Breast Cancer-Specific Mortality in Small-Sized Tumor with Stage IV Breast Cancer: A Population-Based Study

Background Small-sized primary tumor does not always indicate a better prognosis. We hypothesized that very small primary breast tumors with extensive lymph node (LN) metastases represented an aggressive biologic behavior in stage IV disease. Materials and Methods Data between 2010 and 2015 were retrieved retrospectively from the Surveillance, Epidemiology, and End Results database with inclusion criteria of female sex, unilateral, metastatic, and T1/2 invasive ductal carcinoma. Primary study variables included T stage, N stage, grade, metastatic sites, number of involved sites, estrogen receptor status, progesterone receptor status, and human epidermal growth factor receptor 2 status. Kaplan-Meier and adjusted Cox proportional hazards models with interaction terms were used. One-, 2- and 3-year breast cancer-specific mortality (BCSM) was examined according to tumor size. Results We identified 5,340 eligible patients with breast cancer. In multivariate analysis, race, age, grade, molecular subtype, surgery, brain metastases, and liver metastases were found to be independently associated with BCSM. For T1 tumors, the N0, N1, and N2+ groups had the same BCSM. In tumors smaller than 50 mm, the 1-, 2-, and 3-year BCSM did not decline with the decrease of tumor size. For triple-negative breast cancers (TNBCs), the T1a/T1bN2+ group had significantly worse BCSM than any other group did. Conclusion Patients with stage IV cancer with small-sized tumors may have BCSM as high as those with larger tumors. In TNBCs, very small tumors with severe LN involvement are associated with the worst BCSM. Continued efforts are needed to further investigate Ta1/T1bN2 + M1 TNBCs and individualize the treatment for affected patients. Implications for Practice This study revealed that for stage IV breast cancer, smaller primary tumors were not always associated with better breast cancer-specific mortality. This study illustrated that very small triple-negative breast cancers (TNBCs) with extensive regional lymph node involvement may be a surrogate for biologically aggressive disease. Because of poor prognosis of T1a/T1bN2+ TNBCs, there might be an urgent need of more individualized treatment for affected patients. Future correlative studies ought to focus on the genetic and molecular differences in Ta1/T1bN2+ TNBCs that contribute to the biological behavior. Clarification of the regulation mechanism of very small-sized primary TNBCs with metastatic outgrowth in nodes and distant sites will play an integral role in developing targeted therapies.