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The radiation output calibration of external beam radiotherapy machines based on the ionization chamber measurement is an important task of the medical physicists. Earlier, an in-air calibration measurement was practiced for 60Co beam and the water calibration methods were limited to linear accelerators only. In order to have a trace ability of dosimetry between calibration laboratories and users, various protocols were introduced over two decades. The absorbed dose to air calibration) factor was explained in TG-21 and TRS-277 protocols which were later replaced by absorbed dose to water calibration (ND,w)in TG-51 and TRS-398 protocols. In this context, we have compared in-air measurements with in-water measurements using above protocols for the 60Co and I37Cs beams, and the results agree within &2%. The photon beam output values measured using TRS-277 and TRS-398 formalisms were within +2% for the 60Cob beam, 6 MV and 18 MV x-ray photon beams. The measurements using a cylindrical chamber (TRS-277) and parallel plate chamber (TRS-381) have shown good agreement for high energy photons and electrons. The TLD audit carried out for all the photon beams and the results are within &5%.
