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IV.
DERIVATION OF CANCER POTENCY
Basis for Cancer Potency
The studies by Peto et al. (1982; 1984) were used by CDHS (1988) to derive the cancer potency for
NDEA. These studies utilized relatively large numbers of animals (60-240 per group) over a wide dose
range. The Peto et al. (1982) study contained more information about the dose-response at the low
range of experimental doses than the other studies described. Therefore, the cancer potency for NDEA
was calculated from the Peto et al. (1982) study even though the calculated value is lower than other
potency estimates from Lijinsky et al. (1981) or Habs and Schmahl (1980).
Methodology
The study by Peto et al. (1982) showed that several organ sites developed tumors in both sexes of rats
exposed to NDEA. The incidence of hepatocellular neoplasms (histological designation unknown) in
males resulted in the highest potency value when only the 6 lowest doses were considered. Water
consumption by male rats was reported by Peto et al. (1984) to be 41 mL/kg/day. Low-dose group
mortality did not differ significantly from that observed in the control group, therefore no time
corrections were applied to the calculation.
A linearized multistage procedure was used to estimate the cancer potency of NDEA from the Peto et
al. (1982) data in male Colworth rats (Crump et al., 1982). 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 body weight of
human or animal, in this case, 450 grams for male rats.
Using these relationships, a human cancer potency (q human ) of 36 [mg/kg-day] -1 was calculated for
NDEA (CDHS, 1988). An airborne unit risk factor of 1.0E-2 (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.
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