A SURVEY OF METHODS TO CALCULATE MONITOR SETTINGS

Purpose: The results of a survey of how radiation oncology institution s calculate the monitor setting (or time) to deliver the prescribed do se in a photon-beam treatment are presented. Methods and Materials: Th e participants in the mail survey were 94 institutions in the Pediatri c Oncology Group. They were asked to calculate for a hypothetical clin ical case requiring the use of photon beams with corner blocks. A ques tionnaire was also distributed to gather supplemental information. Res ults: Of the 94 institutions whose responses were analyzed, 77% select ed an isocentric setup while the others used a ''fixed-SSD'' approach. The proportions were reversed for the choice of the reference geometr y and the majority of the participants set one monitor unit equal to 1 cGy at depth of electron equilibrium with isocenter placed at the sur face. The tissue-maximum ratio was the most popular quantity for descr iption 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 no menclature and techniques may be desirable.