Calculation of carcinogenic potency from long-term animal carcinogenesis experiments.
Sawyer C., Peto R., Bernstein L., Pike MC.
An index of carcinogenic potency for chemicals tested in chronic animal experiments is described. By analogy with the well-known 'lethal dose 50' (LD50) of quantal bioassay, a 'tumorigenic dose 50' (TD50) may be defined (in the absence both of tumors in the control group and of intercurrent deaths) as that (daily) dose of chemical which gives 50% of the test animals tumors by some fixed age. Tumors in the control (zero-dose) group are handled exactly as for the LD50, and intercurrent deaths are handled by life-table methods. Nonparametric procedures are developed for estimating the TD50 and for constructing confidence intervals. These are based on likelihoods which assume that the tumor hazard is linear in dose.