A COMPARISON OF LIVER-TUMOR DIAGNOSES FROM 7 PCB STUDIES IN RATS

Through a policy assumption, all polychlorinated biphenyls (PCBs) are considered probable human carcinogens by most regulatory agencies base d on experimental studies in rodents where an increased incidence of l iver tumors has been observed. Recognizing that new consensus criteria for the diagnoses of liver tumors in rats had been promulgated, a ree valuation of liver tumor diagnoses from seven PCB studies in rats was undertaken. These seven studies, in which rats were fed PCB mixtures c ontaining 42, 54, or 60% chlorine, were considered to be the best stud ies from which to evaluate the cancer potential of PCB mixtures. The r eevaluation results, where consistent diagnoses now exist across all s tudies, clearly indicate major differences in carcinogenic potential b ased on degree of chlorination. Studies of mixtures with 602 chlorinat ion consistently resulted in a high incidence of liver tumors, whereas studies in which rats were fed mixtures with 54 or 42% chlorination s howed no statistically significant increases in liver tumors. These da ta indicate that continuation of a science policy of assuming that all PCBs are probable human carcinogens with a potency equivalent to the mixture that contains 60% chlorine has no scientific foundation and sh ould be reconsidered. (C) 1994 Academic Press, Inc.