and only onc oF no did not exhibit dung cance“& doath,. Abe fh relationship observed Leiween initial Jung burden and tinie to death with cancer has been often used to infer a threshold burden below which no life shortening uf dogs would be expected. This is shown in Figure 4.6.10 on 4-G 118. fote that the fibrotic deaths there have no bearing on cancer Incidence and inclusicn of those points in the constructing extrapolated curves is a '‘’ _ senseless exercise. flote also that the results are exhibited on-e log-lcg graph wnich virtually obscures all differential detail. Most important, recognize the nature of the experinenit, 7.e., the Jung burdens ware large, the results were saturated, and the number of animals was small. The crudé relationship observed between initial lung burden and time to death with lung cancer does not necessarily imply that a threshold burcen exists for beagles. Quite to the contrary’, the range of exposures above the inferred threshold burden may be interpreted as a region of saturated carcinogenic response, that is a burden regim ein which lung cancer induction in a beagle population approaches 100% during a normal life span. The point - is that the observed time to death is more likely related to the burden, through a population depletion effect, rather than through a burden In the former interpretation appreciable cancer ads dependent latent period. vould be anticipated at lewer burdens. This is again consistent with wh get extensive observations of radioisotope-induced bone tumors in mice, which ‘support the interpretation that "latent period is constant and that the apparent relationship between increasing dose end decreasing time tc death with tumor is due to the effects of dose-level on survival and on tumor expectancy." (See Toxicity of Ra-226 in Mice," HK. Finkel et 23, in Radiation-Induced Cancer, TAEA, Vienna, 1969.) The domain of this comment is broadened here in order to sunvnarize TIN Tr. Ae ereee ‘ab AS ei Si SRRBE AORES e er PRT, PNTe rena oes eps oe kas fing ‘ apame att rhe t re ar NE iaee © Viste :} See A Aa : Lfeee hore ; ROT ge Cnr3" G 4 TOK LGTY FRAT: ae ® » *erAy VO "ee 4 et