Teratomas are germ cell tumors commonly composed of multiple cell types derived from one or more of the 3 germ layers. Inconsistent nomenclature often confuses discussions of various subtypes of teratomas. The word is derived from the Greek teras, meaning monster, which Virchow coined in the first edition of his book on tumors published in 1863.  Teratomas range from benign, well-differentiated (mature) cystic lesions to those that are solid and malignant (immature). Additionally, teratomas may be monodermal and highly specialized. Rarely, within some mature teratomas certain elements (most commonly squamous components) undergo malignant transformation.
In 1831, Leblanc coined the term dermoid cyst in the veterinary literature when he removed a lesion that resembled skin at the base of a horse's skull, which he called a “kyste dermoid.”  Both dermoid and teratoma, terms now more than a century old, remain in general use and often are used interchangeably with various preferences among subspecialties. The earliest implications were that elements similar to skin and its appendages composed dermoids, while teratomas had no such limits. Dermoids now are recognized as often being trigerminal and containing practically any type of tissue.
For those who continue to make a distinction, dermoids are tumors that maintain rather orderly arrangements, with well-differentiated ectodermal and mesodermal tissues surrounding endodermal components. Teratomas, specifically solid teratomas, are essentially devoid of organization; thus, the presence of some degree of organization, a high degree of cellular differentiation, and cystic structure differentiates dermoids from teratomas.  This article focuses on mature cystic teratomas, commonly referred to as dermoid cysts. See the images below.
Teratomas are made up of a variety of parenchymal cell types representing more than 1 germ layer and often all 3. Arising from totipotential cells, these tumors typically are midline or paraxial.  The most common location is sacrococcygeal (57%). Because they arise from totipotential cells, they are encountered commonly in the gonads (29%). By far, the most common gonadal location is the ovary, although they also occur somewhat less frequently in the testes. Cystic teratomas occasionally occur in sequestered midline embryonic cell rests and can be mediastinal (7%), retroperitoneal (4%), cervical (3%), and intracranial (3%).  Cells differentiate along various germ lines, essentially recapitulating any tissue of the body. Examples include hair, teeth, fat, skin, muscle, and endocrine tissue. See images above.
Sacrococcygeal teratomas are the most common tumors in newborns, occurring in 1 per 20,000-40,000 births. [3, 5] A population-based (rather than tertiary referral center) estimate from the United Kingdom found a birth prevalence of 1 per 27,000 live births. 
Mature cystic teratomas account for 10-20% of all ovarian neoplasms. They are the most common ovarian germ cell tumor and also the most common ovarian neoplasm in patients younger than 20 years. They are bilateral in 8-14% of cases. [2, 1, 7, 8]
The incidence of all testicular tumors in men is 2.1-2.5 cases per 100,000 population. Germ cell tumors represent 95% of testicular tumors after puberty, but pure benign teratomas of the testis are rare, accounting for only 3-5% of germ cell tumors. The incidence of all testicular tumors in prepubertal boys is 0.5-2 per 100,000, with mature teratomas accounting for 14-27% of these tumors. It is the second most common germ cell tumor in this population. [9, 10]
Benign teratomas of the mediastinum are rare, representing 8% of all tumors of this region. 
Mature cystic teratomas can result in significant morbidity. Potential complications vary depending on the site of occurrence.
Sacrococcygeal teratomas are commonly diagnosed in the prenatal period, and complications may occur in utero or during or after birth. The outcome after antenatal diagnosis is significantly worse than that for older postnatal surgical series, with survival rates ranging from –54-77%. [12, 13, 14]
Potential complications in utero include polyhydramnios and tumor hemorrhage, which can lead to anemia and nonimmune hydrops fetalis. If significant atrioventricular shunting occurs within the tumor, hydrops may result from high-output cardiac failure. Development of hydrops is an ominous sign. If it develops after 30 weeks' gestation, the mortality rate is 25%. If it is recognized, delivery is recommended as soon as lung maturity is documented. Development of hydrops before 30 weeks' gestation has an abysmal prognosis, with a 93% mortality rate. [12, 15] Makin et al reported that antenatal intervention for the treatment of fetal hydrops did not improve outcomes with neonatal deaths in 6 (86%) of 7 cases.  Hydrops and prematurity are the two main factors that contribute to mortality.
Postpartum morbidity associated with sacrococcygeal teratomas is attributable to associated congenital anomalies, mass effects of the tumor, recurrence, and intraoperative and postoperative complications. Ten to twenty-four percent of sacrococcygeal teratomas are associated with other congenital anomalies, primarily defects of the hindgut and cloacal region, which exceeds the baseline rate of 2.5% expected in the general population. [16, 17, 3]
In one larger series that included 57 cases of benign teratomas over a 40-year period from a single institution, 5 recurrences were documented. Only one of the patients who experienced recurrence did not undergo a coccygectomy, and one patient who was thought to have a benign tumor with immature elements was found to have embryonal carcinoma after the third excision. In this same series, 3 patients had postoperative wound infections and one patient had postoperative pneumonia. The overall survival was 95% and morbidity or mortality rates were consistent over the 40-year period of the study. 
