Monitor Unit Calculations for External Photon and Electron Beams

Based on clinical dose-response data, the ICRU states that dosimetry systems must be capable of delivering dose to an accuracy of 5%. Many factors contribute to both random and systematic deviations in dose delivery, including daily patient setup, target delineation, and dose calculation. The accurate determination of dose per monitor unit (MU) at a single calculation point is an essential part of this process. There are many methods used to determine linear accelerator MUs in the United States. In 1997, the European Society for Therapeutic Radiology and Oncology (ESTRO) published the IAEA’s recommendations for photon beam calculations. Although highly detailed, this document is limited to photon calculations on the central axis, and does not cover asymmetric fields, dynamic wedges or multileaf collimators. Furthermore, the ESTRO methodology is scarcely utilized within this country, due to its extensive use of new nomenclature and lack of formal AAPM endorsement. In the spring of 1999, the Southeast Chapter of the American Association of Physicists in Medicine sponsored a symposium entitled “Monitor Unit Calculations for External Photon and Electron Beams.” Rather than recommending a standard formalism, speakers in the two-day symposium were asked to describe the calculation method they use in a specific clinical situation. The proceedings of this symposium became the framework for the book Monitor Unit Calculations for External Photon & Electron Beams, published last year. This presentation will discuss the major findings of this work.