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 in a population with mammographic screening and in the unscreened women within 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 were grouped by age (25-39, 40-49, 50-59, 60-69 and ≥ 70 years of age) and size of tumor (≤ 10 mm, > 10-≤ 15 mm, and > 15 mm). Poisson regression modeling was used to examine trends by age, tumor size, calendar year and availability of screening. Results. The incidence rate of breast cancer in the total population increased between 1988 and 1996. The greatest increase was seen after 1993 when population-based screening became available. In unscreened women, modeling demonstrated a statistically significant (p < 0.01) 1.5% annual increase in the incidence rate. The annual increase in this rate differed by size of tumor and was approximately 8% (p < 0.01) for small tumors (≤ 10 mm) but not significant for tumors > 10 mm. The greatest increase was in small tumors for women ≥ 50 years of age. Conclusion: The incidence of breast cancer has increased since population-based mammographic screening was introduced in 1994. The rate in unscreened women also showed a significant increase. This was greatest in small tumors for women ≥ 50 years of age. Whether this will translate into an increase in mortality is uncertain and long-term monitoring is required to determine if cohort and period effects impact on the underlying incidence of breast cancer in Victoria.