Primary cutaneous mucinous carcinoma: a case with diagnostic dilemma

Primary Cutaneous Mucinous Carcinoma (PCMC) is a rare adnexal malignancy with indolent behavior but significant diagnostic challenges, particularly in distinguishing it from metastatic mucinous adenocarcinoma. We report a diagnostically complex case of PCMC in a 43-year-old man, initially suspected to represent metastatic breast carcinoma. The patient presented with incidental left axillary lymphadenopathy detected during staging for a small bowel neuroendocrine tumor. Clinical examination revealed a long-standing, painless wart-like axillary skin lesion with recent serous discharge. Imaging demonstrated multiple axillary and intramammary lymph nodes without a definite breast primary. Histopathological analysis of the excised lesion showed mucinous adenocarcinoma with regional nodal metastasis. Immunohistochemistry revealed positivity for ER, PR, GATA3, and mammaglobin, raising strong suspicion for metastatic breast carcinoma. However, absence of a detectable breast lesion and lack of definitive mammary tissue origin created a diagnostic dilemma. Following multidisciplinary review and correlation of clinical, radiological, and pathological findings, a diagnosis of PCMC of the axilla was established. The patient underwent completion axillary lymph node dissection and was commenced on tamoxifen due to hormone receptor positivity. He remains disease-free after 18 months of follow-up. This case highlights the diagnostic difficulty of PCMC, particularly in atypical locations such as the axilla, where it closely mimics metastatic breast carcinoma. Accurate diagnosis requires comprehensive clinicopathological correlation. Recognition of this entity is essential to guide appropriate management.

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