Long-term follow up of malignant transformation of epidermoid cyst at the cerebellopontine angle based on serial imaging findings; A case report and literature review.

We report a case of malignant transformation of epidermoid cysts during long-term follow up more than 10 years with literature review. Epidermoid cysts are benign congenital tumors, accounting for 0.2-1.8% of intracranial tumors. Malignant transformation is extremely rare, with limited reports describing the imaging features and prognosis. A 69-year-old woman had presented with a tumor exhibiting hyperintensity on diffusion-weighted imaging at the left cerebellopontine angle and had been followed-up for the past decade. After 10 years, she developed left facial paralysis and hearing impairment. Contrast-enhanced lesions appeared within the tumor, accompanied by edema. Intraoperative findings revealed a tumor with two distinct components of pearly tumor and hematoma invading the cranial nerves. The pathological diagnosis was squamous cell carcinoma without primary malignancies, suggesting malignant transformation of epidermoid cyst. Although the residual tumor initially seemed dormant after postoperative radiotherapy, the patient developed carcinomatous meningitis. The literature was searched for squamous cell carcinoma without primary cancers showing neuroradiological changes in signal intensity on diffusion- and T2-weighted imaging, revealing 103 cases with a mean interval of 106.1 months from initial diagnosis to malignant transformation of epidermoid cyst. Although postoperative radiotherapy or chemotherapy has been attempted, no standard has been established for adjuvant treatment. The clinical benefits of postoperative adjuvant therapy need to be confirmed in further studies. As neuroradiological alterations on diffusion-weighted imaging or Gd-enhanced T1-weighted imaging during follow-up of epidermoid cyst might indicate malignant transformation of epidermoid cyst, postoperative radiotherapy might be an option worth exploring.