Pathology of the Head and Neck

258 L. Michaels8Fig. 8.24. Lipoma of the internal auditory canal. Note nervebranch passing through adipose tissue near the bottom. Reproducedfrom Michaels and Hellquist [68]figures are unusual. The presence of pleomorphism doesnot denote a malignant tendency. Antoni B areas, probablya degenerated form of the Antoni A pattern, show aloose reticular pattern, sometimes with histiocytic proliferation.Thrombosis and necrosis may be present insome parts of the neoplasm. A mild degree of invasionof modiolus or vestibule along cochlear or vestibularnerve branches may be present even in solitary vestibularschwannomas. Granular or homogeneous fluid exudateis usually present in the perilymphatic spaces ofthe cochlea and vestibule. This may arise as a result ofpressure by the neoplasm on veins draining the cochleaand vestibule in the internal auditory meatus. Hydropsof the endolymphatic system may occur (see above) andin larger tumours there is atrophy of spiral ganglion cellsand nerve fibres in the basilar membrane.The tumour is benign and usually grows slowly. Serioussymptoms and even death may occur, however, dueto damage to cerebral structures if the neoplasm growsto a large size.Neurofibromatosis 2 (Bilateral Vestibular Schwannoma,ICD-O:9540/1): bilateral vestibular schwannomaacoustic neuroma (neurofibromatosis 2, NF2) is, unlikeneurofibromatosis 1 (von Recklinghausen’s disease), notassociated with large numbers of cutaneous neurofibromasand cafe-au-lait spots, but the temporal bone localityof the neural tumour and its bilaterality are inherited asan autosomal dominant trait. This condition has been relatedto a gene localised near the centre of the long arm ofchromosome 22. At autopsy of cases of neurofibromatosis2, neural neoplasms are present in both eighth nervesand other central nerves. There are often many smallschwannomas and collections of cells of neurofibromatousand meningiomatous appearance growing on cranialnerves and on the meninges in the vicinity of the vestibularschwannomas and sometimes even intermixedwith them. The NF2 tumours are histologically similarto those of the single tumours except that the former havemore Verocay bodies and more foci of high cellularity.The NF2 tumours are more invasive, however, tending toinfiltrate the cochlea and vestibule more deeply.As with all schwannomas, the strongest and mostconsistent immunohistochemical reaction is the positivitydisplayed when staining with a polyclonal antibodyagainst S-100 protein is carried out. The vimentinmarker is also usually positive. These findings arecommon to both unilateral vestibular schwannoma andthe schwannomas of NF2. Glial fibrillary acidic proteinand neuron-specific enolase markers are also sometimespositive; the tumours are consistently negative forCD34, a marker widely used for the diagnosis of solitaryfibrous tumours, unless the vestibular schwannoma iswidely degenerated [117].An antibody against Ki67 (MIB1 in paraffin sections)has been utilised in a number of investigations to determinewhether the degree of positivity with this proliferationmarker can be related to the clinical activity of thetumour. It has been demonstrated that tumours 18 mmor smaller in diameter have lower proliferation indicesand growth rates, compared with tumours larger than18 mm [6]. The degree of labelling with the proliferationmarker is higher in cases of NF2 than in those of solitaryvestibular schwannoma [2].Men i ng ioma s (ICD-O:9530/0) are usually intracranialmasses. They arise from arachnoid villi, which aresmall protrusions of the arachnoid membranes into thevenous sinuses. Arachnoid villi may be found in parts ofthe temporal bone, including the inner ear, and on occasionmeningiomas may arise from these structures as primaryneoplasms of the inner ear region. The most likelyposition for a primary inner ear meningioma is in thewall of the internal auditory meatus, where arachnoid villiare normally frequent. The histological appearances ofa meningioma are those of a tumour with a whorled arrangementof cells: meningotheliomatous if the tumourcells appear epithelioid, psammomatous if calcification ofthe whorled masses is prominent and fibroblastic if thetumour cells resemble fibroblasts. Meningiomas as wellas acoustic neuromas may appear in the inner ear in theNF2 syndrome. The meningioma is a slowly growing tumourof the temporal bone that has had a reputation forcomplete benignity. In the temporal bone, however, middleear meningioma sometimes has a strong propensitytowards local recurrence and invasion (see above).Li p o m a s (ICD-O:8850/0) of the internal auditory canaland cerebellopontine angle are rare tumours thatmay be confused clinically with the much commonervestibular schwannoma. On magnetic resonance usingfat-suppressed T1-weighted images after gadoliniumenhancement, however, this tumour displays characteristicsof adipose tissue rather than those of schwannoma.There may be erosion of the walls of the internal auditorycanal as with vestibular schwannoma, and lipomamay appear similar to the latter at operation. Since