Acknowledegement: Dr Louis de Jager and Dr Kathy Taylor from Pathcare for images and review
Macroscopic view of a vestibular schwannoma
A vestibular schwannoma, also known as an acoustic neuroma, is a benign tumour that typically arises from the Schwann cells of the vestibular portion of the eighth cranial nerve (1). From a macroscopic perspective, a vestibular schwannoma appears as a well-defined, round or oval-shaped mass near the internal auditory canal(2). The tumour usually presents as a white to pinkish, firm mass ranging from a few millimetres to several centimetres in diameter.
On the surface of the tumour, there may be areas of cystic degeneration or haemorrhage, giving it a variegated appearance. In larger tumours, compression of surrounding structures such as the brainstem, cerebellum, and cranial nerves may be seen. Additionally, some vestibular schwannomas may have a characteristic ” mushroom” appearance on imaging studies, with the tumour tapering to a point as it extends towards the internal auditory canal (3).
Microscopic view of a vestibular schwannoma
Nils Ragnar Eugène Antoni, a Swedish neurologist and researcher working in the Royal Neurologic Clinic of Stockholm in 1920 under the supervision of the neuropathologist Frithiof Lennmalm, described two distinct patterns of cellular architecture in peripheral nerve sheath tumours, later known as Schwannomas (4). Antoni went on to become a professor of neurology at the Karolinska Institute in Stockholm.
Under the microscope, a vestibular schwannoma typically has two distinct regions: Antoni A and Antoni B.
Antoni A
Antoni A areas are histological patterns commonly found in vestibular schwannomas and benign vestibular nerve tumours (5). These areas are characterised by highly cellular zones with compactly arranged Schwann cells and densely packed collagen fibres. These areas are often associated with rapid tumour growth and increased cell proliferation (more aggressive growth). They may also contain Verocay bodies, unique structures formed by elongated Schwann cell nuclei arranged in parallel rows within a background of eosinophilic cytoplasm.
Antoni B
On the other hand, the Antoni B area is composed of loose, disorganised tissue with a more myxoid appearance. This region often contains many hyalinised blood vessels and stroma, imparting a softer consistency. Antoni B areas may indicate a less aggressive or non-growing tumour. It may also relate to vestibular weakness (6).
The presence of Antoni B areas in vestibular schwannomas is also associated with decreased cellularity, degeneration, and cyst formation within the tumour.
SOX10 immunohistochemical stain of a vestibular schwannoma
SOX10 is a protein (transcription factor) expressed by Schwann cells. In the case of vestibular schwannoma, the SOX10 immunohistochemical stain is used to help differentiate Schwann cells from other cell types (7).
SOX10 is involved in developing and maintaining Schwann cells, which are the cells responsible for forming the myelin sheath around nerves. The tumour primarily comprises Schwann cells in vestibular schwannoma, making the SOX10 stain a valuable tool in confirming the diagnosis. By highlighting the presence of SOX10 protein in the tumour cells, pathologists can accurately identify the tumour as a vestibular schwannoma.
In the following picture, the brown colour is due to SOX10 nuclear staining.
“Ancient” change in a vestibular schwannoma
Over time, microscopic “ancient” change of vestibular schwannoma can occur where the tumour becomes more fibrous and less cellular. This can lead to the formation of areas of calcification, cystic degeneration, and hyalinisation within the tumour (8,9,10)). Individual larger, atypical cells (due to degenerative change) are also frequently observed. Additionally, the tumour may also show signs of chronic inflammation and scarring. These changes can impact the overall appearance and composition of the vestibular schwannoma, making it appear less cellular compared to a “younger” tumour.
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