Desmoplastic Myxoid Tumor of the Pineal Region, SMARCB1-Mutant: Report of 2 Examples of an Extremely Rare Entity.

Desmoplastic myxoid tumor of the pineal region is a newly described tumor included in the CNS WHO 2021, characterized by distinctive location, histopathology, and immunophenotype. We describe 2 patients with this rare tumor. Our patients were 2 women, aged 22 years and 45 years, showing heterogeneously enhancing masses in the pineal region. On histology, the tumors exhibited areas of variable morphology, with the stroma containing myxoid and desmoplastic areas. The tumor cells also had variable morphology, including spindled cells and cells with epithelioid-rhabdoid morphology. Using immunohistochemistry, the tumor cells were positive for SMA, epithelial membrane antigen, and CD34, and were negative for glial fibrillary acidic protein, desmin, cone-rod homeobox, and brachyury. Synaptophysin was positive in 1 tumor. SMARCB1 (INI1) showed loss of nuclear expression. Both tumors showed increased Ki67 proliferative index. The histology and polyphenotypic nature of the tumor pose diagnostic dilemmas. In view of the rarity of the tumor, the management and prognosis of these patients are still not clear, emphasizing the need for reporting additional patients to bring about a better understanding of the nature of the tumor.