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IV.
DERIVATION OF CANCER POTENCY
Basis for Cancer Potency
The study by NTP (1987) was chosen by CDHS (1988) as the key study for the development of a
cancer potency value for 1,4-DCB. In the NTP (1987) study, mice and rats exhibited significant
increases in several types of tumors. The mice were exposed for 5 days/week, resulting in average daily
doses of 0, 214, and 428 mg/kg/day 1,4-DCB. Mice of either sex exhibited a significant increase in
hepatocellular carcinomas or adenomas. The incidence of hepatocarcinomas or adenomas was 17/50,
22/49, and 40/50 in the control, 214, and 428 mg/kg/day groups, respectively. In addition, male rats
showed a significant increase in kidney adenomas and mononuclear cell leukemia. The cancer potency
for 1,4-DCB was calculated from the male mouse hepatocarcinoma and adenoma data.
Methodology
A linearized multistage procedure was used to estimate the cancer potency of 1,4-DCB from the NTP
(1987) data in male B6C3F1 mice (Crump et al., 1982). The concentrations of 1,4-DCB given in the
feed were 0, 214, or 428 mg/kg/day. The premature mortality of animals without tumors was
subtracted from the sample groups. The 95% upper confidence bound on the dose-response slope was
used to derive the human cancer potency value.
The animal cancer potency, q animal , was calculated from the linear slope using the lifetime scaling factor
q animal = q 1 * × (T/T e ) 3 , where T/T e is the ratio of the experimental duration to the lifetime of the animal.
In this case, the scaling factor was equal to 1. An estimated value for the human cancer potency was
determined using the relationship q human = q animal × (bw h /bw a ) 1/3 , where bw is the default body weight
of human or animal (mouse).
Using these relationships, a human cancer potency (q human ) of 0.04 [mg/kg-day] -1 was calculated
(CDHS, 1988). An airborne unit risk factor of 1.1E-5 (µg/m 3 ) -1 was calculated by OEHHA/ATES
from the q human value using the default parameters of 70 kg human body weight and 20 m 3 /day breathing
rate.
V. REFERENCES
California Department of Health Services (CDHS). 1985. Guidelines for Chemical Carcinogen Risk
Assessment and their Scientific Rationale. State of California Health and Welfare Agency, Department
of Health Services, 2151 Berkeley Way, Berkeley, CA..
California Department of Health Services (CDHS). 1988. Proposed Maximum Contaminant Level: 1,4-
Dichlorobenzene (para-Dichlorobenzene). Hazard Evaluation Section, Department of Health Services,
2151 Berkeley Way, Berkeley, CA.
Crump KS. 1982. An improved procedure for low-dose carcinogenic risk assessment from animal
data. J Environ Path Toxicol 5(2):675-684.
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