Answer: Schwannoma

Histology: This neoplasm is well-demarcated and composed of spindle-shaped cells with alternating areas which are more cellular and less cellular. The neuclei of the spindle shaped cells occasionally palisade to form verocay bodies. Of note, mature ganglion cells are seen at the periphery of the lesion.

Discussion: This schwannoma focally abutted against a small ganglion, giving the misleading appearance that ganglion cells were part of the neoplasm. It is otherwise a classical schwannoma. Immunolabeling can be useful in difficult cases: schwannomas will be strongly and diffusely positive for S-100 protein, vimentin, and CD56, and negative for cytokeratin AE1/AE3, desmin, smooth muscle myosin specific, and CD34. More recently, it has been shown that most schwannomas show moderate to strong staining for calretinin, with the extent of staining ranging from focal to diffuse. In contrast, only a small minority (7%) of neurofibromas display focal weak to moderate staining with calretinin.

Reference(s):
– Fine et al., Immunohistochemical staining for calretinin is useful for differentiating schwannomas from neurofibromas. Am J Clin Pathol. 2004 Oct;122(4):552-9.
– Kurtkaya-Yapicier et al, The pathologic spectrum of schwannomas. Histol Histopathol 2003, 18: 925-34.