Scientific analysis of the proposed uses of T25 dose descriptor in chemical carcinogen regulation. An ECETOC Workshop Overview.

The uncertainties that surround the methods used for risk assessment of exposure to carcinogens have been highlighted by a recent document advocating an approach based on the T25 dose (the dose giving a 25% incidence of cancer in an appropriately designed animal experiment). This method relies on derivation of the T25 dose then assesses risk at the exposure dose using proportionality provided by a linear extrapolation (T25/ linear). To promote discussion of the scientific issues underlying methods for the risk assessment of chemical carcinogens, the European Centre for Ecotoxicology and Toxicology of Chemicals (ECETOC) hosted a one-day workshop in Brussels on 10 November 2000. Several invited presentations were made to participants, including scientists from regulatory authorities, industry and academia. In general, it was felt that there was sufficient basis for using the T25 dose as an index of carcinogenic potency and hence as part of the hazard assessment process. However, the use of the T25 in risk assessment has not been validated. The T25/linear and other extrapolation methods based on metrics such as LED10 assume linearity which may be invalid. Any risk calculated using the T25/linear method would provide a precise risk figure similar to the output obtained from the Linearised Multistage (LMS) method formerly used by the Environmental Protection Agency (EPA) in the United States of America. Similarity of output does not provide validation but rather reflects their reliance on similar mathematical approaches. In addition to the T25 issue, evidence was provided that using two separate methods (linearised non-threshold model for genotoxic carcinogens; no-observable-effect level with a safety factor (NOEL/SF) method for all other toxicity including non-genotoxic carcinogens) is not justified. Since the ultimate purpose of risk assessment is to provide reliable information to risk managers and the public, there was strong support at the workshop for harmonisation of approaches to risk assessment for all genotoxic and nongenotoxic carcinogens. In summary, the T25 method has utility for ranking potency to focus efforts in risk reduction. However, uncertainties such as the false assumption of precision and non-linearity in the dose-response curve for tumour induction raise serious concerns that caution against the use of T25/linear method for predicting human cancer risk.