Monitor unit calculations for external photon and electron beams: Report ofthe AAPM Therapy Physics Committee Task Group No. 71

摘要

A protocol is presented for the calculation of monitor units (MU) for photon and electron beams, delivered with and without beam modifiers, for constant source-surface distance (SSD) and source-axis distance (SAD) setups. This protocol was written by Task Group 71 of the Therapy Physics Committee of the American Association of Physicists in Medicine (AAPM) and has been formally approved by the AAPM for clinical use. The protocol defines the nomenclature for the dosimetric quantities used in these calculations, along with instructions for their determination and measurement. Calculations are made using the dose per MU under normalization conditions, D0′, that is determined for each user's photon and electron beams. For electron beams, the depth of normalization is taken to be the depth of maximum dose along the central axis for the same field incident on a water phantom at the same SSD, where D0′ = 1 cGy/MU. For photon beams, this task group recommends that a normalization depth of 10 cm be selected, where an energy-dependent D0′ ≤ 1 cGy/MU is required. This recommendation differs from the more common approach of anormalization depth of dm, withD0′= 1 cGy/MU, although both systems are acceptable within the current protocol. Forphotonbeams, theformalism includes the use of blocked fields, physical or dynamic wedges, and (static)multileaf collimation. No formalism is provided for intensity modulated radiationtherapy calculations, although some general considerations and a reviewof current calculation techniques are included. For electron beams, the formalismprovides for calculations at the standard and extended SSDs using either an effective SSDor an air-gap correction factor. Example tables and problems are included to illustratethe basic concepts within the presented formalism.