Teratomas (from Greek teras, meaning "monster," and -oma, a suffix denoting a tumor or neoplasm) and other germ cell tumors are relatively common solid neoplasms in children. They may occur in both gonadal and extragonadal locations. Locations and specific tumor types depend on the age of the child. These tumors are grouped together because they all appear to arise from postmeiotic germ cells. Most of the malignant tumors produce markers that can be serologically assessed.
Several theories about the origin of these tumors are recognized. The best evidence suggests that most are due to abnormal differentiation of fetal germ cells that arise from the fetal yolk sac. Normal migration of these germ cells may cause gonadal tumors, whereas abnormal migration produces extragonadal tumors.  Teratomas are typically found in the midline or gonads. Frequencies of the most common sites are as follows:
Sacrococcygeal - 40%
Ovary - 25%
Testicle - 12%
Brain - 5%
Other (including the neck and mediastinum) - 18%
By definition, teratomas include components derived from all three embryonic layers: ectoderm, endoderm, and mesoderm. These tissues are foreign to the location in which they are found.
Teratomas may be classified as mature or immature on the basis of the presence of immature neuroectodermal elements within the tumor. Mature tumors (grade 0) have no immature elements. In grade 1 tumors, immature elements are limited to one low-power field per slide; in grade 2 tumors, fewer than four fields are present per slide; and in grade 3 tumors, more than four fields are present per slide.
In the past, survival was linked to the degree of immaturity in the teratoma. Close histologic evaluation of immature teratomas reveals a good correlation between the degree of immaturity and the presence of microscopic foci of frankly malignant elements. These malignant elements are typically yolk sac tumors but may also represent primitive neuroectodermal tumor (PNET). Charoenkwan et al found overexpression of p53 in the more aggressive immature teratomas at all sites. 
The risk of recurrence also appears to be related to the degree of immaturity. Recurrence in a completely resected mature teratoma is less than 10%; in an immature teratoma, recurrence may be as high as 33%.
The likelihood of recurrence depends on the site of the tumor as well as the completeness of resection. The German MAKEI trials suggest that the recurrence rate for immature teratomas can be decreased to 9.5% with chemotherapy.  Sacrococcygeal teratomas are more likely to recur than those in the ovary or other sites. Molecular biologic and cytogenetic studies are providing a firmer scientific basis to these observations.
In 1965, Teilum first suggested the germ cell origin of gonadal tumors.  Since that time, the pathologic classification scheme has evolved to its present state. Germ cells undergo neoplastic transformation as follows:
Suppressed differentiation - Seminoma; dysgerminoma
Differentiation - Initial (embryonal carcinoma); embryonic (mature teratoma and immature teratoma); extraembryonic (choriocarcinoma and endodermal sinus tumor [yolk sac tumor])
Mutter described genetic imprinting as a major factor in the development of some of these tumors.  The developmentally expressed genes insulinlike growth factor 2 (IGF II) and its receptor RNA (H-19); small nuclear riboprotein (SNRPN); mas proto-oncogene; and the tumor suppressor genes WT1 and MASH2 are imprinted, depending on their maternal or paternal origin.
Mutter suggested that these genes or the cells have only the maternal imprint because many teratomas arise from a parthenogenetically activated egg. Therefore, maternally active genes are present in higher than usual concentrations, and maternally inactive products are present at lesser concentrations if at all. These abnormalities may account for the lack of organization of the three germ cell layers.
Oosterhuis et al suggested that tumors may be grouped on the basis of their chromosomal abnormalities as follows  :
Group 1, including immature teratomas and yolk sac tumors - Immature teratomas are usually diploid, whereas yolk sac tumors may be diploid, tetraploid, or aneuploidy; chromosomal aberrations include overrepresentation of chromosomes X, 1, 3, 8, 12, and 14 and underrepresentation of Y and X; deletions in 1p and rearrangements of 3q and 6q may be present; isochromosome 12p (i12p) has been found; an abnormal number of centromeres is frequent in both diploid and aneuploid tumors
Group 2, including most nonseminomatous malignant germ cell tumors and typically including numeric abnormalities in X, 7, 8, 12, and 21 as excess and deletions of Y, 11, 13, or 18 - Once again, isochromosomes 12p with other aberrations of 12p and 1p are present
Group 3, including mature teratomas or mature cystic teratomas - Numeric abnormalities, including extra X, 7, 12, and 15, have been found; no chromosomal structural anomalies have been found
Group 4, including spermatocytic seminoma, a type usually confined to older men - The cytogenetics of this group have not been characterized; as with abnormalities and imprinting patterns, these chromosomal rearrangements can lead to overproduction of certain gene products and underproduction of others; these lead to the abnormal growth characteristics of the tumor
Hara et al suggested that the MAGE gene family of tumor rejection antigens may also be involved in the pathogenesis of these tumors.  These genes appear to be more active in pure seminoma or mixed type of seminomatous elements than in other germ cell tumors. In their limited study of 22 patients, MAGE expression was not correlated with disease progression. It is likely to be only an indicator of maturity or differentiation of the tissues.
