The most common location of extragonadal germ cell tumors occurs in two midline sites: the mediastinum (thymus) and the diencephalop-ineal (pineal and infundibulum) region. The origin of these non-neuroectodermal tumors remains unknown but may be related to the persistence of primordial germ cells which disseminate widely throughout many tissues and organs in the early embryo. These extra-gonadal germ cells typically have an ephemeral existence and undergo an apoptotic death. It is possible that some of these cells may survive and, over time, transform into a neoplasm. This is in contrast to the theory that the presence of these cells is related to a migrational defect. The pineal gland is the most common site of intracranial germ cell tumors (37-45%), followed by the suprasellar region (27-35%), with 10% of tumors involving both regions at presentation

[16,17] (Figure 13.3). Intracranial germ cell tumors occur most frequently between ages 10 and 21 years (70%) and 95% occur before age 33 [16]. These tumors have a marked predominance in males. Jennings et al. (1985) showed that males are 2.2 times more likely to be affected, while Bruce et al. (1995) showed a stronger male preponderance of 8.5:1. Interestingly, there is an equal sex distribution, or perhaps a bias towards females, among suprasellar germ cell tumors [12,16].

Intracranial germ cell tumors represent a heterogeneous group of tumors, divided into germinomas and non-germinomatous germ cell tumors (NGGCT). NGGCT include teratomas, embryonal cell tumors, choriocarcinomas and endodermal sinus tumors (yolk sac carcinoma). Germinomas are the most common intracranial germ cell tumor, comprising between 40 and 65% of all germ cell tumors [16,18]. Teratomas (18-20%), endodermal sinus tumors (5-7%), embryonal cell tumors (3-5%), and choriocar-cinomas (3-4%) are less frequent [16,18]. Furthermore, large series with extensive tissue sampling have shown mixed germ cell histology in up to 25% of germ cell tumors [17]. Thus, it is imperative to establish an adequate histolog-ical diagnosis prior to proceeding with radiation or chemotherapy because the management and prognosis of patients with germ cell tumors vary

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