Germinomas are the most common of germ cell tumors, constituting half of the tumors in the pineal region with a male predominance. (Suprasellar germinoma affects males and females almost equally.) Grossly, the tumor is pinkish-gray, soft, friable, variably demarcated, and often invades the adjacent structures. Histologically, the tumor consists of two types of cells:

• Large polygonal cells have round or spherical nuclei surrounded by abundant cytoplasm. Mitoses are common. The tumor cells are separated in lobules by thin fibrous septa.

• Lymphocytes, the second cell type, form variable collections between the lobules (Fig. 11.33). It is believed that they represent host reaction to the tumor.

Germinoma. A. The tumor in the pineal region infiltrated the midbrain (myelin stain). B. Moderately large tumor cells with clear cytoplasm and prominent nucleus are separated into lobules by fibrous septa that contain large collections of lymphocytes (Cresyl-violet.)

Immunohistochemistry for a-fetoprotein and human chorionic gonadotrophin confirms the germ cell origins of the tumors. Immunoreactivity for placental alkaline phosphatase identifies the malignant cells. The tumors are radiosensitive; some have a more favorable clinical course than do other germ cell tumors.

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