three sources of possible errors will be discussed: 1. discrepancies between the beam data applied in the various calculation systems and actual beam data; 2. systematic errors in the dose calculation method and 3. human mistakes. Quality control, QC, programmes of MU calculations may include various procedures such as a check of the patient data, a check of the machine related input data, an independent recalculation using the same MU calculation program or a completely independent second calculation, without reference to the original one. The data relevant for the calculations should be available for future quality audits. It is therefore recommended that worksheets are designed suitable for both TPS manufacturers and individual institutions. Finally the role of in vivo dosimetry for checking the number of MUs will be elucidated