Revised cobalt-60 depth dose data.

It has long been suspected that the depth-dose characteristics of modern 60Co teletherapy units are no longer well represented by the traditional data of The British Journal of Radiology Supplement 11. Extensive measurements have been undertaken on three modern 60Co units in an attempt to clarify this position. Three different values of source-to-skin distance were studied, namely 50, 80 and 100 cm. Systematic deviations from the traditional data are unambiguous, amounting to up to 3% at 80 cm SSD and as much as 7% at 50 cm SSD. Comparison with calibration results at a large number of institutions lends confidence that the present results are representative of modern treatment units.