[Contralateral breast cancer: metastasis or second primary cancer?]

Metachronous contralateral breast cancer (CBC) is defined as a tumour in the opposite breast which was diagnosed more than 6 months following the detection of the first cancer. We screened, for factors that might predict the risk of developing CBC, a cohort series of 4748 women who had invasive unilateral breast cancer, clinical stage I-IIIa and had been treated at Institut Curie (Paris) between 1981 and 1987. Two hundred and eighty two CBC had been diagnosed with a median follow-up of 80 months. The cumulative rate of CBC was 4.1% at 5 years. We studied relationships between CBC and family history of breast cancer, age at diagnosis of first cancer, menopausal status, tumour size, node involvement, histological type, Scarff Bloom Richardson grade, estrogen and progesterone receptor measurements, as well as the type of primary treatment. Due to competing risks between the occurrence of CBC, local recurrence and metastasis, several options for multivariate analysis were considered. In model I, we focused on the occurrence of CBC, and ignored others events. In model II, only CBC, if first site of failure was taken into account, and in model III we considered others events as time-dependant covariates. Whichever the model we chose, age less than 55 years (RR = 1.40) as well as the presence of lobular type carcinoma (RR = 1.50), were associated with an increased risk of developing a tumour in the contralateral breast. In contrast, the risk of CBC was significantly decreased by adjuvant chemotherapy (RR = 0.54). Neither tumor stage or lymph node involvement influence the risk of CBC. These results suggested that CBC is a second primary breast cancer.