Increasing incidence and constant mortality rates of breast cancer: time trends in Stockholm County 1961-1973.

This study describes trends in breast cancer incidence and survival in Stockholm County during 1961-1973. A discrepancy between increasing incidence and constant mortality rates was reflected in a significantly improved survival of the more recently treated patients. However, no change in survival was found when the patients were classified by axillary node status. The improvement thus seemed entirely the result of a more favorable stage distribution. The shift towards less advanced tumors was mainly caused by an increased age-standardized incidence of node-negative tumors, whereas the incidence of more advanced tumors seemed relatively unchanged. The survival from first distant metastasis was significantly increased; the use of combination chemotherapy might have contributed to this. The increase, however, was only moderate and did not seem to have contributed much to the improved overall survival. It is concluded that several confounding factors must be recognized when time trends in breast cancer are analyzed. A straightforward interpretation of observed changes is therefore not always possible. An increasing detection of tumors with relatively benign biological properties or lead time bias may well contribute to seemingly improved results.