Trends in breast cancer incidence in Greater Bombay: an epidemiological assessment.

Reliable data from the Bombay Cancer Registry show an increase in the age-adjusted incidence rate of breast cancer in women from 17.9 to 24.9 per 100,000 population between 1965 and 1985. By fitting a linear regression model based on the logarithm of the breast cancer incidence rates, it was found that the average percentage increases in crude, age-adjusted and truncated rates over this 20-year period were highly significant. Evaluation of these trends in the light of proven etiological factors suggests that the increase in breast cancer incidence is related to a gradual decrease in the proportion of women having a first child before 20 years of age and to an increase in the proportion of "never married" women. These findings were also applicable to the subgroup of Hindus (70% of the population) who show a significant increase in breast cancer over this period as well as a clear cohort effect, the younger birth cohorts in general having higher rates than the 5-year older cohorts. However, the Muslim and Christian subgroups were found to have stable rates. An earlier study of cervical cancer incidence over the same period showed stable rates among Muslims and Christians but a declining trend among Hindus. Breast cancer is now the leading cancer in women in Bombay, while cancer of the cervix uteri predominates in the rest of the country. Well-designed epidemiological studies are urgently needed to explain the phenomenon and to help control the increase in breast cancer before it assumes the magnitude observed in the developed countries.