Effect of multiple invasive foci on breast cancer outcomes according to the molecular subtypes: a report from the Korean Breast Cancer Society.

BACKGROUND In this study, the prognostic impact of the presence of the multifocal or multicentric tumor (MMT) and its association with molecular subtypes were investigated. PATIENTS AND METHODS We investigated the breast cancer metastasis and survival in patients with multifocal or multicentric invasive foci in the same breast. The study population includes 2882 patients in the Seoul National University Hospital Breast Care Center (SNUHBCC) dataset and 41 179 patients in Korean Breast Cancer Registry (KBCR) dataset. RESULTS From SNUHBCC dataset, we observed a significant role of MMT in developing distant metastasis and death when the tumors were triple-negative subtype. This subtype-specific prognostic importance of MMT in overall survival was also seen in KBCR dataset (HR, 1.32; 95% CI, 1.02-1.69). In tumors <2 cm, the hazard ratios (HRs) for node metastasis and death were similar along the tumor size change in triple-negative subtype, while other subtypes showed a stepwise increment, suggesting the biologic importance of small invasive foci in this subtype. CONCLUSIONS Our results demonstrate the prognostic importance of MMT in patients with triple-negative breast cancers. Small additional invasive foci in triple-negative breast cancer patients should be considered as clinically relevant tumor deposits.