Racial differences in the risk of invasive squamous-cell cervical cancer

To investigate reasons for the higher rates of invasive squamous-cell cervical carcinoma among Blacks than Whites in the United States, we examined data from a case-control study of cervical cancer conducted in five geographic areas of the US, supplemented by incidence data from the Surveillance, Epidemiology, and End Results (SEER) Program, and hysterectomy prevalence data from the Cancer and Steroid Hormone Study. We observed only minor differences between Blacks and Whites in the magnitude of relative risks associated with a long interval since last Pap smear, multiple sexual partners, cigarette smoking, a higher number of births, and low levels of income and education. Thus, differences in the strength of associations contributed little to the higher incidence rate in Blacks, but the prevalence of these risk factors, except for cigarette smoking, was higher in Blacks than Whites. The SEER incidence rate ratio of 2.3 for Blacks compared to whites was increased to 2.7 when incidence rates utilized denominators corrected for prevalence of hysterectomy, while the rate difference increased from 14.9 to 25.8 cases per 100,000 person-years (PY). We estimated further that, after adjustment for prevalence of hysterectomy, the incidence rate for women at the lowest levels of exposure to the risk factors evaluated was 2.2 times higher in Blacks than Whites, but that the corresponding rate difference was only 2.2 cases per 100,000 PYs. Thus, our results suggest that racial differences in the prevalence of exposure to identified risk factors account for most of the difference in incidence rates. It remains to be determined what, as yet unidentified, aspects of lower socioeconomic status contribute to the higher incidence rate in Blacks.