Neurogenic, or neural sheath tumours include neurofibromas, neurilemmomas and Schwannomas. The relationship between schwannomas and neurofibromas has long been controversial, although their common origin from Schwann cells is now accepted. Some regard these tumours as different because of their distinctly different clinical presentations and prognoses. Schwannomas are usually solitary and rarely malignant, while neurofibromas are frequently multiple, often part of von Recklinghausen's disease, with approximately 8% of such cases reported to undergo malignant transformation (2).
The needling of neurogenic tumours may trigger a sharp pain radiating along the nerve and this is a valuable diagnostic sign (4,6).
Malignant neurogenic tumours, despite marked pleomorphism and occasional giant cells, often display nuclear features of recognisable neural tissue, ie a wavy pattern of cells with sharply pointed ends and twisted, very elongated nuclei (5).
Schwannomas are characterised cytologically by the presence of tissue fragments with a fibrillar aspect. Nuclear palisading, sometimes resembling Verocay bodies, may also be present. The cells are frequently elongated, with ovoid nuclei (comma shaped) and a moderate degree of anisokaryosis (4).
The individual neoplastic cells are spindle in shape with tapering ends. The nuclei are uniform in size, oval with rounded ends, showing thin nuclear membrane, and finely granular uniformly distributed chromatin giving them a ground glass appearance. No distinct nucleoli are seen in most cells although some of them show very fine small nucleoli mostly single. The cytoplasm are ill-defined, stain green or greenish-brown and show very fine reddish-brown granules and fibrils (1).
Benign schwannomas characteristically have Antoni A and B tissue, the distinctive pattern of interlacing spindle cells with generally bland nuclei and vascularity (2).
Antoni type A tissue is formed of compactly arranged spindle cells with long oval nuclei which have their long axis arranged parallel to each other resulting in palisade appearance. This type of tissue also shows the Verocay bodies which are formed of parallel bundles of fibres and cells with nuclei polarised at each end. In some of these bodies the cells have a whorled pattern in the center (1).
The Verocay bodies are formed of clumps of neoplastic cells, five to about 100 cells in size. These clumps of "balls" of cells show nuclei that are moulded around each other and in some areas show definite palisading. The center of these organoid rounded balls of cells show brown staining of the cytoplasm while the peripheral parts show greenish-blue staining.. There is a definite tendency of polarization within the cells, with the nuclei pushed to one end of the cell bodies (1).
Antoni type B tissue is formed of spindle and stellate cells arranged in a loose connective tissue matrix and is represented cytologically by the individual spindle-shaped neoplastic cells (1).
Malignant Schwannomas have a varied histology and while they may retain some remnants of distinctive architecture on cytology, for the most part obviously malignant, undifferentiated, spindle-shaped cells without identifying features are produced (2). An accompanying malignant tumour diathesis as well as the cytological features of the individual cells makes the diagnosis of malignancy certain (2).
Although Schwannomas and Neurilemomas are the same tumour, ancient neurilemomas are listed separately because of their characteristic cytological features.
Neurilemomas, especially those of ancient type, may be falsely interpreted as spindle-cell sarcomas, eg fibrosarcomas, synovial sarcomas and hemangiopericytomas. Cytologically the occurrence of large tissue fragments looking like pieces in a jigsaw puzzle is characteristic for neurilemomas. In such fragments, Verocay bodies may be found. Another distinct cytological feature of neurilemomas is the occurrence of fibrillar extracellular substances within cell clusters. The nuclei may be vacuolated. A severe nuclear polymorphism without the corresponding increase in mitotic activity should deter the cytologist from a definite diagnosis of malignancy in soft-tissue tumours. The same should be the case when there is discrepancy between the degree of cell dissociation and the cell polymorphism (3).
Neurofibromas can produce some palisading effect but usually the cells are similar to fibroblasts and do not produce the characteristic Verocay bodies (1).
The single most reliable morphological marker of nerve sheath differentiation is the presence of typical comma-shaped cells, occasionally arranged in palisades, in conjunction with highly characteristic twisted nuclei (7).
Benign tumours tend to give hypocellular smears that show loose tissue fragments with a fibrillar ground substance and more cellular clusters of spindle cells forming Verocay bodies. Malignant tumours show a higher cellularity than their benign counterparts and a more evident and diffuse nuclear pleomorphism and hyperchromasia (7).