DNA flow cytometric analysis indicates that many breast cancers detected in the first round of mammographic screening have a low malignant potential.

An organized mammographic screening program covered 8,690 women from selected birth cohorts (aged 50-59) in the Tampere University Central Hospital district. Forty-four breast cancer cases were detected in the first round of mammographic screening, which is 3.7 times the expected annual number of new cases in this population. To evaluate the proliferative kinetics and biological properties of these cancers, DNA flow cytometric analysis was carried out in 37 of the screen detected cancers (SDCs) using 60 clinically detected stage I-II cancers and 30 screen-detected benign lesions as reference. DNA aneuploidy was observed in 17/37 (46%) of the SDCs as compared to 41/60 (68%) in the clinical controls (p less than 0.05), while all the benign lesions were DNA-diploid. The median S-phase fraction (SPF) in the SDCs was significantly (p less than 0.001) lower (3.5%) than in the clinical controls (9.6%). Differences in SPF persisted in subgroups defined by DNA ploidy and histological type. In stage-I SDCs the median SPF value (2.5%) approached that of benign tumors (1.9%). Our epidemiological and biological data indicate that the first round of mammography predominantly detects prevalent preclinical lesions, some of which are of very low malignant potential. At present such patients may often receive too extensive treatment. DNA flow cytometry could help in the identification of cases which could be treated, for example, by breast-conserving methods.