Review of dosimetric functions for meterset calculations

Mathematical expressions used to calculate doses in a patient, based on data measured in a phantom, have to be simple, understandable, and reliable to minimize possible calculational error. In light of this concern, this paper reviews the dosimetric functions used in meterset calculations to determine the treatment times or monitor units for a prescribed dose. The dosimetric functions are the percent depth dose, the tissue-air ratio, the tissue-phantom ratio, and the inverse square law. This review examined the definition of the dosimetric functions, the inter-relationships among the dosimetric functions, and the mathematical expressions used in meterset calculations for nonstandard source-to-surface distances in a phantom.