The ``microlesion concept'', introduced by D. Grahn in the 1970's, was further explored using published quantitative carcinogenesis data. Laboratory experiments performed by T.C.H. Yang and co-workers and by R.J.M. Fry and co-workers using the Fe ion beam of the Lawrence Berkeley Laboratory Bevalac have resulted in quantitative data on in vitro and in vivo (respectively) carcinogenesis in mouse systems. These data sets were interpreted on the basis of track calculations, and it was found that the action cross section for tumor induction in cultured cells is approximately 0.032 μm2 and that it is about 1/1000th as great in mouse Harderian gland cells. This great difference in carcinogenic sensitivity is a reflection of the biological differences between these two highly promoted systems, neither of which may be quantitatively applicable to humans in space.