Answer: Cellular schwannoma

Histology: The tumor is encapsulated and grows eccentrically along the periphery of a nerve. In the center of the lesion, there is a cystic space. The cellular tumor consists of spindle cells arranged in short bundles and interlacing fascicles with foci of nuclear palisading and whirling of cells. In some areas, the spindle cells form long sweeping fascicles. Mitotic figures are rare. The vessels within the tumor show perivascular hyalinization.

Discussion: Classic schwannomas show a mixture of Antoni A and B areas. Antoni A areas are cellular areas composed of compact spindle cells arranged in short bundles or interlacing fascicles. In these cellular areas, there may be nuclear palisading and whirling of cells. Characteristic Verocay bodies may be seen. Antoni B areas are less orderly and less cellular areas composed of loose matrix with foci of microcystic change and inflammatory cells. This case is an example of a cellular schwannoma that is composed predominately of Antoni A areas that lack Verocay bodies. Given that the lesion is cellular and may sometimes show long, sweeping fascicles, the lesion may be confused with a fibrosarcoma or a leiomyosarcoma. However the low mitotic activity would argue against a malignant diagnosis. Neurofibromas may show cellular areas that resemble Antoni A areas of a schwannoma, however, unlike schwannomas, neurofibromas are not encapsulated and small neuritis can usually be demonstrated within the lesion. Over 100 cases of cellular schwannomas have been reported in the literature, with less than 5% of the patients having recurrences, and none developing metastatic disease.