Risk factors for breast cancer by mode of diagnosis: some results from a breast cancer screening study.

We have investigated factors affecting the probability that a woman with breast cancer participating in a mammographic screening programme will be diagnosed by the screen. Data from a large American case-control study, with subjects drawn from women participating in an annual screening programme, were used. During the screening programme, 409 cases were identified, the mode of diagnosis being screen detection for 331 and interval detection for 78. No significant relationships were found between mode of diagnosis and age, age at menarche, oral contraceptive use, age at first live birth, age at menopause or history of maternal breast cancer. There was a non-significant trend for particular mammographic patterns to be associated with interval detection. However relative risk of breast cancer and probability of interval detection were observed to increase about the time of the menopause. These results suggest that the 3 yearly mammography programme being introduced in the UK might be improved if an extra examination was included around the time of the menopause.