Perifocal Inflammatory Reaction with Volume Fluctuation Caused by Diagnostic Radiation-Induced Regression in Germinoma Makes Histological Diagnosis Difficult despite Its Disappearance following Treatment: A Significant Pitfall and Countermeasures to It

We present a pediatric case of neurohypophyseal germinoma with a perifocal inflammatory reaction (PIR) with volume fluctuation caused by diagnostic radiation-induced regression (DRIR). On-target biopsy failed to confirm the histology because PIR hardly contained any germinoma cells. DRIR-related fluctuation of the tumor volume disguised germinoma as inflammation. We analyzed the cerebrospinal fluid (CSF) and detected a high level of placental alkaline phosphatase (PLAP), which demonstrated the neurohypophyseal lesion to be germinoma and brought the patient from successful radiochemotherapy up to complete remission. PIR adjacent to the germinoma (PIRAG) disappeared completely following radiochemotherapy, although it contained almost no germinoma cells. Examination of the CSF-PLAP level can complement the diagnosis of germinoma and will decrease the risk of misdiagnosis. Neurosurgeons should keep in mind PIRAG, DRIR, and the diagnostic value of CSF-PLAP when germinoma is suspected.