Epidermoid tumor Epidermoid tumor, also known as epidermoid cyst, cholesteatoma or pearloma, which was first reported in 1829. It is a kind of benign tumor with extremely slow growth, accounting for about 1.2% of intracranial tumors, with no differences in gender, age and race. The tumor originates from embryo, but clinical symptoms usually appear in adults, and once found, the tumor is often large. The tumor originates from the ectodermal tissue left in the neural tube during the third to fourth week of embryonic development when the neural tube is separated from the ectoderm and closed. The location of ectodermal tissues left in the neural tube is different, which determines the location of tumor occurrence later. If the ectopic ectodermal tissue is left in the early embryonic stage (when the neural tube is closed), the tumor is mostly located in the midline; if it occurs in the late stage (second cerebral vesicle formation), the tumor is mostly located laterally. Clinically, they are more frequent in the pontocerebellar horn, but also in the fourth ventricle, lateral ventricle, cerebrum, cerebellum, tegmentum, and brainstem. The tumor is mostly burdened, and there are also multiple tumors. 3.Clinical manifestations Clinical symptoms and signs are mostly non-specific. Although the tumor grows slowly, it has strong destructive effect on the tissues around the lesion, and also has inflammatory effect, which may manifest as recurrent episodes of aseptic meningitis, resulting in hydrocephalus and even psychiatric symptoms. Epilepsy can occur in cholesteatoma in the cortical area. Intracerebroventricular cholesteatoma mostly occurs in the lateral ventricular triangle or temporal horn, but rarely occurs in the three or four ventricles. Some of them may occur in the tetralogy of Fallot. They can also occur in the cerebral hemispheres, cerebellum, and brainstem. A small number of patients may have a cranial onset, of which most are located in the midline or near midline. 5. Imaging (1) CT: round-like or irregular-shaped uniform hypodense changes (Figure A), CT values are similar to cerebrospinal fluid, and the lesions resemble arachnoid cysts. In very few patients, the tumor may show CT isointensity or high density, which may be due to calcification of the tumor wall and keratinized debris, spontaneous intra-tumor hemorrhage or significantly increased protein content in the tumor lumen, or some tumors due to secondary infection. Generally, the tumor does not enhance. In a few cases, the contrast agent accumulates in the vessels of the tumor wall and appears as ring or sheet enhancement. (2) MRI: T1 low signal (Figure B), T2 high signal (Figure C), no edematous reaction of brain tissue around the lesion. No enhancement (Figure D). On diffusion imaging, T1 shows high signal and cerebrospinal fluid low signal, which can be used to identify arachnoid cysts (Figure E).