In a more recent series, all 26 patients diagnosed with benign teratomas survived. Seven of 20 patients with long-term follow-up developed neuropathic bladder or bowel disturbances.  Partridge and colleagues studied a series of 45 patients, noting anorectal complications in 29% and urologic complications in 33%. These were associated with both prenatal obstructive findings and therapeutic interventions, as well as Altman classification, perineal reconstruction, and tumor recurrence.  A longitudinal cross-sectional follow-up study found that sequelae developing in childhood tended to improve with time, while functional symptoms reported in adulthood were common in the general population and not significantly increased over a control group. 
Rupture of a cystic teratoma is rare and may be spontaneous or associated with torsion. Most series report a rate of less than 1%, [2, 1] though Ahan et al reported a rate of 2.5% in their report of 501 patients.  Rupture may occur suddenly, leading to shock or hemorrhage with acute chemical peritonitis. Chronic leakage also may occur, with resultant granulomatous peritonitis. Prognosis after rupture is usually favorable, but the rupture often results in formation of dense adhesions.
A recently recognized encephalitis associated with antibodies against the N -methyl D-aspartate receptor (NMDAR) is associated with ovarian mature teratomas. In a series of 400 cases, of which 335 were women, 165 (49%) had tumors and all but six were ovarian teratomas. The syndrome is characterized by a viral-like prodrome followed by a multistage progression of symptoms that includes psychosis, memory deficits, seizures, language disintegration, decreased consciousness, dyskinesias, and autonomic instability. Substantial recovery is usually seen with tumor resection and immunotherapy. 
Autoimmune hemolytic anemia has been associated with mature cystic teratomas in rare cases. In these reports, removal of the tumor resulted in complete resolution of symptoms. Theories behind the pathogenetic mechanism include (1) tumor substances that are antigenically different from the host and produce an antibody response within the host that cross reacts with native red blood cells, (2) antibody production by the tumor directed against host red blood cells, and (3) coating of the red blood cells by tumor substance that changes red blood cell antigenicity. In this context, radiologic imaging of the pelvis may be indicated in cases of refractory hemolytic anemia. [23, 24]
In its pure form, mature cystic teratoma of the ovary is always benign, but in approximately 0.2-2% of cases, it may undergo malignant transformation into one of its elements, the majority of which are squamous cell carcinomas. The prognosis for patients with malignant degeneration is generally poor but dependent on stage and degenerated cell type. [2, 25]
Testicular teratomas occur in children and adults, but their incidence and natural history contrast sharply. Pure teratomas comprise 38% of germ cell tumors in infants and children but only 3% after puberty. In children, they behave as a benign tumor, whereas in adults and adolescents they are known to metastasize. [26, 27] With no documented cases of metastasis, morbidity from prepubertal testicular teratomas is largely limited to surgical or postoperative complications.
During and after puberty, all teratomas are regarded as malignant because even mature teratomas (composed of entirely mature histologic elements) can metastasize to retroperitoneal lymph nodes or to other systems. Rates reported vary from 29-76%. Morbidity is associated with growth of the tumor, which may invade or obstruct local structures and become unresectable. Approximately 20% of patients relapse during surveillance. 
Mature teratomas of the mediastinum, the most common mediastinal germ cell tumor, are benign lesions. They do not have the metastatic potential observed in testicular teratoma and are cured by surgical resection alone. Because of their anatomic location, intraoperative and postoperative complications are the only significant source of morbidity, as other intrathoracic structures are often intimately involved with the tumor. 
Sacrococcygeal teratomas are much more common in females than in males, occurring in a female-to-male ratio of approximately 3-4:1. Most sources report no sex predilection for mediastinal teratomas. Others document a marked male or marked female predominance. Excluding testicular teratomas, 75-80% of teratomas occur in girls. 
The presenting location of teratomas correlates with age.
In infancy and early childhood, the most frequent location is extragonadal, whereas teratomas presenting after childhood more commonly are located in the gonads. 
An increasing number of patients with sacrococcygeal teratomas are diagnosed antenatally. In Gabra’s series this proportion increased from 11% before 1988 to 53% from 1988-2001. Patients presenting later tend to have less obvious external tumors and symptoms of bladder or bowel dysfunction often leads to diagnosis. 
Cystic teratomas of the ovary can occur in persons of any age, although they are diagnosed most frequently during the reproductive years. The peak incidence in most series is age 20-40 years. 
Testicular teratomas may occur at any age but are more common in infants and children. In adults, pure testicular teratomas are rare, constituting 2-3% of germ cell tumors.