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

Objectives To assess the effects of mammographic parenchymal patterns on the 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 Anglia 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 2601 controls. Methods The mammographic parenchymal patterns of breast tissue were assessed according to Wolfe's classification. Statistical analysis was by conditional logistic regression. Results Overall, 67% of patients and 59% of controls were considered to have high risk pattern (P2+DY) mammogram. The risk associated with P2 or DY mammographic patterns compared with N1 was higher for interval cancers (odds ratios (ORs) 2.2 and 2.4 respectively) than for screen detected cancers (ORs 1.7 and 1.1 respectively). For interval cancers in the first 18 months after the last negative mammogram, the risk was particularly high (ORs 3.8 for 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 reduced for high risk parenchymal patterns which are associated with high grade cancers. Changes should aim at improving screening sensitivity for dense parenchymal patterns, and the diagnosis of high grade tumours.