Most medical linear accelerators worldwide are calibrated using ionization chambers that are themselves calibrated by a standards laboratory, or secondary standards laboratory, in a Co-60 beam. Because these chambers are actually used to calibrate high-energy x-ray beams, it has been suggested that calibration against Co-60 is outdated and should be replaced by calibration in linear accelerator beams. This claim is debated in the Point/Counterpoint section of Medical Physics journal September 2015 publication by American Association of Physicists in Medicine.
This can be viewed online here or downloaded here