Letter: Dose error due to a mercury shutter.

One of the early cobalt 60 teletherapy units (Atomic Energy of Canada Ltd., Eldorado Model A) was equipped with a mercury shutter whose design is described in the original paper by Green and Errington (1952). Although most of these machines have been retired from active use, many of the patients who were treated with these machines are among those whose treatment results are presently being assessed. In a current study, at this institute, of patients treated with an Eldorado Model A, some aspects of the influence of the shutter behaviour on the dosimetry were studied. We believe that some of our observations will be of interest to others contemplating similar reviews.