A Confirmed Extrarenal Birt-Hogg-Dubé-Associated Oncocytic Neoplasm.

Birt-Hogg-Dubé (BHD) syndrome is a rare inherited disease characterized by a variety of renal epithelial tumors and oncocytosis, with extrarenal manifestations primarily consisting of pulmonary cysts and cutaneous fibrofolliculomas. Here we report a unique case of a primary extrarenal BHD-associated oncocytic epithelial neoplasm which arose between the duodenum and head of the pancreas. The unusual morphology and immunoprofile of this lesion defied classification as any previously reported entity, despite an extensive diagnostic workup. The immunohistochemical and molecular features indicate the tumor was driven by FLCN loss, and thus a consequence of the underlying germline mutation with a somatic second hit. This tumor is the first reported example of an extrarenal BHD-associated oncocytic epithelial tumor driven by FLCN loss.