Evaluation of breast cancer incidence: is the increase due entirely to mammographic screening?

Objectives: To examine the trends in the incidence rates of breast cancer i n a population with mammographic screening and in the unscreened women with in that population. Methods: Data consisted of incident cases of breast cancer notified to the Victorian Cancer Registry in Victoria, Australia, between 1988 and 1996 and cases detected in the population-based BreastScreen Program. These data we re grouped by age (25-39, 40-49, 50-59, 60-69 and greater than or equal to 70 years of age) and size of tumor (less than or equal to 10 mm, > 10-less than or equal to 15 mm, and > 15 mm). Poisson regression modeling was used to examine trends by age, tumor size, calendar year and availability of scr eening. Results: The incidence rate of breast cancer in the total population increa sed between 1988 and 1996. The greatest increase was seen after 1993 when p opulation-based screening became available. In unscreened women, modeling d emonstrated a statistically significant (p < 0.01) 1.5% annual increase in the incidence rate. The annual increase in this rate differed by size of tu mor and was approximately 8% (p < 0.01) for small tumors (less than or equa l to 10 mm) but not significant for tumors > 10 mm. The greatest increase w as in small tumors for women greater than or equal to 50 years of age. Conclusion: The incidence of breast cancer has increased since population-b ased mammographic screening was introduced in 1994. The rate in unscreened women also showed a significant increase. This was greatest in small tumors for women greater than or equal to 50 years of age. Whether this will tran slate into an increase in mortality is uncertain and long-term monitoring i s required to determine if cohort and period effects impact on the underlyi ng incidence of breast cancer in Victoria.