- A 29 month old boy presented with decreased activity and lethargy and was found to have a large, heterogeneous mass in his posterior fossa compressing the brainstem - He underwent a subtotal resection of the tumor, which was found to be difficult to fully remove due to its attachment to nearby structures - Post-operative imaging showed residual enhancing tumor and diffuse dural enhancement, likely due to intracranial hypotension from the subtotal resection - Pathology revealed the tumor was an atypical teratoid rhabdoid tumor (ATRT), a highly malignant tumor in young children that commonly arises in the posterior fossa

1. Harbor-UCLA Neuroradiology Case Conference Sahar Farzin, M.D. Harbor-UCLA Dept. of Radiology Torrance, CA

2. History: 29 month old boy with decreased activity and lethargy CT Brain without contrast: Ventriculomegaly with hypodensity in the periventricular white matter. Sulci are effaced. Right frontal ventriculostomy catheter in place with distal tip in the right frontal horn of the lateral ventricle. Ill-defined hyperdense mass with central areas of lower attenuation in the posterior fossa

3. Axial FLAIR MR: Ventriculomegaly with abnormal bright signal in the periventricular white matter.

4. Top row: Axial T1WIs without Gadolinium. Bottom row: Axial T1WIs post-Gadolinium.

6. Coronal T1WIs post-Gadolinium: Lobular, heterogeneously enhancing mass with central areas of low signal intensity seen in the midline posterior fossa, arising from the cerebellar vermis. 4 th ventricle is not seen. Ventriculomegaly again noted.

7. Sagital T1WI post-Gadolinium: Lobular, heterogeneously enhancing mass in the posterior fossa compressing and distorting the brainstem (anteriorly)

10. Status post suboccipital craniotomy and subtotal tumor resection T1WIs pre-Gadolinium. There is residual tumor in the superior aspect of the posterior fossa compressing the brainstem. Air and blood products in the operative bed more inferiorly. T1WIs post-Gadolinium. Residual enhancing tumor in the superior posterior fossa. Diffuse dural enhancement. Small amount of pneumocephalus in the operative bed.

11. Sagital T1WI post-Gadolinium. Enhancing residual tumor in the superior posterior fossa, compressing the brainstem anteriorly. Low signal in the mid-inferior posterior fossa consistent with fluid in the operative bed. Diffuse dural enhancement intracranially and in the spinal canal (arrows).

13. Pathologic diagnosis: Atypical Teratoid Rhabdoid Tumor ATRTs are highly malignant, rare tumors seen in young children less than 2 years of age. They are classified as WHO grade IV tumors. 50-60% occur in the infratentorial brain either within the cerebellum or brainstem. Supratentorially they may occur in the cerebral hemispheres, pineal space, septum pellucidum or hypothalamus. The MR imaging features of these tumors include heterogeneous morphology with areas of necrosis, cyst formation, and hemorrhage, as well as heterogeneous enhancement. The major differential consideration in a young child with a heterogeneous, solid posterior fossa tumor arising from the cerebellum is medulloblastoma. ATRTs most commonly arise from a cerebellar hemisphere, rather than the midline vermis. Medulloblastomas commonly arise from the cerebellar vermis. However, in this case the posterior fossa tumor arose from the cerebellar vermis which is an atypical finding for a ATRT. ATRTs can demonstrate leptomeningeal dissemination and prognosis is worse than with medulloblatoma. Usually death occurs within 1 year of diagnosis due to poor response to chemotherapy after a debulking surgery.