Medulloblastoma

A single sagittal image from a CT of the brain demonstrates a heterogeneous slightly hyperdense mass above the bulk of the cerebellum and posterior to the midbrain. Pineal calcifications are displaced superiorly and the mass is clearly below the internal cerebral veins / vein of Galen. 

An aggressive appearing tumor is seen on the pineal region / tectal plate / superior vermis. It measures 37 x 40 x 40 mm, is of slight increased signal intensity on T2 weighted images, with multiple areas of cystic change, and hypointense on T1, with strikingly restricted diffusion, and moderate enhancement of the non cystic components. It fills the tentorial incisura, displaces the tectal plate to the right and anteriorly, extends into the perimesencephalic cistern on the left, and into the medial aspect of the left occipital lobe towards the ependyma of the lateral ventricle. The tumor and displaced superior vermis of the cerebellum cannot be differentiated, indicating either invasion or origin from the cerebellum.

The internal cerebral veins are clearly seen immediately above the main tumor mass as is the pineal galnd. The vein of Galen and straight sinus elevated immediately above the mass appear patent.

There is mild hydrocephalus secondary to aqueduct compression with minimal transependymal edema. Cerebral parenchymal signal intensity and architecture elsewhere normal. This no evidence of CSF spread of tumor in intracranial compartment beyond the main tumor mass.

The mass ( * ) is located below the vein of Galen (blue arrows) and displaces the pineal gland and calcifications (yellow arrows)  anterosuperiorly. 

The patient went on to have a craniotomy and resection of the tumor. 

Histology

The sections show a densely cellular malignant tumor, forming mainly diffuse sheets. Scattered interspersed pale islands and nests of cells are present. Within these islands, the tumor has more linear and cord-like arrangement. The stroma is somewhat more fibrillary. Occasional poorly formed rosettes are identified elsewhere.  The tumor cells have high N/C ratio with enlarged round nuclei, small nucleoli, granular chromatin and scanty cytoplasm. No mature neurons are present.  Sparse mitoses are seen (0-1 mitosis per 10 high power fields).  There are scattered apoptotic bodies throughout the tumor. No endothelial cell hyperplasia is identified.  The background is rich in reticulin fibers, apart from the pale islands. The tumor cells are synaptophysin and B-tubulin positive. There is focal GFAP positivity. The Ki-67 index is about 15%. Neurofilament, chromogranin and CAM5.2
are negative.

The presence of reticulin-poor pale islands has only be described in medulloblastoma and not in pineal parenchymal tumor.

FINAL DIAGNOSIS:

Posterior fossa tumor: Desmoplastic / nodular medulloblastoma (WHO Grade IV).