The patient went on to have a resection.
A thin walled cyst 17x12x8mm. The cyst contains friable dark brown and pale yellow material. The wall is fibrous and measures less than 1mm in thickness. The external surface is partly smooth and partly roughened.
The section shows a cyst with surrounding brain parenchyma and choroid plexus. The lumen contains amorphous eosinophilic material. The cyst is lined by pseudostratified ciliated columnar epithelium. Rare cells have mucinous vacuoles in the cytoplasm. They show no atypical features. The cyst wall is fibrous and it contains foamy macrophages, chronic inflammatory cells with dystrophic calcification. The brain parenchyma is gliotic showing a few Rosenthal fibres. No evidence of malignancy is identified.
The features are those of a colloid cyst.