Metaplastic thymoma

Definition

A thymic tumour composed of foci of neoplastic epithelial cells sharply demarcated from a surrounding bland spindle cell component1.

Synonyms

Thymoma with pseudosarcomatous stroma

Epidemiology

This is a rare type.

Clinical features

There is no association with myasthenia gravis.

Macroscopic appearances

The tumour is circumscribed or encapsulated.

Histopathology

There may be a [peripheral rim of residual thymic tissue. There is no lobulation. The tumour is biphasic, composed of epithelial islands or trabeculae, and sheaves of bland spindle cells in varying proportions. There is no transition between the two components1. The epithelial component is often squamoid with vesicular nuclei. Some nuclei are variously large, empty, hyperchromatic or contain pseudoinclusions. There may be twig-like hyaline material within the epithelial islands. The spindle cells are bland and form short storiform fascicles. In recurrent cases, the spindle cells become atypical. Any lymphocytes are sparse.

Immunohistochemistry

The spindle cell component:

Any lymphocytes are mature T-cells, negative for TdT0.

Cytogenetics

There are few genetic alterations.

Differential diagnosis

• thymic low-grade metaplastic carcinoma

• sarcomatoid carcinoma (carcinosarcoma): shows prominent necrosis, atypia and frequent mitoses.

Management

Prognosis

After complete removal, the behaviour is almost invariably benign1.

References

0 Tumours of the Lung, Pleura, Thymus and Heart. WHO Classification of Tumours. IARC Press 2004.

1 Suster, S., C. A. Moran, et al. (1997). "Thymoma with pseudosarcomatous stroma: report of an unusual histologic variant of thymic epithelial neoplasm that may simulate carcinosarcoma." Am J Surg Pathol 21(11): 1316-23.

This page last revised 4.1.2006.