Case of the Month

A 69 year-old woman presented with an incidental 36x35x30 mm right kidney tumor on a CT scan, centered on the cortex and exophytic. The tumor was grossly circumscribed, mostly solid and tan, with cysts filled with translucent liquid (Figure 1). Microscopically, it showed a compact nested to trabecular pattern, interspersed with areas of sharp edema (Figure 1), and was entirely composed of oncocytic cells with smooth round nuclei with perinuclear halos (Figure 2, inset). It was diffusely and strongly immunoreactive for cytokeratin 7 and negative for CD117 (c-kit).

What is the diagnosis?

1. Hybrid oncocytic tumor
2. Eosinophilic solid cystic renal cell carcinoma
3. Chromophobe renal cell carcinoma, eosinophilic subtype
4. Low-grade oncocytic tumor

Figure 1. Gross photograph of the formalin-fixed specimen.

Figure 2. H&E; 20x magnification. Inset: cytomorphological detail.

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Answer to January/February's Case of the Month

c. Plexiform schwannoma

The biopsy reveals a plexiform architecture with nuclear palisading and Verocay bodies. The tumor is predominantly composed of dense, hypercellular Antoni type A areas. The proliferating spindle cells form interconnecting fascicles and well-encapsulated nodules, with extensive involvement of the dermis and subcutaneous tissue. However, proliferation remains within the confines of the capsule. The morphologic findings are consistent with the diagnosis of plexiform schwannoma. Immunohistochemically, schwannomas stain strongly for S100 protein, SOX-10, and type IV collagen in the Antoni type A areas. They account for 4.3 percent of all schwannomas and have a predilection for the head and neck region (1). Malignant transformation is extremely rare. Plexiform schwanomma has been rarely reported in association with neurofibromatosis type 2 and schwanommatosis. 

Plexiform neurofibromas are composed of multiple interconnecting bundles and fascicles of various sizes and shapes, usually surrounded by a capsule. However, the superficial part is not usually encapsulated and the proliferating fascicles blend into the surrounding dermis. In contrast to schwannoma, it is hypocellular with a myxoid background and lacks the biphasic pattern, but occasionally shows nuclear palisading (2). Neurofibromas stain positive for S100, but there is only staining of scattered cells. Plexiform neurofibroma has an increased probability of malignant transformation into malignant peripheral nerve sheath tumors (MPNST).

MPNST can be multinodular, particularly if the tumor arises from a plexiform neurofibroma. Histologically, it shows fascicles and broad whorls of relatively uniform, spindled cells of varied cellularity, with hyperchromatic, pleomorphic nuclei and perivascular tumor cell accentuation. S100 immunostaining may be helpful, because MPNSTs stain focally for S100 protein in up to 50 percent of cases (3).

Palisaded encapsulated neuroma is a well-circumscribed, partially encapsulated, dermal-based neoplasm composed of well-developed fascicles of Schwann cells and associated axons. Myxoid stroma may be present, with characteristic clefts present within and around the tumor. Multinodular or plexiform growth patterns are uncommon. Immunohistochemistry shows strong S100 and SOX10 positivity in Schwann cells and nerve fibers, whereas neurofilament proteins highlight axons (4)

 Submitted by Muhammad Ahsan, Chughtai Institute of Pathology, Lahore, Pakistan, and Sehar Altaf, Nawaz Sharif Medical College, Lahore, Pakistan.

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