Introduction and general features Most germ cell tumors occur in the gonads; however, their development in extragonadal sites is an intriguing and recognized phenomenon 1, estimated to account for about 1-6% of all cases 1-5. Of the extragonadal sites, the mediastinum is the most common 1,6-8, encompassing 50-70% of adult extragonadal germ cell tumors 9. Conversely, germ cell tumors comprise approximately 10-20% of primary anterior mediastinal tumors (the compartment where they almost exclusively occur) in adults, with the additional primary considerations for this population in this location being thymic lesions, lymphomas, and endocrine tumors (Fig. 9.1) 9-14. The predominance of mediastinal cases in adults with extragonadal germ cell tumors contrasts with the situation in children where the mediastinum gives rise to only 4-7% of the extragonadal germ cell tumors 9,15,16, and the sacrococcygeal region and central nervous system are more common sites of origin 15,17,18. However, germ cell tumors are still estimated to make up as much as 24% of primary anterior mediastinal tumors and 8-18% of primary mediastinal tumors as a whole in children since primary epithelial tumors of the thymus are uncommon in the pediatric population (Fig. 9.2) 9,19-21. Therefore, mediastinal germ cell tumors occur over a wide age range, from neonates to the eighth decade of life 8,10,22. Median and mean ages vary from 23-40 years, reflecting their predominant occurrence in young adults, particularly men 8,10,22. Histologic features of mediastinal germ cell tumors are largely similar to those that occur in the testis and ovary, although they exhibit a number of unique clinicopathologic features and occasional characteristic findings that distinguish them from those of the gonads. In this chapter, we will discuss the incidence, distribution, and pathologic features of the individual histologic types of mediastinal germ cell tumors, with particular emphasis on the most important differential diagnostic considerations related to the mediastinum and points of contrast between these tumors and those of the gonads.
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