Background and purpose: Multifocal/multicentric (MF/MC) breast cancer is frequently encountered in clinical practice. Most studies comparing the heterogeneity of lesions are based on pathological outcomes after upfront surgery. We sought to describe the histological and immunohistochemical characteristics of each lesion in patients with MF/MC breast cancer diagnosed by core needle biopsy. Methods: We retrospectively reviewed the tumor type

grade

and immunohistochemical characteristics ［estrogen receptor (ER)

progesterone receptor (PR)

human epidermal growth factor receptor 2 (HER-2) and Ki-67］ of ipsilateral MF/MC breast cancer patients and assessed the heterogeneity among the lesions. Molecular phenotype was determined by immunohistochemistry. Results: A total of 75 MF/MC breast cancer patients were included. At the time of the manuscript writing

51 patients proved to be lymph node positive either by fine needle aspiration cytology or sentinel lymph node biopsy. Intertumoral heterogeneity in tumor type and grade was detected in 9 of 75 patients (12.0%). Intertumoral heterogeneity in ER

PR

 HER-2 and Ki-67 was detected in 1 (1.3%)

10 (13.3%)

1 (1.3%) and 9 (12.0%) patients

respectively. Intertumoral heterogeneity in molecular phenotype was detected in 10 patients (13.3%). Immunohistochemistry tests carried out according to current guideline may result in suboptimal treatment in 4 patients (5.3%). Conclusion: Histological and immunohistochemical heterogeneity can be found among lesions of ipsilateral MF/MC breast cancer diagnosed by core needle biopsy. Immunohistochemistry tests carried out in the index tumor only may result in suboptimal treatment in these patients.