Pineal parenchymal tumor of intermediate differentiation presenting with SIADH-related hyponatremia: a case report and diagnostic challenge

Abstract Pineal region tumors are rare, accounting for less than 1% of intracranial neoplasms, and often present significant diagnostic challenges. Among them, Pineal Parenchymal Tumors of Intermediate Differentiation (PPTID) represent an uncommon subtype with overlapping radiologic features that may mimic more common pineal lesions. We report the case of a 56-year-old male who presented with new-onset generalized tonic–clonic seizures, drowsiness, and altered sensorium secondary to severe hyponatremia. Magnetic resonance imaging revealed a well-defined lobulated mass in the pineal region with obstructive hydrocephalus, initially raising the radiologic suspicion of germinoma. The patient underwent midline suboccipital craniotomy with gross total resection of the lesion. Histopathological examination showed sheets of uniform small cells with attempted rosette formation and low mitotic activity. Immunohistochemistry demonstrated diffuse synaptophysin positivity with a low Ki-67 labeling index (~ 4%) and negativity for glial markers. Molecular testing revealed a KBTBD4 alteration, supporting the diagnosis of PPTID with low-grade histologic features. The patient subsequently received adjuvant focal radiotherapy to a total dose of 54 Gy in 30 fractions and has remained asymptomatic during six months of follow-up, with no evidence of recurrence on imaging. This case highlights the importance of integrating radiologic, histopathologic, and immunohistochemical findings for the accurate diagnosis of PPTID, as radiology alone may be misleading. Early recognition of this rare entity is essential for optimal surgical management and timely adjuvant therapy. Reporting such cases contributes to a better understanding of the clinical course, diagnostic nuances, and therapeutic outcomes associated with PPTID and helps strengthen the existing literature on pineal region tumors.