Background: The presence of epithelial layer along with myoepithelial layer is the basis of benignity for any breast lesion. This study sought to determine if the Value of cytokeratin 5/6 in benign and malignant breast lesions is diagnostic. Cytokeratins are intermediate filaments present in all epithelial cells so it’s positivity in myoepithelial cells differentiate benign lesions from malignant lesions. Material and Methods: Hospital based observational study, carried out on biopsy samples received. Tissue were fixed in formal saline, paraffine blocks were prepared and stained with Haematoxylin and Eosin (H&E). Immunohistochemical (IHC) staining for cytokeratin 5/6 was applied on paraffin embedded sections of 40 samples using avidine biotin peroxidase technique with mouse antihuman polyclonal D5/16B4 antibody and visualized with DAB. The pattern and intensity of staining of tumor cells was observed and graded. Findings: 40 cases of breast lesions were taken, out of which 26 were benign and 14 were malignant. All 26(65%) cases of benign lesions comprising of fibroadenoma and fibrocystic disease were strongly positive. Out of 14(35%) cases 13 cases of IDC(NOS) and 1 case of medullary carcinoma breast 07 (53.8%)cases of IDC(NOS) were CK 5/6 positive with variable staining index. Benign lesions were strongly positive as compare to malignant lesions. Conclusion: Benign lesions are strongly positive for CK5/6 but if malignant lesions are positive for CK5/6, it indicates poor outcome due to basal molecular subtype and have an aggressive behavior.

Keywords: Breast lesions; Cytokeratin 5/6; Immunohistochemical staining; Observational study; Fibroadenoma, fibrocystic disease.