Posterior fossa atypical meningioma
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MRI of the brain demonstrates a large left sided extra-axial posterior fossa mass. It is isointense to the cerebellum on T1 weighted images, and hyperintense on T2. Following contrast administration it vividly and homogeneously enhances. There is no evidence of calcification or hemorrhage, however it does modestly restrict on DWI.
The mass invades and occludes the transverse and proximal sigmoid sinus as well as invades into the skull, with a single nodular focus passing through the outer table and into the subcutaneous tissues of the scalp. It also extends superiorly through the tentorium into the supratentorial compartement.
The cerebellum is markedly distorted by the mass and has edema within it. Somewhat surprisingly there is no hydrocephalus despite effacement of the fourth ventricle.
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This patient went on to have a posterior fossa craniotomy and excision of the mass.
Microscopic Description: Sections show meningioma, and lamellar bone and skeletal muscle infiltrated by meningioma showing increased cellularity. The tumour cells are arranged in sheets with whorl formation. Areas of tumour necrosis are seen, and some large aggregates of foamy macrophages are noted. The individual tumour cells display increased nuclear pleomorphism, prominent nucleoli, and mitotic figures are seen. The mitotic index is approximately 4 mitoses per 10 high power field.
Final Diagnosis: Invasive meningioma (atypical meningioma - WHO grade II).
Distinguishing an atypical meningioma from a hemangiopericytoma is difficult on imaging. When a large mass such as this invades bone, giving both as a differential is prudent.