Thymic low grade metaplastic carcinoma

Synonym

Biphasic thymoma, mixed polygonal and spindle cell type³

These tumours closely resemble thymoma with pseudosarcomatous stroma. However, the epithelial component is described as carcinomatous with variable atypia and mitotic activity. the spindle cell component shows a gradual transition from the epithelial component. Moreover, the spindle cell component is positive for cytokeratins and for EMA. There may be mesenchymal metaplasia with expression of actin, desmin, or neural markers (chromogranin, S-100 or GFAP).

Immunohistochemistry

		epithelial component	spindle cell component
	cytokeratins	positive1	positive1
	EMA	positive1	positive1
	vimentin	positive1	positive1
	actin	2/41	2/41
	Chromogranin	1/4 (minority of cells)1	1/4 (minority of cells)1
	S-100	1/4 (minority of cells)1	1/4 (minority of cells)1
	GFAP	1/4 (minority of cells)1	1/4 (minority of cells)1
	NSE	1/4 (minority of cells)1	1/4 (minority of cells)1

Differential diagnosis

• Type A thymoma

• Thymic carcinosarcoma / sarcomatoid carcinoma.

Prognosis

One of four patients developed a recurrence.

References

¹Yoneda, S., A. Marx, et al. (1999). "Low-grade metaplastic carcinomas of the thymus: biphasic thymic epithelial tumors with mesenchymal metaplasia--an update." Pathol Res Pract 195(8): 555-63.

²Shimosato Y et al. Tumors of the mediastinum. In: Rosai J, Sobin LH, eds. Atlas of Tumor Pathology. 3rd Edn. Armed Forces Institute of Pathology 1997;93-115