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Monitor unit calculations as part of beam commissioning for a radiation treatment planning system

The purpose of this paper is to illustrate the need for validating the calculation of monitor units as part of the process of commissioning a photon beam model in a treatment planning system. Three examples are provided in which this validation identified subtle errors, either in the dose model or in the implementation of the dose algorithm. These errors would not have been detectable if the commissioning process only compared relative dose distributions. A set of beam configurations, with varying rectangular field sizes, secondary blocking, wedges, and SSDs, was established to validate monitor unit calculations for two beam models in two treatment planning systems. Monitor units calculated using the treatment planning systems were compared with monitor units calculated from point dose calculations (TMR tables). When discrepancies occurred, the dose models and the code were analyzed to identify the causes of the discrepancies. Discrepancies in monitor unit calculations were both significant (up to 5%) and systematic. Analysis of the dose computation software found (1) a coordinate system transformation error, (2) mishandling of dose-spread arrays, (3) differences between dose calculations in the commissioning software and the planning software, and (4) shortcomings in modeling of head scatter. Corrections were made in the beam calculation software or in the data sets to overcome these discrepancies. Consequently, we recommend incorporating a comprehensive validation of monitor unit calculations as part of any beam commissioning process.