Incidence of metachronous contralateral breast cancer in the Canton of Zurich: a population-based study of the cancer registry

PURPOSE To examine the incidence and characteristics of metachronous contralateral breast cancer (CBC) among women in the Canton of Zurich, Switzerland. METHODS For 1980-2006, patients with unilateral invasive breast cancer (UBC) were analysed for metachronous CBC. Poisson's regression was used to estimate incidence rates of metachronous CBC according to age, year of diagnosis, follow-up period since first breast cancer and morphology. RESULTS Of 16,323 patients with UBC, 700 (4.3 %) developed a second malignant tumour of the opposite breast. Median age at first breast cancer was lower in the CBC group than in the full cohort. Median interval time between first and second breast cancer was 5.5 (interquartile range 2.6-10.1) years. Incidence rate at age 20-29 was 1006 (95 % confidence interval, CI, 452-2238) cases per 100,000 person-years and decreased to 299 (199-450) at 80-84. Age-adjusted incidence rates according to period of diagnosis decreased from 618 (530-721) for 1980-1984 to 329 (217-500) cases per 100,000 person-years for 2005-2006. Incidence rate ratio of CBC for lobular carcinoma was 1.28 (95 % CI 0.99-1.67) adjusted by age group and period of diagnosis compared to ductal carcinoma. CONCLUSIONS In our study, incidence rates for CBC are comparable with findings from the literature. A reduction in the incidence of metachronous CBC, thought to be due to adjuvant therapies, is seen in our data. In our cohort, younger age and lobular carcinoma were associated with an increased risk of CBC