Neurothekeoma

Definition

Neurothekeomas are subdivided into (i) myxoid/hypocellular, (ii) cellular and (iii) mixed subtypes. The myxoid variant shows evidence of neuro-sustentacular differentiation, while the status of the mixed and cellular variants is less certain.

It has been argued that nerve sheath myxoma is a different entity and should not be classed as a myxoid form of neurothekeoma.

Epidemiology

Neurothekeomas occur across a wide age range, including children, with a peak in the second decade and a female predominance of 2:1.

Clinical features

Tumours occur at a wide range of anatomical sites, with the head the most common site.

Macroscopic appearances

The tumour forms a firm papular or nodular mass.

Histopathology

Neurothekeomas are situated within the dermis or subcutis and are non-encapsulated. Involvement of skeletal muscle is unusual outside the face. They consist of multiple small nodules separated by collagenous stroma or myxoid. The cells within the nodules have a whorled pattern. Individual cells are spindled or epithelioid with eosinophilic cytoplasm. Osteoclast-like giant cells may be present, usually sparse. There is variable cytological atypia. Mitotic rates are variable.

Subtypes:

• cellular neurothekeoma: comprise up to 10% myxoid stroma

• mixed neurothekeoma; 10% to 50% myxoid stroma

• myxoid neurothekeoma: more than 50% myxoid stroma

Immunohistochemistry (with nerve sheath myxoma for comparison)

Differential diagnosis

• Melanocytic naevus, particularly Spitz naevus; are positive for S-100, Melan A, tyrosinase and HMB45, as well as for MIFT, PGP9.5 and NKI/C3.

• Nerve sheath tumour

• Plexiform fibrohistiocytic tumour; occurs most often on the extremities. There is sparing of the upper dermis, lack a prominent myxoid stroma; immunohistochemistry is poorly documented.

• Reticulohistiocytoma (solitary epithelioid histiocytoma) lacks a packeted architecture; is strongly positive for CD68 and CD163.

• True nerve sheath myxoma: has a peak incidence in the fourth decade, is typically found on the extremities, particularly the hands; head and neck lesion are uncommon. They are highly myxoid, with cords of cells that are positive for S-100 and GFAP. There may be residual perineural cells positive for EMA.

• Superficial angiomyxoma (cutaneous myxoma): are highly myxoid tumours with mildly pleomorphic mono-and multi-nucleated cells and spindle tumour cells. They are often positive for CD34, sometimes for actin.

Management

Complete excision.

Prognosis

Lesions may recur if inadequately excised.

References

1 WB Laskin et al. The "Neurothekeoma": immunohistochemical analysis distinguishes the true nerve sheath myxoma from its mimics. Hum Pathol 2000;31:1230-1241.

2 Fetsch JF, Laskin WB, Hallman JR, et al. Neurothekeoma: an analysis of 178 tumors with detailed immunohistochemical data and long-term patient follow-up information. Am J Surg Pathol 2007; 31:1103-14

©SMUHT/PW Bishop