A small primary tumor does not necessarily correspond to a favorable prognosis. We hypothesized that stage IV breast cancers with very small primary tumors accompanied by extensive lymph node metastases may reflect an aggressive tumor biology. Data spanning 2010 through 2015 were collected in a retrospective manner from the Surveillance, Epidemiology, and End Results (SEER) database. Eligible cases were limited to women with unilateral metastatic invasive ductal carcinoma classified as T1 or T2. Key variables examined in the study encompassed T stage, N stage, tumor differentiation grade, locations of metastases, count of metastatic sites, estrogen receptor (ER) status, progesterone receptor (PR) status, and HER2 status. Analyses involved Kaplan-Meier survival curves and multivariable Cox regression models that included interaction terms. Rates of breast cancer-specific mortality at 1, 2, and 3 years were assessed with respect to primary tumor size. A total of 5,340 eligible breast cancer patients were included in the analysis. Multivariate analysis identified race, age, tumor grade, molecular subtype, surgical treatment, and the presence of brain or liver metastases as independent predictors of breast cancer-specific mortality (BCSM). Among T1 tumors, patients with N0, N1, or N2+ disease exhibited similar BCSM. For tumors smaller than 50 mm, reductions in tumor size did not correspond to lower 1-, 2-, or 3-year BCSM. Notably, in triple-negative breast cancers (TNBCs), the T1a/T1bN2+ subgroup demonstrated significantly higher BCSM compared with all other groups. Stage IV breast cancer patients with small primary tumors may experience breast cancer-specific mortality (BCSM) comparable to those with larger tumors. In triple-negative breast cancers (TNBCs), very small tumors accompanied by extensive lymph node involvement are linked to the poorest BCSM. Further research is warranted to better understand T1a/T1bN2+ M1 TNBCs and to guide individualized treatment strategies for these patients. This study demonstrates that in stage IV breast cancer, smaller primary tumors do not necessarily confer a survival advantage in terms of breast cancer-specific mortality (BCSM). Notably, very small triple-negative breast cancers (TNBCs) with extensive regional lymph node involvement appear to reflect an aggressive tumor biology. Given the poor prognosis associated with T1a/T1bN2+ TNBCs, there is an urgent need for more individualized treatment strategies. Future studies should investigate the genetic and molecular characteristics underlying T1a/T1bN2+ TNBCs and clarify the mechanisms driving metastatic progression from small primary tumors, which may inform the development of targeted therapies.
