of choice is complete surgical removal (i.e., complete resection). teratomas are normally well-encapsulated and noninvasive of surrounding tissues, hence they are relatively easy to resect from surrounding tissues. exceptions include teratomas in the brain, and very large, complex teratomas that have pushed into and become interlaced with adjacent muscles and other structures. prevention of recurrence does not require en bloc resection of surrounding tissues. chemotherapy for malignant teratomas, usually, surgery is followed by chemotherapy. teratomas that are in surgically inaccessible locations, or are very complex, or are likely to be malignant (due to late discovery and/or treatment) sometimes are treated first with chemotherapy. follow-up although often described as benign, a teratoma does have malignant potential. a uk study of 351 infants and children diagnosed with "benign" teratoma reported 227 with mt, 124 with it. five years after surgery, event-free survival was 92.2% and 85.9%, respectively, and overall survival was 99% and 95.1%. a similar study in italy reported on 183 infants and children diagnosed with teratoma. at 10 years after surgery, event-free and overall survival were 90.4% and 98%, respectively. depending on which tissue(s) it contains, a teratoma may secrete a variety of chemicals with systemic effects. some teratomas secrete the "pregnancy hormone" human chorionic gonadotropin (βhcg), which can be used in clinical practice to monitor the successful treatment or relapse in patients with a known hcg-secreting teratoma. this hormone is not recommended as a diagnostic marker, because most teratomas do not secrete it. some teratomas secrete thyroxine, in some cases to such a degree that it can lead to clinical hyperthyroidism in the patient. of special concern is the secretion of alpha-fetoprotein (afp); under some circumstances, afp can be used as a diagnostic marker specific for the presence of yolk sac cells within the teratoma. these cells can develop into a frankly malignant tumor known as yolk sac tumor or endodermal sinus tumor. adequate follow-up requires close observation, involving repeated physical examination, scanning (ultrasound, mri, or ct), and measurement of afp and/or βhcg. epidemiology ovarian tumors by incidence and risk of ovarian cancer, with mature cystic teratoma at bottom and immature teratoma at right. embryonal teratomas most commonly occur in the sacrococcygeal region; sacrococcygeal teratoma is the single most common tumor found in newborn humans. of teratomas on the skull sutures, about 50% are found in or adjacent to the orbit. limbal dermoid is a choristoma, not a teratoma. teratoma qualifies as a rare disease, but is not extremely rare. sacrococcygeal teratoma alone is diagnosed at birth in one out of 40,000 humans. given the current human population and birth rate, this equals five per day or 1800 per year. add to that number sacrococcygeal teratomas diagnosed later in life, and teratomas in other locales, and the incidence approaches 10,000 new diagnoses of teratoma per year. other animals ovarian teratomas have been reported in mares, mountain lions, and canines. teratomas also occur, rarely, in other species. use in stem cell research pluripotent stem cells including human induced pluripotent stem cells have a unique property of being able to generate teratomas when injected in rodents in the research laboratory. the roots of this observation has been attributed to leroy stevens of the jackson laboratory. in 1970, stevens noticed that the cell populations that gave rise to teratomas were very similar to the cells of very early embryos. for this reason, the so-called "teratoma assay" is one of the gold-standard validation assays for pluripotent stem cells. because differentiated human pluripotent stem cells are being developed as the basis for numerous regenerative medicine therapies, there is concern that residual undifferentiated stem cells could lead to teratoma formation in injected patients, and researchers are working to develop methods to address this concern. new research has looked at utilizing the human teratoma in chimeric animal studies as a promising platform for modeling multi-lineage human development, pan-tissue functional genetic screening, and tissue engineering.