Volume 26, Issue 9 p. 1847-1870
Radiation treatment physics
Open Access

AAPM's TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams

Peter R. Almond

Peter R. Almond

Brown Cancer Center, Louisville, Kentucky 40202

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Peter J. Biggs

Peter J. Biggs

Department of Radiation Oncology, Massachusetts General Hospital, Boston, Massachusetts 02114

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B. M. Coursey

B. M. Coursey

Ionizing Radiation Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899

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W. F. Hanson

W. F. Hanson

M.D. Anderson Cancer Center, University of Texas, Houston, Texas 77030

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M. Saiful Huq

M. Saiful Huq

Kimmel Cancer Center of Jefferson Medical College, Thomas Jefferson University, Philadelphia, Pennsylvania 19107

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Ravinder Nath

Ravinder Nath

Yale University School of Medicine, New Haven, Connecticut 06510

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D. W. O. Rogers

D. W. O. Rogers

Ionizing Radiation Standards, National Research Council of Canada, Ottawa K1A OR6, Canada

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First published: 07 September 1999
Citations: 1,250

Abstract

A protocol is prescribed for clinical reference dosimetry of external beam radiation therapy using photon beams with nominal energies between urn:x-wiley:00942405:media:mp8691:mp8691-math-0001 and 50 MV and electron beams with nominal energies between 4 and 50 MeV. The protocol was written by Task Group 51 (TG-51) of the Radiation Therapy Committee of the American Association of Physicists in Medicine (AAPM) and has been formally approved by the AAPM for clinical use. The protocol uses ion chambers with absorbed-dose-to-water calibration factors, urn:x-wiley:00942405:media:mp8691:mp8691-math-0002 which are traceable to national primary standards, and the equation urn:x-wiley:00942405:media:mp8691:mp8691-math-0003 where urn:x-wiley:00942405:media:mp8691:mp8691-math-0004 is the beam quality of the clinical beam, urn:x-wiley:00942405:media:mp8691:mp8691-math-0005 is the absorbed dose to water at the point of measurement of the ion chamber placed under reference conditions, urn:x-wiley:00942405:media:mp8691:mp8691-math-0006 is the fully corrected ion chamber reading, and urn:x-wiley:00942405:media:mp8691:mp8691-math-0007 is the quality conversion factor which converts the calibration factor for a urn:x-wiley:00942405:media:mp8691:mp8691-math-0008 beam to that for a beam of quality urn:x-wiley:00942405:media:mp8691:mp8691-math-0009 Values of urn:x-wiley:00942405:media:mp8691:mp8691-math-0010 are presented as a function of urn:x-wiley:00942405:media:mp8691:mp8691-math-0011 for many ion chambers. The value of urn:x-wiley:00942405:media:mp8691:mp8691-math-0012 is given by urn:x-wiley:00942405:media:mp8691:mp8691-math-0013 where urn:x-wiley:00942405:media:mp8691:mp8691-math-0014 is the raw, uncorrected ion chamber reading and urn:x-wiley:00942405:media:mp8691:mp8691-math-0015 corrects for ion recombination, urn:x-wiley:00942405:media:mp8691:mp8691-math-0016 for temperature and pressure variations, urn:x-wiley:00942405:media:mp8691:mp8691-math-0017 for inaccuracy of the electrometer if calibrated separately, and urn:x-wiley:00942405:media:mp8691:mp8691-math-0018 for chamber polarity effects. Beam quality, urn:x-wiley:00942405:media:mp8691:mp8691-math-0019 is specified (i) for photon beams, by urn:x-wiley:00942405:media:mp8691:mp8691-math-0020 the photon component of the percentage depth dose at 10 cm depth for a field size of urn:x-wiley:00942405:media:mp8691:mp8691-math-0021 urn:x-wiley:00942405:media:mp8691:mp8691-math-0022 on the surface of a phantom at an SSD of 100 cm and (ii) for electron beams, by urn:x-wiley:00942405:media:mp8691:mp8691-math-0023 the depth at which the absorbed-dose falls to 50% of the maximum dose in a beam with field size urn:x-wiley:00942405:media:mp8691:mp8691-math-0024 urn:x-wiley:00942405:media:mp8691:mp8691-math-0025 on the surface of the phantom urn:x-wiley:00942405:media:mp8691:mp8691-math-0026 urn:x-wiley:00942405:media:mp8691:mp8691-math-0027 for urn:x-wiley:00942405:media:mp8691:mp8691-math-0028 cm) at an SSD of 100 cm. urn:x-wiley:00942405:media:mp8691:mp8691-math-0029 is determined directly from the measured value of urn:x-wiley:00942405:media:mp8691:mp8691-math-0030 the depth at which the ionization falls to 50% of its maximum value. All clinical reference dosimetry is performed in a water phantom. The reference depth for calibration purposes is 10 cm for photon beams and urn:x-wiley:00942405:media:mp8691:mp8691-math-0031 cm for electron beams. For photon beams clinical reference dosimetry is performed in either an SSD or SAD setup with a urn:x-wiley:00942405:media:mp8691:mp8691-math-0032 urn:x-wiley:00942405:media:mp8691:mp8691-math-0033 field size defined on the phantom surface for an SSD setup or at the depth of the detector for an SAD setup. For electron beams clinical reference dosimetry is performed with a field size of urn:x-wiley:00942405:media:mp8691:mp8691-math-0034 urn:x-wiley:00942405:media:mp8691:mp8691-math-0035 urn:x-wiley:00942405:media:mp8691:mp8691-math-0036 urn:x-wiley:00942405:media:mp8691:mp8691-math-0037 for urn:x-wiley:00942405:media:mp8691:mp8691-math-0038 cm) at an SSD between 90 and 110 cm. This protocol represents a major simplification compared to the AAPM's TG-21 protocol in the sense that large tables of stopping-power ratios and mass-energy absorption coefficients are not needed and the user does not need to calculate any theoretical dosimetry factors. Worksheets for various situations are presented along with a list of equipment required.