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IV.
DERIVATION OF CANCER POTENCY
Basis for Cancer Potency
Cancer potency factors for BCME were derived from male Sprague-Dawley rat respiratory tract tumor
data (Kuschner et al., 1975; Leong et al., 1981). Cancer potency values are based on the most
sensitive site, species and study demonstrating carcinogenicity of a particular chemical, unless other
evidence indicates that the value derived from that data set is not appropriate (CDHS, 1985). The
Kuschner et al. (1975) study used relatively high exposure levels of BCME. The exposure levels used
in the Leong et al. (1981) study were lower, and the dose-response exhibited is highly non-linear.
Therefore, a cancer potency estimated from the Leong et al. (1981) data set may be more
representative of low-dose rate potency. For low dose exposures to BCME (below 1 ppb), the
potency value was calculated from dose-response data published by Leong et al. (1981); for periodic
high dose exposures (at or above 1 ppb BCME), the potency was derived from the study by Kuschner
et al. (1975) (CDHS, 1988).
Methodology
Cancer potency factors (q 1
*
) were derived using a linearized multistage procedure (CDHS, 1985) with
the dose-response data for male Sprague-Dawley rat respiratory tract tumors (Kuschner et al., 1975)
and nasal tumors (Leong et al., 1981). Absorbed doses were calculated assuming complete absorption
of inhaled BCME, using an inspiration rate of 0.29 m 3 /day for Sprague-Dawley rats. The dose from a
continuous exposure to 1 ppb BCME (4.7 µg/m 3 ) would therefore be 1.36 µg/day, or 2.6 µg/kg-day.
The cancer potency factors (q 1 * ) derived from the Leong et al. (1981) and Kuschner (1975) data sets
were 8.9 (mg/kg/day) -1 and 47 (mg/kg/day) -1 , respectively. Surface area scaling was employed to
transform animal cancer potency factors to human cancer potency factors, using the relationship (q human
= q animal * (bw h / bw a ) 1/3 ), where q human is the human potency, q animal is the animal potency, and bw h and
bw a are the human and animal body weights, respectively. Body weight values used for humans and
Sprague-Dawley rats were 70 kg and 0.52 kg, respectively. The human cancer potency factors (q 1 * )
derived from the Leong et al. (1981) and Kuschner (1975) data sets were 45.6 (mg/kg/day) -1 and 240
(mg/kg/day) -1 , respectively. The unit risk factor was derived by OEHHA/ATES from the low dose
exposure cancer potency value using a reference human body weight of 70 kg and an inspiration rate of
20 m 3 /day.
V. REFERENCES
California Department of Health Services 1988. Risk Specific Intake Levels for the Proposition 65
Carcinogen: Bis(chloromethyl)ether. Reproductive and Cancer Hazard Assessment Section, Berkeley,
CA.
California Department of Health Services (CDHS) 1985. Guidelines for Chemical Carcinogen Risk
Assessment and Their Scientific Rationale. CDHS, Health and Welfare Agency, Sacramento, CA.
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