A case control study of breast cancer risk and exposure to injectable progestogen contraceptives. Methods and patterns of use among controls

Abstract

Objective: To describe the patterns of use of injectable progestogen contraceptives (IPCs) among coloured and black women in the Western Cape. These data are part of an ongoing study in the Western Cape, the main aim of which is to explore the relationship between IPCs and breast cancer.

Design: A population-based case-control study of breast cancer risk in relation to the use of IPCs among coloured and black women.

Setting: The Western Cape, including the Cape metropole and surrounding rural areas.

Study subjects: All coloured and black women with newly diagnosed breast cancer, resident in the study area and below age 55 years, who present at either of the two tertiary care hospitals in the Western Cape are recruited. Controls are a sample of hospitalised patients representative of the populations from which the patients are drawn. Cases are frequency-matched to controls according to cross-tabulation of age, ethnic group and residential area in a ratio of approximately 1:3.

Measurements: Questionnaires are administered by trained nurse interviewers, information is elicited on a wide range of variables, including sociodemographic variables, medical history, family history of breast disease, lifetime history of all methods of contraception and use of non-contraceptive female steroids, reproductive variables, cigarette smoking, alcohol consumption and other potentially confounding variables.

Results: Between January and December 1994, 122 incident cases and 389 controls were enrolled. Ever-use of IPCs among the controls was 72% (N = 280) and use for 5 years or more was 30% (N = 117). Use of IPCs in the distant past was common, with 61% (N = 232) of all controls having initiated use 10 or more years previously. Current use was also high (19%). Other contraceptive methods were used far less commonly.

Conclusion: Coloured and black women in South Africa have been using and continue to use IPCs for more commonly and for longer periods than women anywhere else in the world. It is therefore especially important to evaluate the risk of breast cancer and other health effects of IPCs. The rates of use identified in this study ensure that there will be adequate statistical power to evaluate long-term use, use in the distant past and current use of IPCs.