Unilateral Ovarian Metastasis from an Occult Breast Primary Presenting as a Primary Ovarian Neoplasm: A Case Report

Metastatic ovarian neoplasms may closely mimic primary ones, particularly when unilateral. Metastasis from breast carcinoma is recognised, but presentation as an isolated ovarian mass without an identifiable primary lesion is rare and diagnostically challenging. A 43-year-old female presented with a unilateral right adnexal mass, clinically and radiologically suggestive of primary ovarian malignancy. Histopathological examination revealed a tumour composed of monomorphic cells with focal signet ring morphology, raising differential diagnoses including primary ovarian tumours and metastatic carcinoma. A systematic immunohistochemical panel was applied. Tumour cells were negative for calretinin and inhibin, excluding sex cord-stromal tumours, including signet ring stromal tumours, and negative for Napsin A, ruling out the possibility of clear cell carcinoma. Absence of Cytokeratin 20 (CK20) and CDX2 expression argued against a gastrointestinal origin. Diffuse CK7 and Epithelial Membrane Antigen (EMA) positivity prompted further evaluation, which demonstrated strong Oestrogen Receptor (ER), Progesterone Receptor (PR) and GATA-binding protein 3 (GATA3) expression along with loss of E-cadherin. These findings were consistent with metastatic invasive lobular carcinoma of the breast. Subsequent clinical and radiological evaluation failed to identify a primary breast lesion, confirming occult breast carcinoma. This case demonstrates the importance of immunohistochemistry in resolving diagnostic dilemmas and highlights that metastatic disease should be considered even in unilateral ovarian tumours with ambiguous morphology.