Joint International Conference on Long-term Experiments ...

The question of determining a “safe” dose has been badly distorted in the statistical
literature. Traditionally, toxicologists did not determine a “safe” dose by considering doses
at which no animals had a particular lesion. It was not a case of naively assuming that
seeing nothing in a small number of animals meant that nothing would occur. Instead, the
toxicologist examined the high-dose animals to determine a pattern of lesions that could be
associated with the compound at test. Then, he or she projected a set of putative precursors
of this syndrome of lesions. The traditional “maximum safe dose” was the dose at which
none of the lesions nor any of their putative precursors were seen in any animal. The
statisticians assumed that each animal was given a 0 (no lesion) or 1 (lesion). In fact, each
animal is scored in a complicated but biologically meaningful way. A “safe” dose was one
in which all the animals had scores less than the least score that could be associated with a
treatment effect.
The first toxicological problem, describe the “toxic” effect as a function of dose, is a
much more difficult one. We have addressed this problem elsewhere but only by suggesting
multivariate statistical methods that might be useful. If we do not use maximum tolerated
doses and reduce the length of studies to 12 months, then we may be able to describe a dose
response for frank carcinogens like the nitrosamines. But, when we use very high doses and
follow the animals for most of their natural life span, so that the “effect” is to shift lesion
patterns, increasing the incidence of one and decreasing the incidence of another, there is no
clear “direction” to the graph of response versus dose. To “solve” this problem, we will
need to think of dose response in some different fashion or we will have to abandon the
current design of these studies because they produce uninterpretable results.
APPENDIX: STATISTICAL METHODOLOGY
The individual animal tabulations in many of the NTP studies define organ and organsystems
that were routinely examined. In addition, there is a category of any organ not
routinely examined but for which histopathology was run because of a grossly observable
lump or bump. In the tabulation, individual neoplastic lesions are described in terms of
tissues type, degree of malignancy, and (possible) source. Tissue type designations tend to
be pathologist specific, so these were ingnored in this analysis. Instead, for each organ or
organ system a given animal was scored as follows:
0= no neoplastic lesion
1= worst neoplastic lesion was benign
2= worst neoplastic lesion was malignant but in situ
3= worst neoplastic lesion was invasive or metastatic
This produced a vector of 20-25 numbers for each animal (depending on sex and
species). An additional component was added by including the number of weeks on trial for
that individual animal.
All the animals in a given sex X species X AZO dye group, including the controls, the lowdose
animals, and the high-dose animals, were combined, and the first and second principal
components were computed for the 150 points.
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