ABSTRACT
Teletherapy cobalt-60 units were first used for patient treatment in 1951 in Canada (Green
and Errington 1952; Johns et al. 1952). Although megavoltage x-ray units had been available
for some years prior to this date, they were not in widespread use, whereas, cobalt units became
the mainstay of external beam therapy for the next 30 years or so world-wide.
