A Unique Case of Mediastinal Teratoma with Mature Pancreatic Tissue, Nesidioblastosis, and Aberrant Islet Differentiation: a Case Report and Literature Review.

Mediastinal teratomas with elements of mature pancreatic tissue are rare. Only a very few cases of pancreatic tissue with nesidioblastosis in teratoma have been reported. Here, we report a case of a 12-year-old male who presented with pleural effusion and was revealed to have a large anterior mediastinal mass. Biopsy of the mass revealed benign mature teratoma. After biopsy, the teratoma ruptured into the right thoracic cavity. It was then excised and sent to pathology for further evaluation. Preoperatively, there was no evidence of hyperinsulinemia or hypoglycemia. Postoperatively, there was no change in blood glucose levels. Histologically, the mass showed large areas of mature pancreatic tissue flanking a small intestine-like structure. Numerous endocrine cell islets, poorly defined groups of neuroendocrine cells and ductular-insular complexes characteristic of nesidioblastosis were dispersed in the exocrine pancreatic parenchyma. In addition, other parts of the tumor containing keratinizing squamous epithelium with cutaneous adnexal glands, small intestine, and bronchus including cartilage and respiratory epithelium were observed. Some islets contained two or more cell types while others were monophenotypic. Immunohistochemical staining showed pronounced expression of pancreatic polypeptide, moderate expression of somatostatin and insulin and nearly complete absence of glucagon-containing cells. The selective deletion of glucagon might hold clues to an important regulatory mechanism in pancreatic development.