Pineal gland differentiation in mature teratoma: An under-recognized condition and potential pitfalls for overdiagnosis.

We herein report three cases of mature teratomas with pineal gland differentiation, which is a less recognized phenomenon. Case 1 was a 6-year-old male with a neck mass, Case 2 was a 23-year-old female with a retroperitoneal mass, and Case 3 was a 45-year-old female with a retroperitoneal mass. Each case showed the typical macroscopic and histological findings of mature teratoma, such as solid and cystic lesions mainly lined with a mature squamous epithelium. All cases also showed glial differentiation. Small foci of lobulated cell nests were detected in the center of or adjacent to mature glial tissue. Cells had a clear to pale eosinophilic cytoplasm with small round nuclei. Immunohistochemically, cells were positive for synaptophysin, neurofilament protein with a perivascular "club-shaped swelling" pattern, and cone-rod homeobox protein. To the best of our knowledge, this is the first report of pineal gland differentiation arising in mature teratoma, which may be easily overlooked or misdiagnosed as somatic-type tumors, particularly neuroendocrine tumors. To avoid overtreatment, pathologists need to be aware that pineal gland differentiation may occur in mature teratomas.