Hematologic malignancies associated with germ cell tumors.

Germ cell tumor (GCT)-associated hematologic malignancies present a unique challenge to hematologists and hematopathologists. As most GCTs are of gonadal origin, only a small percentage occur at extragonadal sites in the midline. Extragonadal GCTs are believed to originate from the ectopic primordial germ cells that fail to migrate to the urogenital ridge during development. An overactive KIT pathway and overexpression of genes on chromosome 12p are strongly implicated in GCT development. Approximately 54% of extragonadal GCTs are located in the anterior mediastinum. This is disproportionally high among the midline structures, presumably due to a favorable microenvironment for GCT development in the developing thymus. The mediastinal nonseminomatous GCTs have two unique features. First, they are often refractory to current treatment modality with the worst prognosis among GCTs of all sites. Second, they have a tendency to give rise to secondary hematologic neoplasia. The outcome is grave for patients with GCT-associated hematologic malignancies. As standard chemotherapy used to treat their bone marrow-derived counterparts has been ineffective, the best treatment modality to achieve long-term survival is allogeneic hematopoietic stem cell or cord blood transplant for a very limited number of cases.