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cellular neurothekeoma

Friday 5 April 2013

Cellular neurothekeomas are distinctive benign cutaneous tumors of uncertain histogenesis.

Cellular neurothekeomas have a predilection for the upper limbs and head and neck of pediatric and young adult females and rarely recur following incomplete excision.

There is no good evidence that these lesions show nerve sheath differentiation and the nomenclature will likely change when the tumor cell lineage is better defined.

Atypical histologic features (including pleomorphism, infiltration of subcutis, and a high mitotic rate) seem to have no clinical significance.

Epidemiology

In a study of 133 cases of cellular neurothekeoma, Hornick and Fletcher reported patient ages ranging from 1 to 65 years, with a mean age of 25 years. The male-to-female ratio was 1:1.8. (PMID:17325474)

There is a 1.8:1 female predominance, with a mean age of 25 years (84% <40 y).

Mean tumor size is 1.1 cm (range: 0.3 to 6 cm; 90% <2 cm).

The tumors arise most often on the upper limb (35%) or head and neck (33%).

Fifty-two percent of the tumors are limited to the dermis, and 48% also involve superficial subcutaneous tissue.

Microscopy

Histologically, most cases are poorly marginated; 25% infiltrate fat, and 8% entrapp skeletal muscle (mostly situated on the face).

Nearly all tumors have a lobulated or micronodular architecture and are composed of nests and bundles of epithelioid to spindled cells with palely eosinophilic cytoplasm, often separated by dense hyaline collagen;

13% show focally sheetlike areas, and 4% are notably plexiform.

Myxoid stroma is observed in 29% tumors; 8% are predominantly myxoid.

4% show marked stromal hyalinization.

Osteoclastic giant cells are seen in 15% of cases.

The mean mitotic rate is 3 per 10 high power fields; 21% have > or =5 per 10 high power fields.

Most tumors show mild cytologic atypia in the form of nuclear variability and small nucleoli; 25% contain notably pleomorphic cells.

Microscopy

- Microscopy

  • The cellular subtype is a poorly circumscribed tumor involving the superficial dermis with possible extension into the superficial subcutis.
  • Infiltration into fat and entrapment of skeletal muscle can be seen.
  • The tumor consists of nests and fascicles of epithelioid cells with vesicular nuclei containing scant myxoid stroma.
  • Due to the fact that the myxoid element determines the size of tumor nodules, the nodules of cellular neurothekeoma are smaller than those of the myxoid subtype.
  • Although the myxoid variant of neurothekeoma is hypocellular, the cellular subtype is hypercellular.
  • Sclerotic collagen is seen more markedly in cellular than myxoid tumors.
  • Osteoclastic giant cells are seen in a minority of cases.
  • Perineural and vascular invasion may rarely be seen in cellular neurothekeoma.
  • Cellular neurothekeoma can be histologically mistaken for melanoma and is, therefore, important to recognize.
  • Also included in the histologic differential diagnosis are Spitz nevi, smooth muscle tumors, fibrohistiocytic tumors, and carcinomas.

- Desmoplastic and atypical variants of cellular neurothekeoma

  • In a study of 133 cellular neurothekeomas, 74 cases (56%) demonstrated at least one atypical feature.
  • Mitotic activity may be present but is usually low, with a mean of 3 per 10 high power fields (HPF) reported in one study.

Immunochemistry

All tumors are reactive for NKI-C3.

89% express neuron-specific enolase,

57% showed at least focal staining for smooth muscle actin.

Rare cases are focally desmin positive.

All tumors are negative for S-100 protein.

Differential diagnosis

- plexiform fibrohistiocytic tumor
- benign fibrous histiocytoma
- low-grade sarcoma.

Prognosis

About 10% of tumors recur locally (often situated on the face), after a mean of 18 months; these tumor have been marginally excised or have involved excision margins in all cases with available information. No other clinical or pathologic features correlated with recurrence.

Follow-up studies of atypical cellular neurothekeomas have not shown that the presence of increased number of mitoses or cellular atypia correlates with any clinically significant endpoints.

Large tumor size, atypical histologic features (high mitotic rate, pleomorphism, infiltration of adipose) have been shown to have no clinical significance.

Therefore, it is regarded that cellular neurothekeomas behave in a benign fashion and only occasionally recur, generally in the setting of incompletely excised lesions on the face.

See also

- cellular neurothekeoma

Reference

- Cellular neurothekeoma: detailed characterization in a series of 133 cases. Hornick JL, Fletcher CD. Am J Surg Pathol. 2007 Mar;31(3):329-40. PMID: #17325474#