Summary Between 1946 and 1976 over 9,000 women with breast cancer were seen within one year of diagnosis at the A. Maxwell Evans Clinic (AMEC) in Vancouver, British Columbia. By 1978, 275 had a subsequent diagnosis of a second primary in the contralateral breast: 100 were diagnosed within 1 year, and 175 after 1 year of the first primary. Two separate comparison groups of AMEC patients with unilateral breast cancer were selected to identify risk factors for bilateral breast cancer and to determine the incidence. The average annual incidence rates for a second primary in the contralateral breast were 5.0, 4.1 and 3.0 per 1,000 women for women less than 45 years, 45-54 years, and over 55 years of age at diagnosis of first primary breast cancer, respectively. These rates remained stable for at least 15 years after the diagnosis of the first primary. Two risk factors were found for bilateral cancer within 1 year of the first primary, histologic diagnosis of lobular carcinoma and absence of pathologic involvement of axillary nodes; one risk factor was found for bilateral breast cancer after 1 year of the first primary, family history of breast cancer.With longer survival rates from breast cancer the risk increases that a woman will develop a second breast malignancy. Information is needed about this likelihood and the characteristics of women at high risk, especially when considering such issues as prophylactic contralateral mastectomy. This study was undertaken to determine factors which influence the incidence of second primary tumours of the contralateral breast. MethodOver 9,000 women had their first histologically confirmed primary breast cancer diagnosed between 1946 and 1976 and were registered for treatment within one year of diagnosis at the A. Maxwell Evans Clinic (AMEC) in Vancouver, British Columbia. Two hundred and seventy-five of these women subsequently had an invasive second primary diagnosed in the contralateral breast prior to November 1978 and they comprised the bilateral cancer group ("cases"). Two comparison groups ("controls") were selected from the remaining women with unilateral breast cancer: one comparison group, the "5% sample", was a randon 5% sample of these women; the second comparison group, the "matched sample", consisted of 275 women individually matched with the cases by age (±2 years), year of diagnosis of the first primary (± 1 year) and survival (required to be greater than the elapsed time between the diagnosis of the first and second primary breast cancers in the case).The medical records of the women in the bilateral cancer group and two comparison groups were reviewed in 1978, restricting the observations on risk factors to those available at the time of referral for the first breast primary. None were lost to follow-up. Information was collected on recognised risk factors for both unilateral and bilateral breast cancer and clinical details of the first primary breast cancer. A case-control study design was then used to identify the risk factors associated with bilateral brea...
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