Thymic squamous cell carcinoma, epidermoid carcinoma

A thymic tumour showing squamous differentiation, as seen in other organs and lacking an infiltrate of immature T-cells.

Epidemiology

These tumours are rare but are the most common type of thymic carcinoma. The frequency is higher in Asia than in the West.

Clinical features

There may be paraneoplastic polymyositis but not myasthenia gravis, unless the carcinoma has arisen by progression from a thymoma, nor red cell aplasia.

Radiology

Macroscopic appearances

The tumour is not encapsulated and lacks fibrous septa. There is often necrosis or haemorrhage. Invasion of lung, pericardium and major vessels is common. Most tumours are Masaoka stage III or IV at the time of presentation.

Histopathology

These tumours consist of epithelial lobules separated by fibrous bands permeated by inflammatory cells. The squamous cells show intercellular bridges and keratinisation (with Hassall's corpuscle-like keratotic pearls). There is definite cytological atypia. Broad bands of fibrohyaline stroma separate the tumour nests. There is not the radial pattern of cells at the edge of tumour nests common in bronchogenic carcinomas. Any lymphocytes are mature and admixed with plasma cells.

Immunohistochemistry

	Cytokeratin	1/12
	AE1/AE3	13/133
	EMA	1/12
	CD5	positive0
	CD70	positive0
	CD117	positive0
	Chromogranin Synaptophysin CD56	positive alone or in combination in two-thirds of cases0

Ultrastructure

Cytogenetics

There are gains of chromosomes 1q, 8, 11, 17q and 18, with losses of 3p, 6, 16q and 17p1.

Differential diagnosis

Squamous cell carcinoma metastatic from other sites usually lack the organisation into lobules separated by fibrous bands, as seen in thymic squamous cell carcinoma. Squamous carcinomas of other organs are negative for CD5 and CD700. Nasopharyngeal carcinomas may be positive for CD700.
Thymomas: negative for CD5 (except for some of B3 type) and are infiltrated by immature T-cells.
If associated with a cyst, needs to be differentiated from pseudopapillary growth seen in multilocular thymic cysts.

Management

Prognosis

Metastases occur to lymph nodes, bone, lung, liver and brain. The prognosis, along with that of basaloid carcinoma, is better than for other thymic carcinomas.

References

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

1 J Rosai et al. Histological typing of tumours of the thymus. WHO International histological classification of tumours. Springer-Verlag, second edition, 1999.

2 Chalabreysse, L., B. Etienne-Mastroianni, et al. (2004). "Thymic carcinoma: a clinicopathological and immunohistological study of 19 cases." Histopathology 44(4): 367-74.

3 Nonaka D, Henley JD, Chiriboga L, et al. Diagnostic utility of thymic epithelial markers CD205 (DEC205) and Foxn1 in thymic epithelial neoplasms. Am J Surg Pathol 2007; 31:1038-44

This page last revised 6.1.2006.