Seminoma of the mediastinum

Definition

A germ cell tumour identical to the corresponding gonadal tumour.

Epidemiology

Primary mediastinal seminomas accounts for 8% of extragonadal germ cell tumours and 9 to 39% of mediastinal germ cell tumours. Mediastinal seminoma is almost exclusively found in males, with most cases occurring the in the third or fourth decade of life.

Clinical features

Presentation is usually secondary to the mass effect. Unlike non-seminomatous tumours, seminomas of the mediastinum are not associated with Klinefelter's syndrome.

Macroscopic appearances

Most tumours are circumscribed, without infiltration of adjacent structures.

Histopathology

The morphology is identical to that of gonadal seminomas. They are composed uniform sheet of round or polygonal cells. The cytoplasm contains copious glycogen. The nuclei show one or more large nucleoli. There is often a background of lymphocytes, plasma cells, sometimes eosinophils. There may be granulomatous inflammation. Occasionally, the inflammation may mask the seminoma. Syncytiotrophoblastic cells are occasionally present, but, in contrast to choriocarcinoma, there is no cytotrophoblastic component.

Residual thymic tissue may undergo cystic change or hyperplasia.

Seminoma may occur as a component of a mixed germ cell tumour. Spermatocytic seminoma has not been reported in the mediastinum.

Immunohistochemistry

	PLAP	80-90%0
	Vimentin	70%0
	CD117	often positive0
	pancytokeratin	70% (staining is often weak and focal)0
	bHCG	positive in any syncytiotrophoblastic cells0
	CEA	negative0
	EMA	negative0
	AFP	negative0

Mediastinal seminomas differ from those of the testes in more often expressing cytokeratins (Cam5.2: 80% versus 21%), PLAP (93% versus 50%) and vimentin (70% versus 46%).

Differential diagnosis

• Mediastinal metastases from gonadal primary seminoma is unlikely in the absence of retroperitoneal deposits.

• Metastatic melanoma

• Lymphoma

• Thymoma

• Thymic carcinoma

• Clear cell carcinoma

Management and Prognosis

Metastases occur to cervical and abdominal lymph nodes, lung, pleura, chest wall, liver, adrenals, bone and brain. The prognosis is better than for non-seminomatous mediastinal germ cell tumours. The five year survival is 90% with combination chemotherapy.

References

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

This page last revised 19.1.2006.

©SMUHT/PW Bishop