Verifying monitor unit calculations for tangential whole breast fields in 3-dimensional planning

An essential component of quality assurance in radiation therapy is verifying the accuracy of monitor unit calculations. For tangential breast fields, monitor unit differences between primary calculations and second checks are usually larger than considered acceptable at other anatomical sites. A simple model to reconcile the differences between sophisticated and simple algorithms is presented, based on estimating the the volume irradiated by the field, replacing the breast contour with a rectangular block having an equal volume but a new field width which provides almost equivalent scatter to the prescription point. This analysis can also assist the treatment planning physicist in selecting a tolerance window for verifying monitor unit calculations for tangential breast fields.