title: Monitor unit calculations for external photon and electron beams: Report of the AAPM Therapy Physics Committee Task Group No. 71.
abstract: 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, D'0, 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 D'0 = 1 cGy/MU. For photon beams, this task group recommends that a normalization depth of 10 cm be selected, where an energy-dependent D'0 ≤ 1 cGy/MU is required. This recommendation differs from the more common approach of a normalization depth of dm, with D'0 = 1 cGy/MU, although both systems are acceptable within the current protocol. For photon beams, the formalism includes the use of blocked fields, physical or dynamic wedges, and (static) multileaf collimation. No formalism is provided for intensity modulated radiation therapy calculations, although some general considerations and a review of current calculation techniques are included. For electron beams, the formalism provides for calculations at the standard and extended SSDs using either an effective SSD or an air-gap correction factor. Example tables and problems are included to illustrate the basic concepts within the presented formalism.
authors.affiliation: Department of Physics, Mary Bird Perkins Cancer Center, Baton Rouge, Louisiana 70809.
authors.forename: John P
authors.initials: JP
authors.lastname: Gibbons
authors.affiliation: Department of Radiation Oncology, Mayo Clinic, Rochester, Minnesota 55905.
authors.forename: John A
authors.initials: JA
authors.lastname: Antolak
authors.affiliation: Department of Radiation Physics, UT M.D. Anderson Cancer Center, Houston, Texas 77030.
authors.forename: David S
authors.initials: DS
authors.lastname: Followill
authors.affiliation: Department of Radiation Oncology, University of Pittsburgh Cancer Institute, Pittsburgh, Pennsylvania 15232.
authors.forename: M Saiful
authors.initials: MS
authors.lastname: Huq
authors.affiliation: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri 63110.
authors.forename: Eric E
authors.initials: EE
authors.lastname: Klein
authors.affiliation: Department of Radiation Oncology, University of Michigan, Ann Arbor, Michigan 48109.
authors.forename: Kwok L
authors.initials: KL
authors.lastname: Lam
authors.affiliation: Department of Radiation Oncology, Virginia Commonwealth University, Richmond, Virginia 23298.
authors.forename: Jatinder R
authors.initials: JR
authors.lastname: Palta
authors.affiliation: Department of Radiation Oncology, Cancer Centers of North Carolina, Raleigh, North Carolina 27607.
authors.forename: Donald M
authors.initials: DM
authors.lastname: Roback
authors.affiliation: Department of Medical Physics, Fletcher-Allen Health Care, Burlington, Vermont 05401.
authors.forename: Mark
authors.initials: M
authors.lastname: Reid
authors.affiliation: Department of Radiation Oncology, University of Minnesota, Minneapolis, Minnesota 55455.
authors.forename: Faiz M
authors.initials: FM
authors.lastname: Khan
mesh_terms: D000465:Algorithms; D004583:Electrons / Q000627:therapeutic use*; D006801:Humans; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D017785:Photons / Q000627:therapeutic use*; D011829:Radiation Dosage; D011874:Radiometry / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation* / Q000379:methods*