427 Endo Lymphatic Sac Tumor Beyond Von-Hippel Lindau syndrome: A Rare Tumor in a Sporadic Setting

Introduction/Objective Endolymphatic sac tumors (ELSTs) are rare, locally aggressive neoplasms that arise from the endolymphatic sac and duct, typically located in the posterior petrous portion of the temporal bone. Despite their bland histological appearance, these tumors often behave destructively invading nearby structures such as the mastoid, semicircular canals, cranial nerves, and even the cerebellopontine angle. Patients may present with a constellation of symptoms, including hearing loss, vertigo, tinnitus, and cranial nerve deficits, depending on the extent of invasion. ELSTs are most commonly seen in patients with von Hippel–Lindau (VHL) disease, a hereditary cancer syndrome. However, sporadic cases without any underlying genetic predisposition can occur and are often diagnostically challenging due to the tumor’s rarity and nonspecific presentation. Misdiagnosis is common, with lesions being mistaken for other entities such as paragangliomas, metastatic thyroid carcinoma, or middle ear adenomas. Here, we report a sporadic case of ELST in an elderly patient, emphasizing the importance of considering this rare diagnosis even outside the usual VHL context. Methods/Case Report A 67-year-old Caucasian woman with a history of multiple sclerosis presented with progressive right-sided facial weakness, first noted in 2018. Over several years, her facial palsy gradually worsened. Imaging in 2025 revealed a 1.8 × 1.8 × 1.5 cm lesion in the right petrous temporal bone compressing the facial and vestibulocochlear nerves, with a mild mass effect on the adjacent cerebellum. She underwent surgical excision of the lesion. Histological analysis revealed papillary structures lined by a single layer of cuboidal-to-columnar epithelial cells with distinct borders. The background showed granulation tissue, inflammatory cells, and focal hemorrhage. No cytologic atypia, necrosis, or mitotic activity was identified. Immunohistochemistry confirmed an epithelial neoplasm, showing diffuse positivity for cytokeratin and PAX8. The tumor had a low Ki-67 proliferation index. Stains for TTF-1 and CD10 were negative, helping to exclude metastatic thyroid or renal carcinoma. Given the known link between ELSTs and VHL syndrome, genetic testing was performed but returned negative supporting a diagnosis of a sporadic endolymphatic sac tumor. Results Not applicable. Conclusion While endolymphatic sac tumors (ELSTs) are most commonly associated with von Hippel–Lindau (VHL) syndrome, they can also occur sporadically, as demonstrated in this case. Their rarity and overlapping features with other skull base tumors often make diagnosis challenging. This case underscores the importance of maintaining a broad differential diagnosis for petrous bone lesions, particularly in elderly patients without a known genetic predisposition. Histopathologic evaluation, along with a thorough immunohistochemical panel, was essential in distinguishing this tumor from its histologic mimics. Positivity for PAX8 and cytokeratin, a low Ki-67 proliferation index, and the absence of markers indicative of metastatic disease were key in confirming the diagnosis. Although ELSTs are histologically low-grade, they are often locally aggressive and can result in significant clinical morbidity. Timely surgical resection remains the cornerstone of treatment. In this patient, gradual neurological improvement postoperatively highlights the potential for favorable outcomes in sporadic cases when accurate diagnosis and prompt intervention are achieved. Furthermore, genetic testing proved valuable in guiding long-term follow-up and counseling.