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total); an untreated control group was included in the study (Gothoskar et al., 1970). Interim sacrifices
were performed at 6, 9 and 13 months. An increased incidence of liver tumors (hepatomas) was noted
in both males and females. Tumor incidence data is listed in Table 1.
Table 1.
Thioacetamide-induced hepatoma incidence in male and female Swiss mice
(Gothoskar et al., 1970)
Dose group Average dose 1
(mg/kg-day)
Tumor incidence
male control 0 0/6
male treated 36 6/6
female control 0 0/6
female treated 39 6/7
1. Doses as reported by Gold et al. (1984).
2. Tumor incidences as reported by Gold et al. (1984).
IV.
DERIVATION OF CANCER POTENCY
Basis for Cancer Potency
Gold et al. (1984) list results from the study of thioacetamide by Gothoskar et al. (1970) in male and
female Swiss mice. A total of 89 mice of both sexes were fed a diet containing 0.03% thioacetamide
for 6, 9, 13 or 17 months. The group studied for 17 months consisted of 12 control mice (6 male and 6
female) and 13 treated mice (6 male and 7 female). Hepatomas were seen in all treated male mice,
precluding estimation of the upper bound on potency in these animals. Females were slightly less
sensitive; six of the seven dosed female mice developed hepatomas. Because this is the only doseresponse
data available in Gold et al., the data for the females are used to derive potency (see Table 1).
The value presented here may be an underestimate, but is the best value currently available (Cal/EPA,
1992).
Methodology
Expedited Proposition 65 methodology (with cross-route extrapolation) was used to derive a cancer
potency factor. A unit risk factor was then calculated by OEHHA/ATES from the cancer potency
factor using a reference human body weight of 70 kg and an inspiration rate of 20 m 3 /day.
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