Case Report: Hidden danger in breast eczematous lesions: cutaneous metastasis from silent lung adenocarcinoma misdiagnosed as Paget's disease.

Background: Cutaneous metastasis from lung adenocarcinoma is uncommon and may clinically mimic primary dermatologic or breast diseases, particularly when it involves the nipple-areola complex. Such presentations are diagnostically challenging, especially in patients without respiratory symptoms or a smoking history. Case presentation: We report the case of a 69-year-old woman with no smoking history or pulmonary symptoms who presented with a rapidly enlarging erythematous, edematous, and vesicular plaque involving the left nipple-areola complex and adjacent breast skin. Breast ultrasonography suggested Paget's disease of the breast, and an initial diagnosis of mammary Paget's disease was entertained. However, the rapid progression, extensive vesiculation, and absence of an underlying breast mass prompted a skin biopsy. Histopathology revealed dermal infiltration by atypical adenocarcinoma cells. Immunohistochemistry showed positivity for cytokeratin 7, thyroid transcription factor-1, and Napsin A, and negativity for p63 and INSM1, supporting a diagnosis of metastatic lung adenocarcinoma. Chest computed tomography revealed a right upper lobe lung mass with multiple pulmonary metastases, pleural effusion, lymphadenopathy, and suspected bone metastasis. The final diagnosis was stage IVB lung adenocarcinoma with cutaneous metastasis. Molecular testing was declined, and the patient subsequently developed respiratory failure and died four days after intensive care unit admission. Conclusion: This case underscores that rapidly progressive nipple-areolar eczematous or vesicular lesions should not be presumed to represent Paget's disease, even in the absence of pulmonary symptoms. Early skin biopsy and multi-marker immunohistochemistry are essential to distinguish cutaneous metastasis from primary breast disease and facilitate timely oncologic evaluation.