A survey of methods to calculate monitor settings.

Abstract

The results of a survey of how radiation oncology institutions calculate the monitor setting (or time) to deliver the prescribed dose in a photon-beam treatment are presented. The participants in the mail survey were 94 institutions in the Pediatric Oncology Group. They were asked to calculate for a hypothetical clinical case requiring the use of photon beams with corner blocks. A questionnaire was also distributed to gather supplemental information. Of the 94 institutions whose responses were analyzed, 77% selected an isocentric setup while the others used a "fixed-SSD" approach. The proportions were reversed for the choice of the reference geometry and the majority of the participants set one monitor unit equal to 1 cGy at depth of electron equilibrium with isocenter placed at the surface. The tissue-maximum ratio was the most popular quantity for description of beam characteristics, but several other choices were common. Different names were sometimes used for the same concept. The variation among institutions made the review time-consuming, but no serious systematic errors were detected. Some standardization of nomenclature and techniques may be desirable.