Verifying monitor unit calculations for tangential breast fields

An essential component of quality assurance in radiation therapy is verifying the accuracy of monitor unit calculations. Differences between between sophisticated algorithms using 2.5D or 3D calculations and simpler Monitor unit check algorithms assuming a flat water phantom must be expected. For many anatomical sites such differences are small and of little or no consequence in the context of monitor unit verification. However, for tangential breast fields the discrepancies are considerably larger than those that would generally be considered acceptable. A simple model to reconcile the differences between sophisticated and simple algorithms is presented based on replacing the breast contour with a triangular or elliptical contour and using this to estimate an equivalent rectangle providing equivalent scatter to the prescription point. The elliptical approximation reconciles the observed differences in calculated monitor units. The analysis we present can assist the treatment planning physicist in selecting a method and tolerance window for verifying monitor unit calculations for tangential breast fields.