Growing Teratoma Syndrome Arising from a Pineal Mixed Germ Cell Tumor in a 12-Year-Old Boy.

Introduction: Pineal region tumors in children are rare and present significant diagnostic and therapeutic challenges due to their deep location, varied histology, and overlapping radiologic features. Mixed germ cell tumors (MGCTs) are particularly rare and may demonstrate complex clinical behavior. Case Presentation: We report the case of a 12-year-old boy who presented in August 2024 with progressive headache and nausea. Brain magnetic resonance imaging (MRI) revealed a large pineal mass. Biopsy with endoscopic third ventriculostomy revealed an MGCT composed of 90% immature teratoma and 10% germinoma, with a Ki-67 index of 25%. Serum alpha-fetoprotein (AFP) was mildly elevated (23.2 ng/mL), while beta-human chorionic gonadotropin levels were normal. The patient received six cycles of chemotherapy according to the ACNS1123 protocol for non-germinomatous germ cell tumors. The MRI after chemotherapy showed increased tumor size with cystic/necrotic components and normalization of AFP. Subsequent surgical resection revealed a mature teratoma without malignant elements, consistent with growing teratoma syndrome (GTS). Notably, the Ki-67 index had decreased from 25% at diagnosis to 5% after resection. Given the initial histology and lack of cerebrospinal fluid analysis, adjuvant craniospinal irradiation of 36 Gy with a 54-Gy boost to the tumor bed was delivered according to ACNS0122. Conclusion: The patient tolerated treatment well and remains in good clinical and radiological condition. This case underscores the importance of recognizing GTS and individualizing treatment in pediatric pineal MGCTs.