Prognostic implications of estrogen receptor pattern of both tumors in contralateral breast cancer

Estrogen receptor (ER) status is important for breast cancer survival, it is however unclear how prognosis of contralateral breast cancer (CBC) is affected by ER-status of the two tumors. We conducted a large, population-based study of ER-status of both tumors in CBC patients and its influence on prognosis. The cohort consisted of all women diagnosed with CBC in Stockholm, Sweden during 1976-2005, with information on ER-status from medical records (N = 933). Prognosis was modeled as incidence rates of distant metastasis via Poisson regression. The proportion of CBCs with both cancers of the same ER-status was significantly larger than expected by chance. For synchronous (simultaneous) cancers the prognosis was significantly affected by the combined ER-status of both tumors (p = 0.01). Compared to unilateral breast cancer patients the incidence rate ratio (IRR) for patients with double ER-positive tumors was 1.25 (95 % CI: 0.88-1.76), for ER-discordant tumors 2.19 (95 % CI: 1.18-4.08) and for double ER-negative tumors 3.95 (95 % CI: 1.77-8.81). For metachronous (non-simultaneous) cancers, women with double ER-positive tumors had similarly bad prognosis (IRR = 2.95; 95 % CI: 2.39-3.64) as women with double ER-negative tumors (IRR = 2.88; 95 % CI: 1.83-4.52). Both shorter time span between first and second cancer and endocrine therapy for the first cancer further worsened prognosis of women with double ER-positive metachronous CBC. For synchronous CBC patients, ER-pattern of both tumors is an important prognosticator, while among metachronous CBC patients, double ER-positive tumors confer equally bad prognosis as double ER-negative cancers. Our results indicate that this might be due to endocrine therapy resistance.