We present a pediatric case of a rapidly expanding third ventricle germ cell tumor (GCT). A 14-year-old boy suffered from gradual-onset central diabetes insipidus (DI) and received desmopressin treatment. Magnetic resonance imaging (MRI) showed nonspecific findings of the pituitary-hypothalamic axis. Nine months after the initial DI diagnosis, he developed progressively worsening headache. MRI demonstrated a third ventricle tumor causing noncommunicating hydrocephalus, although an MRI 16 weeks before admission did not show the lesion. We performed gross total resection (GTR) of the tumor in 2 stages: a translamina terminalis approach and an extended transsphenoidal approach. The lesion was histologically diagnosed as immature teratoma with some germinoma. His noncommunicating hydrocephalus resolved after surgery. Through postoperative radiochemotherapy (whole ventricle: 23.4 Gy/13 fractions, tumor bed: 27.0 Gy/15 fractions, and 3 courses of carboplatin-etoposide), he has was in complete remission at the 3-year follow-up and has continued his high school program. This case suggests the following: (1) a mixed GCT originating from the neurohypophysis/infundibulum can show rapidly expansive growth in a child with central DI
(2) GTR and adjuvant radiochemotherapy can result in a good therapeutic outcome in rapidly expanding GCT
and (3) the extended transsphenoidal approach is a complementary approach to transcranial resection of anterior third ventricle GCTs.