A clinicopathological analysis of papillary endolymphatic sac tumor in inner ear

Abstract

Background Endolymphatic sac tumor (ELST) is a rare tumor originating from endolymphatic epithelium of inner ear. This tumor exhibits low-grade malignancy with benign histopathological appearance and clinically destructive behavior which occurs in the skull base and frequently invades the posterior petrous bone, the mastoid, semicircular canal, cerebellopontine angle structures and cranial nerve. The presence of intracranial ELST always makes the diagnosis challenge for clinicians and pathologists. Herein we describe a case of ELST in skull base. The clinicopathology of this tumor and its differential diagnosis are discussed. Methods The clinical manifestation of a patient with primary ELST occurring in right cerebellopontine angle was presented retrospectively. Resected mass was routinely paraffin-embedded and stained with hematoxylin and eosin. Dako EnVision immunohistochemical staining system was used to detect the tumor antigen expressions, including cytokeratin (CK), vimentin (Vim), epithelial membrane antigen (EMA), carcinoembryonic antigen (CEA), synaptophysin (Syn), chromogranin A (CgA), S-100 protein (S-100), glial fibrillary acidic protein (GFAP), thyroglobulin (TG), thyroid transcription factor-1 (TTF-1) and Ki-67. Results A 32-year-old male patient presented with 20-year history of progressive hearing loss. MRI scan revealed an expansile lytic lesion of the mastoid process of the right petrous bone, measuring 4.20 cm × 3.30 cm × 2.00 cm, occupied the right cerebellopontine angle with infiltration of surrounding dura mater. But the lesion did not break the dura mater and invade the brain parenchyma. Craniotomy was performed and the tumor was removed totally. Histological examination revealed a papillary, cystic or glandular architecture in mass. The papillary and glandular structures were lined by a single layer of flattened cuboidal-to-columnar cells. The stroma of the papillary fronds was richly vascularized and chronically inflamed. There were minimal cellular pleomorphism and rare mitotic activity and necrosis was not observed. There were cystic glandular spaces filled with colloid-like material which was remarkably similar to thyroid tissue. Immunohistochemical staining showed that the tumor cells were diffusely positive for CK (AE1/AE3), Vim, EMA and focally positive for CEA, but negative for Syn, CgA, S-100, GFAP, TG and TTF-1. The Ki-67 index was less than 1%. The colloid-like material showed positive reactivity with periodic acid⁃Schiff (PAS). Based on clinical presentation and histological findings, a final histological diagnosis of primary papillary ELST in cerebellopontine angle was made according to the criteria of WHO classification. The patient had not received radiotherapy or chemotherapy and attended follow-up for 3 months, without any neurological deficit or signs of recurrence. Conclusion ELST is rare skull base tumor originating from endolymphatic epithelium within the vestibular aqueduct, characterized clinically by slow growth with local invasion and bone destruction. Due to the rarity of this tumor, it can easily be confused with other tumors of skull base with papillary architecture such as chorioid plexus papilloma, papillary ependymoma, papillary meningioma, middle ear adenoma, and metastatic papillary carcinoma of the thyroid. Complete excision is the best guarantee against tumor recurrence, while recurrence may occur due to subtotal resection.