Mammographic parenchymal patterns and mode of detection: implications for the breast screening programme

Objectives-To assess the effects of mammographic parenchymal patterns on th e risk of breast cancer detected at first screen, second screen, and in the interval between these two screens. Settings-A nested case-control study within a screening cohort in East Angl ia was designed. The study group comprised 502 patients with cancer at the prevalence screening round, 198 patients with interval cancer, and 175 with cancer at the first incidence screen. These patients were matched with 260 1 controls. Methods-The mammographic parenchymal patterns of breast tissue were assesse d according to Wolfe's classification. Statistical analysis was by conditio nal logistic regression. Results-Overall, 67% of patients and 59% of controls were considered to hav e high risk pattern (P2+DY) mammogram. The risk associated with P2 or DY ma mmographic patterns compared with N1 was higher for interval cancers (odds ratios (ORs) 2.2 and 2.4 respectively) than for screen detected cancers (OR s 1.7 and 1.1 respectively). For interval cancers in the first 18 months af ter the last negative mammogram, the risk was particularly high (ORs 3.8 fo r P2 and 4.1 for DY compared with N1). The high risk associated with P2 and DY patterns was concentrated on invasive ductal grade III cancers (ORs 2.7 and 3.8) rather than grade I or II cancers (ORs 1.6 and 1.2). Conclusions-The study strongly suggests that screening effectiveness is red uced for high risk parenchymal patterns which are associated with high grad e cancers. Changes should aim at improving screening sensitivity for dense parenchymal patterns, and the diagnosis of high grade tumours.