Case Report: Intracranial epidermoid cyst accompanied by bone hyperplasia forming a bone encapsulation.

Intracranial epidermoid cysts are not true neoplasms; rather, they represent congenital benign lesions characterized by the presence of stratified squamous epithelium, commonly referred to as pearl tumors or cholesteatomas. These cysts exhibit slow growth within the cranial cavity, primarily manifesting as compressive changes and rarely inducing adjacent hyperostosis. Of particular note, the bone encapsulation phenomenon with distinct morphological features, resulting from hyperostosis, is extremely rare, with hardly any similar cases reported in the existing literature. This report provides a detailed description of an epidermoid cyst case located in the frontotemporal region. The patient, a 53-year-old female, presented with a 20-day history of dizziness, headache, accompanied by nausea and vomiting, and was subsequently admitted to our hospital for intracranial space-occupying lesion (left frontal lobe). Postoperative histopathological examination confirmed the diagnosis of epidermoid cyst with intralesional hemorrhage, inflammatory granulation tissue proliferation, and fibrosis. Radiological imaging and intraoperative observations revealed reactive hyperostosis surrounding the cyst, with partial formation of bone encapsulation. The formation of this bone hyperplasia and the bone encapsulation with a special shape is related to the synergistic action of inflammatory stimulation and mechanical pressure.