Epidemiologic and molecular risk factors for contralateral breast cancer among young women

The strongest identified risk factor for second primary contralateral breast cancer (CBC) is the age at diagnosis of a woman's first primary breast cancer (Chen et al, 1999). The younger a woman is diagnosed with an initial breast cancer, the greater her risk is of developing CBC. In a cohort study of 41 109 US women diagnosed with a first breast cancer, women diagnosed before the age of 45 years had a 5.4-fold greater risk of CBC than expected. Women of 45–54 and 55+ years of age had 3.4- and 1.3-fold greater risks of CBC, respectively (Harvey and Brinton, 1985). Smaller studies from England (Prior and Waterhouse, 1978), Sweden (Adami et al, 1985), Japan (Murakami et al, 1987), Germany (Brenner et al, 1993), and Slovenia (Volk and Pompe-Kirn, 1997) have reported similar results, finding that women <45 or <50 years of age have 3.0–9.9-fold greater risks of CBC than expected. A family history of breast cancer and having a first breast tumour with a lobular histology have also been shown to increase CBC risk, while treatment of a first primary breast cancer with chemotherapy or tamoxifen has each been shown to decrease CBC risk (Chen et al, 1999). Although numerous studies have evaluated these and other risk factors for CBC, few have had sufficient power to evaluate epidemiologic or molecular risk factors among women diagnosed with a first breast cancer at a young age. Thus, little is known about what factors predispose young women diagnosed with a first breast cancer to develop CBC. To evaluate risk factors for CBC among women diagnosed with a first breast cancer at age younger than 45 years, we followed 1285 participants in a population-based case–control study, who were diagnosed with breast carcinoma from 1983 through 1992 in the Seattle-Puget Sound area of western Washington State for the occurrence of CBC.