Cytokeratin 7 and TRPS1 Staining in Basal Cell Carcinoma of the Breast in a Breast Cancer Patient: A Potential Diagnostic Pitfall

Summary: Breast carcinoma represents the most common visceral malignancy to metastasize to/invade the skin, and its recognition poses significant therapeutic and prognostic implications. A 56-year-old woman was referred to our institution for management of biopsy-proven breast carcinoma (Invasive carcinoma with lobular features, grade 1, ER+/PR+/HER2-equivocal). Breast MRI revealed a 4.8cm mass with extension to the nipple-areola. Examination additionally revealed a 6mm thin erythematous scaly papule, located 10cm lateral to the nipple. Due to concern for involvement by breast carcinoma, a punch biopsy was performed with pre-ordered cytokeratin 7(CK7), per protocol, revealing CK7-diffusely positive interconnecting basaloid strands extending from the epidermis, with peripheral palisading and scattered apoptotic bodies, arising in a loose stroma with associated inflammation. Additional workup showed patchy/weak TRPS1 staining, strong/diffuse p40 staining, and negative staining for ER, PR, and mammaglobin. The findings were most consistent with a superficial basal cell carcinoma. CK7 expression has been described in basal cell carcinoma arising on photoprotected sites with abundant apocrine glands, supporting theories that some of these lesions may arise from glandular epithelium. Similarly, although once considered specific for breast carcinoma, TRPS1 has been recently described in cutaneous neoplasms as well. Careful correlation with morphologic findings and additional stains is therefore important.