LTD10 Values for Chemicals Evaluated as Carcinogenic in the CPDB

For cancer risk assessment, researchers and regulatory agencies are currently using a variety of approaches to the use of tumor incidence data from animal cancer tests to estimate low-dose human risk. Measures of potency include q1*, TD50, TD25, TD10, or TD05. The EPA carcinogen risk assessment guidelines (2005) select a benchmark dose approach, which is generally the dose to induce tumors in 10% of animals (TD10). In order to account for experimental variability, a lower 95% confidence limit on TD10, designated LTD10, generally is used for risk assessment.

Due to the relatively narrow range of doses in animal cancer tests and the limited range of statistically significant tumor incidence rates, estimates of potency are constrained to a relatively narrow range of values. Because of this constraint, simple, quick estimates of the lower 95% confidence limit on TD10 (LTD10) can be readily obtained from the CPDB, using the reported values of TD50 and its lower 99% confidence limit. Since LTD10 is derived from TD50, it is standardized to the lifespan of the species, e.g., whenever experiments are terminated in rats or mice earlier or later than 2 years, LTD10 like TD50, is standardized by extrapolation to 2 years.

Methods used to estimate LTD10 values from results in the CPDB are described in Statistical Methods for Estimating LTD10. LTD10 values can be calculated for any target site in any single experiment of interest in the CPDB by using the TD50 and its lower confidence limit and the formula in Statistical Methods for Estimating LTD10.

The LTD10 Excel Spreadsheet

We have estimated LTD10 values for the 805 chemicals in the CPDB that have been evaluated as positive by the author of at least one published experiment. Methods for classification of positivity are the same as those for TD50 in the Summary Table by Chemical. LTD10 values are provided in an Excel spreadsheet. Chemical names are listed alphabetically, and Chemical Abstracts Service Registry (CAS) number is also reported. The harmonic mean of LTD10 values for each positive chemical is reported for rats, mice, hamsters, dogs and nonhuman primates.

In the Excel spreadsheet, for chemicals that induce tumors in more than one experiment, the LTD10 value is a Harmonic Mean. If there are both positive and negative experiments in a species, the negative results are ignored.

Additional information on each chemical is reported in the spreadsheet in several superscripts. For example, an “m” superscript indicates that there is more than one positive experiment on the test agent in the species, and an “n” indicates that while at least one published author evaluated the chemical as carcinogenic, none of the results in the species are statistically significant.

The Excel spreadsheet of LTD10 values has 3 worksheets: The first worksheet has introductory text and describes the superscripts. The second worksheet includes the superscripts, and the third worksheet excludes the superscripts in order to facilitate use of the LTD10 values for reading into databases or in further calculations.

See also the Summary Table by Chemical, which reports the target organs and the harmonic mean of TD50 in each species.

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Last updated: August 2, 2007

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