What the Eyelid Can Tell You: The Unexpected Initial Presentation of De Novo Stage <scp>IV</scp> Breast Carcinoma

A 66-year-old female presented with seven months of progressive right upper eyelid (RUL) drooping and thickening of her right lower eyelid (RLL). MRI revealed soft tissue enhancement of the RUL and RLL pre-septal planes without posterior extension. Biopsy revealed poorly cohesive carcinoma infiltrating in a linear architectural pattern with foci of signet ring cell forms. Positivity for mucicarmine, keratin CAM5.2, CK7, GATA3, BRST-2, mammaglobin, and ER supported a metastatic breast carcinoma to the eyelid without a previously known primary site. E-cadherin and p120-catenin membranous staining was suggestive of a ductal breast carcinoma with lobular features as the initial presentation of de novo stage IV breast carcinoma. Subsequent follow-up with oncology revealed a palpable right breast mass with associated lymphadenopathy. Estrogen receptor PET scan showed disease in the right breast, right axilla, left cervical nodes, calvarium, and orbit. Biopsy of the right breast lesion confirmed a carcinoma histopathologically and immunohistochemically identical to that found in the eyelid biopsy. This case's histopathological features of invasive ductal breast carcinoma masquerading as invasive lobular carcinoma exemplify the challenging complexity of mixed disease.