A survey of methods to calculate monitor settings

Purpose: 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. Methods and Materials: 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. Results: 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. Conclusion: 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.