Meningioma (atypical) with extensive bony invasion
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Bone window CT demonstrates marked expansion of the right skull vault with irregular margins and generalised lucency. It results in marked mass effect with shift of the midline to the left.
The patient went on to have a resection.
Macroscopic description: A semi-circular piece of recognizable skull bone grossly involved on one surface by an infiltrating soft, tan, bosselated tumour in the centre of the piece. The skull bone measures 7 x 9 cm in the maximum two diameters, while it ranges in thickness from 0.6 up to 1.5 cm nearest the tumour involvement. The diameter of the tumour within the centre of the skull bone is 5 x 4 cm, and the tumour appears to involve the bone resection margin.
Microscopic description: Representative sections from the specimens received in four containers reveal classic features of meningioma with prominent whorling effects sometimes hyalinized whorls and occasional psammoma bodies. Some of the hyalinized whorls show peculiar radial arrangement of collagen fibers that stain green with the Masson trichrome stain. The tumour cells are arranged mostly in fibroblastic growth pattern admixed sometimes with a meningothelial pattern with indistinct cell borders.
Cytologically, there is focal enlargement of the nuclei in association with prominent nucleoli and focal areas of small-cell change. Apoptotic bodies are readily identified and mitotic activity averages 5 mitotic figures/10 high-power field. There are no areas of micronecrosis. At the periphery of the tumour fragments there is extensive prominent transmural tumor invasion of blood vessels, but no luminal tumour emboli are identified. Sections including skeletal muscle tissue and bone trabeculae confirm invasion by meningioma. The tumor is noted to invade the dura on both sides. MIB-1 immunostaining shows a considerable number of cells in the mitotic cycle.
FINAL DIAGNOSIS: Atypical meningioma with diffuse infiltration of dura, skeletal muscle and bone.
This case demonstrates the importance of accurate histological assessment, as the tumor shows atypical histologic features, such as mitotic activity averaging 5 mitotic figures/10 high-power fields, small-cell changes, prominence of nucleoli, and sometimes hypercellularity.
Under the WHO classification this meningioma is classified as WHO grade 2, which likely indicate a higher recurrence rate and more aggressive behavior than benign meningioma.
It is tempting to use the term intraosseous meningioma for tumours such as this, but infact finding involvement of the arachnoid layer and dura with tumour suggests that this is an intracranial meningioma (often of the en plaque morphology) with bony involvement. True 'primary intraosseous meningiomas' have no dural involvement.