Intracranial growing teratoma syndrome following radiotherapy--an unusually fulminant course. 
Residual radiologically progressive masses following multimodality treatment of malignant mixed intracranial germ cell tumors are described.  Often these enlarge, and this is called the growing teratoma syndrome.  A similar phenomenon during radiotherapy alone has not been described.  A 5-year old boy presented with features of raised intracranial pressure.  Imaging revealed a posterior third ventricular mass, which was biopsied endoscopically.  A review of the scanty tissue was suggestive of a pineal parenchymal tumor, and hence radiation was planned.  After just ten fractions, he developed rapid neurological deterioration.  Repeat imaging raised a possibility of a teratomatous tumor.  He underwent emergency excision.  However, he had a stormy postoperative course and succumbed to deep venous infarction.  Histology revealed a purely mature teratoma.  Though a growing teratoma syndrome has been described following chemotherapy, no such report while on radiation exists.  Ours could be the first such reported case.  We discuss the possible mechanisms with a review of the literature.