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

Abstract

Background: 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.

Methods: 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).

Results: 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.

Conclusion: 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.

PIP: The 3492 women with a first primary breast cancer diagnosed in 1960-89 at the Cancer Institute in Chennai, India, were enrolled in a cohort study to identify risk factors for contralateral breast cancer. The 788 women who died or developed contralateral breast cancer within 12 months of initial breast cancer diagnosis were excluded from the cohort. 17,317 women-years of observation (mean follow-up time, 7.4 years) on the remaining women were accrued by the end of 1994. The incidence of metachronous contralateral breast cancer was 2.3/1000 women-years. The age-adjusted incidence of contralateral breast cancer among women with unilateral breast cancer was 7 times the incidence per single breast in the general population (relative risk (RR), 7.4; 95% confidence interval (CI), 4.8-11.4). The risk of bilateral breast cancer was significantly increased over that for primary breast cancer among women with an affected mother (RR, 4.5; 95% CI, 1.1-19.6) and those 21-25 years of age at first childbirth compared with younger women (RR, 2.8; 95% CI, 1.2-6.7). A significant negative association between contralateral breast cancer and hormone therapy was recorded (RR, 0.3; 95% CI, 0.1-0.6). The risk factors age at menarche, number of children, age at menopause, menopausal status, and radiotherapy for the primary breast cancer did not significantly differ for second compared with first breast cancers.