Presentation
Progressive lower-limb weakness over 3 weeks. Two-week history of morning vomiting. No bulbar signs.
Patient Data
Large posterior fossa tumor centered on the fourth ventricle causing significant local mass effect and hydrocephalus. Predominantly low signal on T1 with multiple foci of T1 hyperintensity. T2 hyperintense with associated diffusion restriction. Limited enhancement following contrast. No other lesion.
Spinal imaging. No metastases.
PATHOLOGY
The intraoperative diagnosis of an embryonal tumor has been confirmed on the paraffin section. Microscopy reveals a highly cellular malignant tumor composed of densely packed cells with round to oval hyperchromatic nuclei surrounded by scanty cytoplasm.
Immunohistochemistry reveals patchy positivity for synaptophysin within the tumor cells. GFAP highlights the adjacent normal brain tissue but there are also some tumor cells which are positive. The tumor cells show focal positivity for SMA and patchy positivity for EMA. The tumor cells are negative with INI1. Beta-catenin immunohistochemistry reveals strong membranous and cytoplasmic staining within the tumor cells. NeuN is positive within normal-appearing neurons. S100 shows strong positivity within the normal brain, but here are occasional S100 positive tumor cells. Neurofibrillary protein highlights the normal brain parenchyma. The MIB1 proliferation index is very high.
Posterior fossa - Embryonal atypical teratoid/rhabdoid tumor, WHO Grade 4
Case Discussion
A posterior fossa tumor in a child that shows diffusion restriction has a relatively narrow differential. The first tumor that you think about is a medulloblastoma. However, in this age-group, you must put AT-RT first.
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