Should 60Co tissue-air ratios be re-evaluated?

This note has shown that, in contrast with the claims of the Bjaerngard work (1988,1989), there is no inconsistency in defining a normalized tissue-air ratio (TAR) at depth of maximum build-up, dm, and zero field size. The adjustment to the Bjaerngard equations necessary to demonstrate this means that published TAR for non-zero field sizes should be increased by about 3%. Two of the difficulties in using this approach are (i) the problem of dealing with a TAR defined at zero depth, as opposed to dm, and (ii) the non-linearity of TAR plots at constant values of S/d. These are discussed. (author)