Incidence and Prognosis of Synchronous and Metachronous Bilateral Breast Cancer

<jats:sec><jats:title>Purpose</jats:title><jats:p> Because the incidence of breast cancer is increasing and prognosis is improving, a growing number of women are at risk of developing bilateral disease. Little is known, however, about incidence trends and prognostic features of bilateral breast cancer. </jats:p></jats:sec><jats:sec><jats:title>Patients and Methods</jats:title><jats:p> Among 123,757 women with a primary breast cancer diagnosed in Sweden from 1970 to 2000, a total of 6,550 developed bilateral breast cancer. We separated synchronous (diagnosed within 3 months after a first breast cancer) and metachronous bilateral cancer, and analyzed incidence and mortality rates of breast cancer using Poisson regression models. </jats:p></jats:sec><jats:sec><jats:title>Results</jats:title><jats:p> The incidence of synchronous breast cancer increased by age and by 40% during the 1970s, whereas the incidence of metachronous cancer decreased by age and by approximately 30% since the early 1980s, most likely due to increasing use of adjuvant therapy. Women who developed bilateral cancer within 5 years and at age younger than 50 years were 3.9 times (95% CI, 3.5 to 4.5) more likely to die as a result of breast cancer than women with unilateral cancer. Women with a bilateral cancer diagnosed more than 10 years after the first cancer had a prognosis similar to that of a unilateral breast cancer. Adjuvant chemotherapy of primary cancer is a predictor of poor survival after diagnosis of early metachronous cancers. </jats:p></jats:sec><jats:sec><jats:title>Conclusion</jats:title><jats:p> We found profound differences in the incidence trends and prognostic outlook between synchronous and metachronous bilateral breast cancer diagnosed at different ages. Adjuvant chemotherapy therapy has a dual effect on metachronous cancer: it reduces the risk, while at the same time it seems to worsen the prognosis. </jats:p></jats:sec>