Sections show fragments of a cystic lesion composed of abundant lamellated anucleate keratin debris arranged in a wavy onion-skin pattern. The cyst wall consists of thin fibrovascular tissue lined by bland stratified squamous epithelium. Focal stromal congestion and acute hemorrhage are present. No skin adnexal structures are identified. No cellular atypia, increased mitotic activity, or malignancy is seen.
Non-enhancing, lobulated extra-axial mass centered within the posterior fossa and fourth ventricle; radiologically highly characteristic of an epidermoid cyst or infected/dirty arachnoid cyst.
Soft tissue fragments collectively measured 4.3x4x0.8 cm, totally embedded.
Brain, posterior fossa mass, biopsy: