Important increase of invasive lobular breast cancer incidence in Geneva, Switzerland

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Abstract
A recent paper from the United States reported a sharp and unexplained increase in invasive lobular breast cancer incidence since 1977 (Li et al., Cancer 2000;88:2561–9). We investigated if this trend was also present in Geneva, Switzerland, where breast cancer incidence is one of the highest in Europe. We analyzed trends in breast cancer incidence according to histologic subtype, age and stage, to clarify the pattern. Our population‐based study includes all histologically confirmed invasive breast carcinomas (n = 6,247) recorded between 1976 and 1999 at the Geneva Cancer Registry. Breast histology was classified as ductal carcinoma, lobular carcinoma and other. Incidence trends were studied by log‐linear regression analyses. Models including effects of age, period and birth cohorts were used to describe rising incidence trends. The incidence of ductal carcinoma increased 1.2% per year (ptrend < 0.001) from 85.2 to 110.1/100,000. This increase concerned women aged 50–69 years and early‐stage tumors. Lobular cancer incidence increased disproportionately (14.4% per year, ptrend < 0.01) and rose from 2.9 to 20.5/100,000. This increase affected all age categories and both localized and advanced stages. In addition, a strong age‐cohort effect was present (p < 0.05), and women aged 50–59 years born after 1944 experienced the most marked increase. Our study shows a disproportionate increase of lobular breast cancer incidence compared to ductal cancer incidence. Contrary to ductal cancer, trends for lobular cancer are unlikely to be explained by increased use of screening mammography. Other explanations must be researched, in particular the role played by hormone replacement therapy.