The concept of teratoma with malignant transformation indicates the development of non–germ cell malignancies within a teratoma. Among 641 patients in the MAKEI protocols 83/86/89/96, nine patients were identified with this finding.  Five patients presented with a carcinoma, two presented with glial tumors, and two with embryonal tumors. Resection and chemotherapy were typically used. Because these tumors are quite rare, response to treatment is difficult to generalize. 
When platinum-based chemotherapy–resistant tumors are evaluated, between one third and one half of tumors exhibit microsatellite instability.
Sacrococcygeal teratoma occurs in 1 in 30,000-70,000 live births. The female-to-male predominance is 4:1. Ovarian teratomas are almost as common, whereas testicular teratomas are about one third less frequent. The overall incidence of malignant germ cell tumors is approximately 3% of all childhood malignancies, or approximately 3 cases per million population per year. The frequency of all germ cell tumors has increased over the last several decades.
Mortality for congenital teratomas depends on gestational age and the size and location of the tumors. Survival of preterm infants younger than 30 weeks' gestation with sacrococcygeal teratoma is only 7%, whereas the survival for infants older than 30 weeks' gestation is 75%.
Rapid early growth is associated with the yolk-sac phenotype and carries a poorer prognosis.  Early tumors are frequently large relative to the size of the infant and may induce congestive heart failure. Cervical teratomas may frequently lead to airway problems and death when they are large.
Before current developments in chemotherapy, the 10-year survival rate for malignant germ cell tumors ranged from 25% for embryonal carcinoma to 75% for dysgerminoma. Today, overall survival rates are greater than 90%.
Now that survival of most patients is approaching 100%, the morbidity of treatment, particularly fertility, must be addressed. Because these patients are often prepubertal, standard means of collecting and preserving sperm or ova are not applicable. Techniques for preserving normal gonadal tissue and maturing it to produce viable sperm and ova must be developed.
Long-term follow-up of these patients is necessary because of the possibility of chronic pulmonary disease, hearing loss, and other long-term toxicities of the chemotherapy used in treating these lesions.
No racial predispositions for these tumors are known.
Sacrococcygeal teratoma has a 4:1 female-to-male predominance. In other germ cell tumors, the female-to-male ratio is roughly 2:1 in children.
Sacrococcygeal teratomas are congenital. Those with a significant external component (see the image below) are identified at birth. Tumors without an external component (Altman type 4) are discovered later. When the tumors are resected before the patient is aged 2 months, 7-10% are malignant. After that age, the risk of malignancy greatly increases to more than 50% by age 1 year.
The incidence of ovarian germ cell tumors (see the image below) increases with age and peaks around age 15-19 years. When girls younger than 15 years were examined, fewer than 10% of tumors occurred in girls younger than 5 years, 20% were found in girls aged 5-9 years, and more than 70% were found in girls aged 10-14 years. 
Benign ovarian tumors, largely teratomas, predominate. Roughly 70% of malignant ovarian tumors in childhood are germ cell tumors, one quarter are epithelial, and the remainder are stromal tumors. The ratio of germ cell tumors to epithelial malignancies decreases with increasing patient age.
Chromosomal abnormalities also appear to be related to age at presentation for teratomas. In girls younger than 5 years, no chromosomal abnormalities were found, whereas older girls often have gains of 12p and chromosomes 7 and 8.
Testicular germ cell tumors in childhood are split between teratomas and yolk sac tumors (see the image below). They are more common from birth to age 5 years. From age 6 years until puberty, testicular tumors are exceedingly uncommon. Thereafter, the incidence increases, with a more adultlike tumor pattern with seminomas gradually becoming the predominant histology.
Both teratomas and yolk sac tumors may be associated with contralateral in situ dysgenesis in 9% of patients compared with 0.5% of otherwise healthy males. Contralateral tumors are often found. These are occasionally synchronous but are more often metachronous. Ongoing surveillance of the contralateral testis is therefore needed.
Among malignant germ cell tumors, yolk sac tumors predominate until patients are aged 14 years. Few tumors of any type are diagnosed in children aged 5-9 years. For malignant teratomas, yolk sac tumors, and all germ cell tumors, respective rates by age group are as follows:
Age group 0-4 years - 0.45 case, 3.66 cases, and 4.16 cases per million population
Age group 5-9 years - 0.12 case, 0.12 case, and 0.12 case per million population
Age group 10-14 years - 0.05 case, 1.30 case, and 1.77 case per million population
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