^{1,a)}, John A. Antolak

^{2}, David S. Followill

^{3}, M. Saiful Huq

^{4}, Eric E. Klein

^{5}, Kwok L. Lam

^{6}, Jatinder R. Palta

^{7}, Donald M. Roback

^{8}, Mark Reid

^{9}and Faiz M. Khan

^{10}

### 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, , 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 = 1 cGy/MU. For photon beams, this task group recommends that a normalization depth of 10 cm be selected, where an energy-dependent ≤ 1 cGy/MU is required. This recommendation differs from the more common approach of a normalization depth of d m , with = 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.

I. INTRODUCTION II. CALCULATION FORMALISM II.A. Photons II.A.1. Monitor unit equations II.A.2. Field-size determination II.A.3. Radiological depth determination II.B. Electrons II.B.1. Monitor unit equations II.B.2. Field-size determination III. DETERMINATION OF DOSIMETRIC QUANTITIES III.A. Dosimetry equipment III.A.1. Ionization chambers III.A.2. Phantoms III.B. Measurements of dosimetric quantities III.B.1. Measurements of dosimetric quantities: Photon beams III.B.2. Measurements of dosimetric quantities: Electron beams IV. INTERFACE WITH TREATMENT-PLANNING SYSTEMS V. MU CALCULATIONS FOR IMRT FIELDS V.A. Calculation methodologies V.B. Task group recommendations VI. QUALITY ASSURANCE VII. SUMMARY OF RECOMMENDATIONS VIII. EXAMPLES VIII.A. Photon calculations VIII.B. Electron calculations

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### 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, , 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 = 1 cGy/MU. For photon beams, this task group recommends that a normalization depth of 10 cm be selected, where an energy-dependent ≤ 1 cGy/MU is required. This recommendation differs from the more common approach of a normalization depth of d m , with = 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.

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