Case History

36-year-old male presented with a three-month history of back pain and ataxia. A chest radiograph demonstrated a right hilar mass. Chest CT demonstrated a mass measuring 5.3 x 5.2 cm in the right mediastinum. (Figure 1) A CT guided fine-needle aspiration was performed. The Papanicalaou stained slide revealed a cellular smear consisting of fragile large, round, discohesive cells with scant cytoplasm. (Figure 2) The nuclei had finely granular chromatin with variably sized nucleoli. (Figure 3) Histiocytes, giant cells, lymphocytes, plasma cells, and granulomas were present in the background. (Figure 4)

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Diagnosis & Discussion
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Figure 1	Figure 2
Figure 3	Figure 4
Figure 5

Diagnosis: Primary Mediastinal Seminoma

Discussion:
The mediastinum is the most common site of origin of extragonadal germ cell tumors that usually arise in the anterior mediastinum. Seminoma in the mediastinum occurs almost exclusively in men, usually young adults. It is the most common malignant germ cell tumor in the mediastinum.

Fine-needle aspirations of germ cell tumors typically demonstrate a population of seminoma cells and inflammatory cells. The seminoma cells are large, round, poorly cohesive, and fragile. The nuclei are large with fine chromatin. Single or multiple nucleoli of varying sizes can be present. The cytoplasm is clear with punched-out vacuoles and well-defined outlines. Chronic inflammation is variable which can include lymphocytes, plasma cells, and eosinophils. Well-formed granulomas and multinucleated giant cells are common.

One of the characteristic features of seminomas is the presence of a tigroid background, consisting of interwoven, lacy material, best appreciated in air-dried slides. Unfortunately this feature was not present in our case. The tumor cells will stain positive with PLAP (placental alkaline phosphatase), are usually negative to cytokeratins and uniformly negative to EMA (epithelial membrane antigen).

The differential diagnosis of mediastinal seminoma is broad and may include a lymphoproliferative disorder, thymoma, metastatic lung carcinoma or a granulomatous inflammatory process.

Due to the primitive appearance and poor cohesion of the seminoma cells, large cell lymphoma should be considered in the differential diagnosis. However, large cell lymphomas are usually PAS negative, lack the tigroid background, and do not form true tissue clusters. The background lymphocytes of seminoma are reactive in nature and will stain positive with LCA. This stain could possibly lead to a misdiagnosis of lymphoma. However, careful interpretation of LCA in this case is of great important to avoid this pitfall.

The dual cell population of seminoma may be reminiscent of thymoma, but the epithelial cells of thymoma will be negative for PAS and PLAP.

Metastatic poorly differentiated lung carcinoma should also be considered in the differential diagnosis. In these tumors, the clinical history and radiologic findings are very important. Additionally, seminomas are PLAP positive and TTF-1 negative.

Occasionally, granulomata could be a prominent feature of seminoma that may lead to the diagnosis of granulomatous inflammatory process. However, the presence of large seminoma cells is the clue to the correct diagnosis.

The correct diagnosis of seminoma is extremely important because in contrast with most other malignant neoplasm, these tumors can be treated to induce remission, and even cure.

Surgical biopsy correlation: The patient underwent mediastinal biopsy, which revealed seminoma. (Figure 5)

Acknownledgement:
This case is submitted By: Rafael Rodriguez, MD, and Alaa Afify , MD from the Department of Pathology, University of California , Davis , Medical Center , Sacramento , CA