Risk factors for contralateral breast cancer in Chennai (Madras), India.

<bold>Background: </bold>This is the first cohort study conducted in India to identify risk factors for contralateral breast cancer (CBC) among patients with first primary breast cancer.<bold>Methods: </bold>Patients with first primary breast cancer diagnosed in 1960-1989 at the Cancer Institute (WIA) in Chennai, India, were followed-up until 31 December 1994. The risk of CBC was assessed among unilateral breast cancer (UBC) patients who survived for >12 months following the diagnosis of breast cancer and did not develop a second cancer (n = 2665) and among those who developed a CBC > or =12 months after the diagnosis of breast cancer (n = 39).<bold>Results: </bold>The age-adjusted incidence of CBC among women with UBC was seven times the incidence (per single breast) in the general population. Among women with UBC the relative risk (RR) was 4.5 (95% CI: 1.1-19.6) comparing those with and without a history of breast cancer in the mother, and 2.8 (95% CI: 1.2-6.7) comparing age at first birth 21-25 versus earlier. The RR was 0.3 (95% CI: 0.1-0.6) comparing those with and without hormone therapy for their UBC. Radiotherapy for the UBC had no significant effect on the incidence of CBC.<bold>Conclusion: </bold>Positive family history of breast cancer and later age at first childbirth emerged as stronger risk factors for CBC than UBC. Hormone therapy reduces the risk of CBC. [ABSTRACT FROM AUTHOR]