PRINCIPLES OF TOXICOLOGY

296 CHEMICAL CARCINOGENESIS
evaluated for cocarcinogenic responses rather than attributing all of the activity to the chemical being
tested.
Issues Associated with the Histopathological Examination
In some instances, perhaps more so in years past, the histopathological examination of the slides taken
from the control animals have not been examined as rigorously as those slides taken from the animals
administered the test compound. While at first it might seem that more attention should be paid to those
slides where the potential change is anticipated, this can lead to results that are an artifact of the
examination. For example, if all animals during the test became infected by a viral organism, and if
this infection affected the background cancer incidence in a particular organ of the animal, then placing
a greater emphasis on the “exposed” slides might lead one to reach erroneous conclusions. In this
situation the pathologist might identify more tumors in exposed animals simply because of the more
extensive microscopic search of the exposed tissues even though equivalent numbers of infectioninduced
tumors might exist in both control and exposed animals.
Other aspects of the histopathological examination may affect the outcome of the study. For
example, what organs should be examined? Should we evaluate organs like the Zymbal glands of rats
if humans have no anatomic correlate? What relevance should be attached to results where only benign
tumors, or tumors that behave benignly, are elicited? What relevance should be attached to a chemical
that increases the tumor incidence in one organ while decreasing the tumor incidences in other organs,
particularly if the total cancer/tumor risk of the animal group does not increase? Should we attach the
same significance to these results? (Note: Here the extrapolation to humans would essentially be no
net changes in the population’s risk of cancer.) As only one chemical example of this phenomenon,
PCBs, a chemical of considerable regulatory restriction and interest, has been observed in several
studies to produce liver tumors in rats, and relatively low exposure guidelines have been developed for
this chemical on the basis of such data. However, two general findings in these studies were that the
total tumor incidence in exposed animals was not increased (because the prevalence of other tumor
types were decreased) and that these tumors did not behave like malignant masses; in fact, the exposed
animals lived on average longer that did the control animals.
One final facet of this issue is the fact that over the years the pathological descriptions (criteria for
classifying pathological changes as tumors) have evolved. This means that chemicals using more
modern descriptions might be viewed as having lower tumor incidences than they would if their
evaluation occurred in years past. While this difference does not affect whether the test was considered
to have produced a positive finding for carcinogenicity, it does affect the tumor incidence reported in
the test, which, in turn, affects the perceived potency of the chemical as measured by the cancer slope
factors derived from the tumor incidence that was reported. Thus, the perceived potency of a chemical
carcinogen, as measured by its cancer slope factor, may differ according to which pathological criteria
were used.
Dietary and Caloric Restrictions
Over the years we have come to realize that nutritional status and caloric intake during the test can
affect the test results. In most test protocols the rats are fed ad libitum; that is, they are given constant
access to food in the cage. Given the already restricted activities that can occur within these cages and
the propensity of animals to eat as often as allowed, the animals tested under these conditions are
generally obese animals during much of their lifetime. Studies with a number of different chemicals
have shown that obesity can inflate the final tumor incidence that is observed; that is, there are a number
of chemicals that, when administered at the same dose, will result in those animals placed on a normal
caloric or restricted caloric intake to have significantly lower tumor rates than observed in animals fed
ad libitum. Since ad libitum feeding is the general rule in chronic animal test procedures, the results
of many studies have been inflated or possibly made statistically significant by the mere fact that the
animals were allowed to ingest more food than their bodies need. Similarly, some chemicals might