Lipochoristomas (lipomatous tumors) of the acoustic nerve

Archives of Pathology & Laboratory Medicine, Nov 2003 by Wu, Sandy S, Lo, William W M, Tschirhart, Donald L, Slattery, William H III, Et al

Context.-Lipochoristomas (lipomatous choristomas) are rare tumors of the acoustic nerve (cranial nerve VIII/vestibulocochlear nerve) within the internal acoustic canal and sometimes the cerebellopontine angle, and are histogenetically believed to be congenital malformations. Their clinically indolent behavior has recently prompted a more conservative management protocol in a quest for maximal nerve/hearing preservation. This approach contrasts sharply with that for the common internal acoustic canal/cerebellopontine angle tumors, the neuroepithelial neoplasms (acoustic schwannomas and meningiomas), which behave more aggressively and have more prominent clinical manifestations. Owing to their rarity, the clinicopathologic features of cranial nerve VIII lipochoristomas have been obtained mainly through case reports.

Objective.-We present the clinicopathologic features of 11 cases of lipochoristomas of cranial nerve VIII.

Design.-The 11 cases were documented between 1992 and 2003. We performed complete clinical reviews with histologic, histochemical, and immunohistochemical analyses of formalin-fixed, paraffin-embedded tumor samples.

Results.-The patients were 8 men and 3 women with hearing loss of the right ear (5 patients) or the left ear (6 patients). No patient had bilateral tumors. All lipochoristomas histologically possessed mature adipose tissue admixed with varied amounts of mature fibrous tissue, tortuous thickwalled vessels, smooth muscle bundles, and skeletal muscle fibers, the latter verified with immunohistochemistry.

Conclusions.-The histomorphologic and immunophenotypic evidence showed that these tumors are better characterized as choristomas than as simple "lipomas," as they have been labeled in the past. Their overall nonaggressive clinical nature in addition to the characteristic radiologie and histomorphologic findings are important clinicopathologic features for the pathologist to recognize and differentiate, especially during frozen section evaluations, in order to direct the neurosurgeon to a more appropriate conservative therapeutic intervention.

Neoplasms of the internal acoustic canal (IAC) and cerebellopontine angle (CPA) often involve the acoustic nerve (cranial nerve VIII [CNVIII]/vestibulocochlear nerve). The vast majority are of neuroepithelial nosology, comprised mostly of acoustic schwannomas (approximately 80%-90%) and meningiomas (approximately 10%),1-4 while uncommon tumors include epidermoid/dermoid cysts, arachnoid cysts, and a variety of rare benign and malignant neoplasms. Lipochoristomas (lipomatous choristomas), previously known as lipomas, are rare lesions of the IAC/CPA, comprising 0.1% of all IAC/CPA tumors.1-11 Many have been discovered incidentally. They are slow-growing lesions with clinically indolent behavior and have been surmised to be congenital malformations rather than neoplasms.12

Clinical features of CNVIII lipochoristomas have mostly been examined in case studies and extensive reviews.5,7 Complete pathologic evaluations are uncommon." Results from larger series with thorough histomorphologic analyses have yet to be documented. Our study represents the experience at our institution during the past decade. The clinicopathologic features of 11 cases of lipochoristomas of the IAC are described. (During the same 10-year period, 2537 cases of acoustic schwannomas and meningiomas were identified from our surgical pathology archive; 11/ 2537 = 0.43%.) The histomorphology, immunohistochemistry, and clinical behavior of the tumors are cataloged. A comparison is made with the information available in the English language literature.

MATERIALS AND METHODS

A complete review of medical records and radiographic films was performed. The pathology specimens included formalin-fixed, paraffin-embedded tissue blocks from the surgically excised lipochoristomas or from tissue biopsies. Serial sections were produced from each block, and hematoxylin-eosin-stained slides were made. Additional trichrome histodiemically stained slides and immunohistochemical analyses (S100, glial fibrillary acidic protein, smooth muscle actin, muscle-specific actin, desmin, CD34, and epithelial membrane antigen) were made in 7 of the 11 cases for which the individual cell types needed to be validated further. The antibodies were produced by Cell Marque (Hot Springs, Ark), available as prepackaged kits made and distributed by Ventana Medical Systems, Inc (Tucson, Ariz). All immunoreactions were performed according to the manufacturer's guidelines, using the ES Automated Slide Stainer from Ventana in combination with Ventana DAB (diaminobenzidine) Detection Kits. Tissue slices mounted on polylysine-treated slides were deparaffinized (70[degrees] oven x 2 hours), immersed in xylene, bar-coded for automated processing, and serially rehydrated in progressively weaker ethanol solutions (100% to 80%) and lastly in water. The slides were finally immersed in Ventana Wash Solution. Inhibitor solution was applied, followed by primary antibodies (Table 1) and biotinylated secondary antibodies. No pretreatment was needed for any of the immunoreactions. Avidin-streptavidin enzyme conjugate was then used, followed by detection with the stable chromogen DAB and copper DAB enhancer.