Metastatic Plasmacytoid Urothelial Carcinoma Masquerading as a Primary Signet Ring Cell Carcinoma of the Duodenum.

Plasmacytoid urothelial carcinoma can histologically mimic gastrointestinal signet ring cell carcinoma, a potential diagnostic pitfall resulting in improper clinical management. We present a rare case of a malignant duodenal ulcer due to metastasis from plasmacytoid urothelial carcinoma. Only by histological and retrospective immunohistochemical comparison with the primary bladder tumor was this revealed as a metastasis from a plasmacytoid urothelial carcinoma. This case report highlights the importance of clinical correlation and comparison with any previous pathology specimens, the limitations of immunohistochemical staining, and the utilization of both old and new immunohistochemical tools when differentiating signet ring cell carcinomas of primary sites versus potential metastases.