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

Abstract

Taking mu p as the attenuation coefficient of the primary radiation and d is the depth in tissue, Bjarngard et al (1989) showed that plots of TAR (d, S) exp ( mu pd) against S, for given values of the ratio S/d, were approximately linear and converged to 1.00 at zero field size, using the unmodified values of published TAR data. This observation was used to support the proposition that the zero-field-size values of TAR, TAR(d, 0), should be reduced. Bjarngard and Petti (1988) also suggested that a normalized TAR(dm, 0) was inconsistent with the notion that TAR(d, 0) describes primary dose. The present author shows that there is another way to interpret Bjarngard's analysis, which is consistent with a normalized TAR(dm, 0). The consequence of this alternative analysis is that non-zero-field-size TAR should be increased.