Verifying monitor unit calculations for tangential whole‐breast fields in three‐dimensional planning

Verification of the accuracy of monitor unit calculations is an essential component of quality assurance in radiation therapy. For tangential breast fields, monitor unit differences between primary calculations and second checks are usually larger than would be considered acceptable at other anatomic sites. Here, we present a simple model to reconcile the differences between sophisticated and simple algorithms, based on estimating the volume irradiated by the field, replacing the breast contour with a rectangular block having an equal volume, but using a new field width that 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. PACS numbers: 87.53.Kn, 87.53.Tf, 87.53.Xd