Biologic effect of grid cobalt 60 radiation.

In a recent clinical study a group of advanced cancer patients were treated with focused telecobalt grid therapy. The relative biologic effect of grid therapy compared to homogeneous irradiation was studied. White Swiss mice were exposed in comparable groups to homogeneous and grid cobalt 60 radiation. The ratio of the grid dose to the homogeneous dose required to produce a median 30 day lethality was found to be 2.4. This factor, however, was dependent upon total dose and varied from 2.9 at lower doses to 2.2 at higher doses. Analysis of mean survival times of the 2 groups of mice revealed a ratio of 2.5 at the LD 50/30 dose level. The volume dose for grid exposure was found to be 1.9 times less than that delivered by equal homogeneous doses. Thus, a sparing effect of grid irradiation was suggested by the discrepancy of the ratios of 1.9 and 2.4. The ratio of 2.9 is close to that found to be clinically effective in the clinical cancer study. (auth)