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Please note there is a correction to page 1862. The updated version was published online 11 July 2002.

A protocol is prescribed for clinical reference dosimetry of external beam radiation therapy using photon beams with nominal energies between ⁶⁰Co 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, N, which are traceable to national primary standards, and the equation D = Mk_QN, where Q is the beam quality of the clinical beam, D is the absorbed dose to water at the point of measurement of the ion chamber placed under reference conditions, M is the fully corrected ion chamber reading, and k_Q is the quality conversion factor which converts the calibration factor for a ⁶⁰Co beam to that for a beam of quality Q. Values of k_Q are presented as a function of Q for many ion chambers. The value of M is given by M = P_ionP_TPP_elecP_polM_raw, where M_raw is the raw, uncorrected ion chamber reading and P_ion corrects for ion recombination, P_TP for temperature and pressure variations, P_elec for inaccuracy of the electrometer if calibrated separately, and P_pol for chamber polarity effects. Beam quality, Q, is specified (i) for photon beams, by %dd(10), the photon component of the percentage depth dose at 10 cm depth for a field size of 10×10 cm² on the surface of a phantom at an SSD of 100 cm and (ii) for electron beams, by R₅₀, the depth at which the absorbed-dose falls to 50% of the maximum dose in a beam with field size 10×10 cm² on the surface of the phantom (20×20 cm² for R₅₀>8.5 cm) at an SSD of 100 cm. R₅₀ is determined directly from the measured value of I₅₀, 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 0.6R₅₀–0.1 cm for electron beams. For photon beams clinical reference dosimetry is performed in either an SSD or SAD setup with a 10×10 cm² 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 10×10 cm² (20×20 cm² for R₅₀>8.5 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. ©1999 American Association of Physicists in Medicine.