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Abstract

A 65-year-old male with a history of pulmonary adenocarcinoma presented with new cutaneous nodules suspicious for metastases. The primary lung tumor was histologically confirmed as an acinar-pattern adenocarcinoma exhibiting central necrosis, spread through airspaces, lymphatic invasion, and satellite foci. Immunohistochemistry of the lung tumor showed cytokeratin 7 (CK7) positivity but was negative for thyroid transcription factor-1 (TTF-1), napsin A, cytokeratin 20 (CK20), CDX-2, and NKX3.1. This profile is atypical, as most lung adenocarcinomas express TTF-1 and napsin A. Biopsy of the cutaneous nodules revealed a dermal infiltrate of atypical gland-forming epithelial cells consistent with metastatic adenocarcinoma. Intracellular mucin was present in the tumor cells as was, on review, present in the original lung tumor. Immunophenotyping of the skin lesions matched the lung primary: CK7 positive, TTF-1 and napsin negative. This corroborated metastatic pulmonary origin despite absence of classic lung markers, with the knowledge that mucin producing lung adenocarcinomas are frequently negative for TTF-1. This case highlights a diagnostic pitfall for dermatopathologists when evaluating skin metastases from pulmonary adenocarcinoma with unusual immunoprofiles. The lack of TTF-1 and napsin A expression may cause confusion with upper gastrointestinal or pancreaticobiliary primaries, underscoring the importance of clinical correlation, thorough histomorphologic assessment, and broad immunohistochemical panels. Recognition of this rare immunophenotype pattern in cutaneous metastatic adenocarcinomas avoids misdiagnosis and guides proper patient management.