Trends in breast cancer aggressiveness before the introduction of mass screening in southeastern Netherlands 1975-1989.

Objective: The increased incidence of breast cancer in the southeastern Netherlands was accompanied by markedly improved relative survival and stable mortality. We investigated whether the average aggressiveness of tumors changed over time in a population-based study, before the introduction of mass screening.
Methods: The mitotic activity index (MAI) was determined retrospectively for 1051 consecutive patients diagnosed with invasive, non-metastatic breast cancer in 1975, 1981, 1988, and 1989. Trends over time, and effects of age, tumor size and lymph node status were examined by univariate and multivariate regressions.
Results: Age-adjusted incidence of low MAI tumors changed from 35/100,000 in 1975 to 45/100,000 in 1988-89, an increase of 30% (P = 0.01), the incidence of tumors with a high MAI increased about 20% (P = 0.28), from 25 to 29/100,000. For small tumors (T1) the odds for a high MAI was lower in 1981 (OR: 0.80; 95% CI: 0.37-1.73) and 1988-89 (OR: 0.66; 95% CI: 0.35-1.23) compared to 1975. Among T3 and T4 tumors the odds increased to 2.03 (95% Cl: 0.71-5.86) in 1981 and 2.16 (0.76-6.18) in 1988-89.
Conclusion: Although the incidence of tumors with low aggressive potential increased, the incidence of high MAI tumors also increased. Stable breast cancer mortality rates in the face of increasing incidence rates during the period 1975-89 cannot be attributed solely to changes in tumor aggressiveness; early diagnosis and better treatment may also have contributed.