Basal Cell Carcinoma With Ductal Differentiation Presenting as a Large Unilocular Cyst on the Abdomen

We report an unusual case of basal cell carcinoma (BCC) with ductal differentiation presenting as a large unilocular cyst on the abdomen of an 83-year-old Japanese man. An initial punch biopsy showed part of the bilayered cyst wall, suggesting a sweat gland neoplasm. However, complete excision revealed a unilocular cyst continuous with underlying basaloid tumor nests showing peripheral palisading, stromal clefting, and partial BerEP4 expression, consistent with BCC. Both the luminal and outer layers of the cyst also showed partial BerEP4 expression. The luminal layer was positive for epithelial membrane antigen (EMA) and carcinoembryonic antigen, indicating that the unilocular cyst was a genuine cyst rather than a pseudocyst secondary to mucin deposition or tumor necrosis. The outer layer lacked myoepithelial markers including SRY-box transcription factor 10 and smooth muscle actin, suggesting that it represented part of the BCC rather than myoepithelium. Overall, we diagnosed the lesion as BCC with ductal differentiation forming a cyst with an EMA-positive lining. BCC with ductal differentiation has been reported, but cases with large unilocular cysts are uncommon. This case broadens the histologic spectrum of BCC and emphasizes the importance of complete excision and comprehensive immunohistochemical analysis in cystic lesions to avoid misdiagnosis.