October 2011: A 49 year old woman with a large thigh mass

History: A 49 year-old woman presented with a left thigh mass. It was completely excised and found to be well circumbscribed. It weighed 13 grams, and was relatively homogeneous gray-to-tan. It lacked hemorrhage and/or necrosis.

Microscopically, the tumor had well-defined border (Fig. 1, 2) and grew in a whorled lobular or “finger print” pattern (Fig. 3). High density regions of cells were juxtapositioned with lowly cellularity regions (Fig. 4). The cells were polygonal, epithelioid and spindled, with intervening collagen (Fig. 5). The nuclei were bland, and lacked pleomorphism or significant mitotic activity (Fig. 6).

Immunohistochemistry showed strong S100 positivity and negative staining for cytokeratin.

Diagnosis: Cellular neurothekeoma, thigh

Michelle Iverson, PSF, and Donald R. Chase, MD
Department of Pathology and Human Anatomy,
Loma Linda University and Medical Center, Loma Linda, California
California Tumor Tissue Registry, Loma Linda, California

Discussion: Neurothekeoma (NT) is a term used by Gallager and Helwig in 1980 to reference a unique neural tumor. As described, NT is a superficial dermal tumor that typically occurs in the upper extremities of the head and neck, mostly in the first two decades of life. It is comprised of epithelioid cells and spindle cells which arrange into nodules with a whorled or somewhat spiral fascicular pattern. Collagen is often seen in association with the cellularity. Nuclear atypia and mitotic figures are rare, and when they do occur, the clinical significance is uncertain. Treatment consists of excision and clinical monitoring. The tumors are considered low-grade without metastatic potential, although local reoccurrence has been described.

Rosati, in 1986, described a cellular variant of myxoid neurothekeoma. This variant appears morphologically the same as classic neurothekeomas, however the immunohistochemical profile is shows S100 is be negative and NKI-C3 and NSE to be positive.

Although the tumor can usually be diagnosed on its unique growth patterns, mimickers include:

• Nerve sheath myxoma which has a greater myxoid component, and is multilobular with a prominent fibrous periphery. The cells are more spindled and stellate. They are S100 and GFAP positive.

• Melanocytic neoplasms (especially Spitz nevus) show downward growth which is not seen in cellular neurothekeoma. They show epithelioid and spindle cells in a nested pattern.

• Plexiform fibrohistiocytic tumor is a multinodular, biphasic tumor composed of histiocytic cells and osteoclast-like giant cells with fascicles of fibroblastic cells in-between.

Suggested reading:

Hornick, JL, Fletcher, CDM. Cellular neurothekeoma: detailed characterization in a Series of 133 cases. Am J Surg Pathol 2007; 31:329.

Barnhill, R.L., Mihm M.C. Jr. Cellular neurothekeoma: a distinctive variant of neurothekeoma mimicking nevomelanocytic tumors. Am J Surg Pathol 1990; 14:113.

Calonie, E., Wilson Jones, E., Smith N.P. Cellular ‘neurothekeoma’; an epitheloid variant of pilar leiomyoma? Morphological and immunohistochemical analysis of a series. Histopathology 1992; 20:397.

Fetsch, JF, et al. Neurothekeoma: an analysis of 178 tumors with detailed immunohistochemical data and long-term patient follow-up information. Am J Surg Pathol 2007; 31:1103.

Gallager, R.L., Helwig, E.B. Neurothekeoma-benign cutaneous tumour of neural origins. Am J Clin Pathol 1980; 74:759.

Fetsch, JF, et al. Nerve sheath myxoma: a clinicopathologic and immunohistochemical analysis of 57 morphologically distinctive, S-100 protein- and GFAP-positive, myxoid peripheral nerve sheath tumors with a predilection for extremities and a high local recurrence rate. Am J Surg Pathol 2005; 29:1615.

Angervall, L., Kindblom, L-G., Haglid, K. Dermal nerve sheath myxoma. A light and electron microscopic, histochemical and immunohistochemical study. Cancer 1984; 53:1752.

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