Abstract
Endolymphatic sac tumours (ELST) are rare and slow-growing neoplasms that develop out of the endolymphatic sac of the temporal bone. Even though histologically low grade, they show locally aggressive behaviour, and may affect important neurovascular structures of the cerebellopontine angle. These tumours can present with auditory or vestibular dysfunction, and they are usually radiologically indistinguishable from other cerebellopontine angle lesions. We present a rare case of a left cerebellopontine angle mass, which was a left endolymphatic sac tumour based on histopathological examination and immunohistochemical analysis. Under microscopy, the arrangements of the papillae of the cells demonstrated cuboidal to columnar eosinophilic to clear cytoplasmic cells, with no mitosis or necrosis. Immunohistochemistry showed focal positivity for CA IX, AE1/AE3, and CK7, and negativity for the rest (GFAP, SOX10, PAX8, TTF1, EMA, and CK20). Ki-67 (MIB-1) labelling index indicated low proliferative activity at 127/113, which is 1-2%. This tumour should be recognised, as its clinical and radiological presentations may be similar to those of other lesions of the cerebellopontine angle, and it is extremely rare. The diagnosis and surgical care should be done early to avoid local invasion and neurological complications.
