Occurrence and outcome of de novo metastatic breast cancer by subtype in a large, diverse population.

Purpose: To examine the occurrence and outcomes of de novo metastatic (Stage IV) breast cancer, particularly with respect to tumor HER2 expression.

Methods: We studied all 6,268 de novo metastatic breast cancer cases diagnosed from 1 January 2005 to 31 December 2011 and reported to the California Cancer Registry. Molecular subtypes were classified according to HER2 and hormone receptor (HR, including estrogen and/or progesterone receptor) expression. Multivariable logistic regression was used to estimate odds ratios (ORs) and 95 % confidence intervals (CIs) of Stage IV versus Stage I-III breast cancer; Cox proportional hazards regression was used to assess relative hazard (RH) of mortality.

Results: Five percent of invasive breast cancer was metastatic at diagnosis. Compared to patients with earlier stage disease, patients with de novo metastatic disease were significantly more likely to have HER2+ tumors (HR+/HER2+: OR 1.29, 95 % CI 1.17-1.42; HR-/HER2+: OR 1.40, 95 %CI 1.25-1.57, vs. HR+/HER2-). Median survival improved over time, but varied substantially across race/ethnicity (Asians: 34 months; African Americans: 6 months), neighborhood socioeconomic status (SES) (highest: 34 months, lowest: 20 months), and molecular subtype (HR+/HER2+: 45 months; triple negative: 12 months). In a multivariable model, triple negative (RH 2.85, 95 % CI 2.50-3.24) and HR-/HER2+ (RH 1.60, 95 % CI 1.37-1.87) had worse, while HR+/HER2+ had similar, risk of all-cause death compared to HR+/HER2- breast cancer.

Conclusions: De novo metastatic breast cancer was more likely to be HER2+. Among metastatic tumors, those that were HER2+ had better survival than other subtypes.