Primary Cutaneous Mucinous Carcinoma: A Case with Diagnostic Dilemma

Abstract Introduction: Primary cutaneous mucinous carcinoma (PCMC) is a rare slow-growing neoplasm of the skin with an indolent behaviour. PCMC is slightly more common in men than women and typically affects people at 50 to 70 years old. The usual clinical presentation of PCMC in adults is asymptomatic, solitary erythematous nodule, papule, or cyst of variable sizes typically occurring on the face, scalp or in the axilla. Histologically, PCMC is indistinguishable from metastatic mucinous adenocarcinoma of breasts, gastrointestinal tracts, lungs, and salivary glands. This case report presents a case of PCMC, mimicking a skin metastasis from a male breast cancer. Case Presentation: A 43-year-old Indian gentleman presented with large matted left axillary lymph nodes. Clinical examination showed a wart-like skin lesion (2x2 cm) at the medial aspect of the left axillary region, with multiple matted enlarged left axillary lymph nodes. No abnormalities on bilateral breast examination. He then underwent wide local excision of the skin lesion and excision of the matted left axillary lymph nodes. The axillary skin lesion showed adenocarcinoma with mucinous differentiation and the excised lymph nodes confirmed metastatic mucinous adenocarcinoma. The immunohistochemistry (IHC) of the tumour was positive for mammaglobin, oestrogen receptor (ER), progesterone receptor (PR) and GATA-3. Hence, the initial diagnosis of metastatic primary breast carcinoma was favoured; despite no breast tissue was identified histologically from the axillary specimen. Given this diagnostic dilemma, the slides were sent for a second opinion to the histopathology experts. After further evaluation and multidisciplinary team discussion, the final diagnosis for him was PCMC. Conclusion: PCMC is a rare tumour with challenging clinical presentation and histological diagnostic dilemma. Therefore, a multidisciplinary team approach with strong clinical and radiological correlation is warranted to differentiate PCMC from a case of metastatic breast carcinoma, as the IHC may overlap.