|Classification and external resources|
A dermoid cyst is a cystic teratoma that contains developmentally mature skin complete with hair follicles and sweat glands, sometimes clumps of long hair, and often pockets of sebum, blood, fat, bone, nails, teeth, eyes, cartilage, and thyroid tissue. Because it contains mature tissue, a dermoid cyst is almost always benign. The rare malignant dermoid cyst usually develops squamous cell carcinoma in adults; in infants and children it usually develops an endodermal sinus tumor.:781
Some authors use the term dermoid cyst as a frank synonym for teratoma, meaning any teratoma, regardless of its histology or location. Others use it to mean any mature, cystic teratoma. These uses appear to be most common in gynecology and dermatology.
A dermoid cyst can occur wherever a teratoma can occur.
A dermoid cyst is a mature cystic teratoma containing hair (sometimes very abundant) and other structures characteristic of normal skin and other tissues derived from the ectoderm. The term is most often applied to teratoma on the skull sutures and in the ovaries of females.
Periorbital dermoid cysts 
Dermoid cysts can appear in young children, often near the lateral aspect of the eyebrow (right part of the right eyebrow or left part of the left eyebrow). These are sometimes watched and sometimes excised. An inflammatory reaction can occur if the dermoid cyst is disrupted.
Dermoid cysts can recur if not completely excised. Sometimes, complete excision is not practical if in a dumbbell configuration where the cyst extends through a suture line in the skull.
Spinal Dermoid Cysts 
Spinal dermoid cysts are benign ectopic growths thought to be a consequence of embryology errors during neural tube closure. Their reported incidence is extremely rare, accounting for less than 1% of intramedullary spinal cord tumours. It has been proposed that a possible 180 cases of spinal dermoid tumours have been identified over the past century in the literature.
Various hypotheses have been advanced to explain the pathogenesis of spinal dermoids, the origin of which may be acquired or congenital.
- Acquired or iatrogenic dermoids may arise from the implantation of epidermal tissue into the subdural space i.e. spinal cutaneous inclusion, during needle puncture (e.g. lumbar puncture) or during surgical procedures on closure of a dysraphic malformation.
- Congenital dermoids, however, are thought to arise from cells whose position is correct but which fail to differentiate into the correct cell-type. The long-time held belief was that the inclusion of cutaneous ectodermal cells occurred early in embryonic life, and the displaced pluripotent cells developed into a dermoid lesion.
Spinal abnormalities, e.g. intramedullary dermoid cysts may arise more frequently in the lumbosacral region (quite often at the level of the conus medullaris) and may be seen with other congenital anomalies of the spine including posterior spina bifida occulta as identified by the neuroradiological analysis.
Treatment for dermoid cyst is complete surgical removal, preferably in one piece and without any spillage of cyst contents. Marsupialization, a surgical technique often used to treat pilonidal cyst, is inappropriate for dermoid cyst due to the risk of malignancy.
The association of dermoid cysts with pregnancy has been increasingly reported. They usually present the dilemma of weighing the risks of surgery and anesthesia versus the risks of untreated adnexal mass. Most references state that it is more feasible to treat bilateral dermoid cysts of the ovaries discovered during pregnancy if they grow beyond 6 cm in diameter. This is usually performed through laparotomy or very carefully through laparoscopy and should preferably be done in the second trimester.
Differential diagnosis 
A small dermoid cyst on the coccyx can be difficult to distinguish from a pilonidal cyst. This is partly because both can be full of hair. A pilonidal cyst is a pilonidal sinus that is obstructed. Any teratoma near the body surface may develop a sinus or a fistula, or even a cluster of these. Such is the case of Canadian Football League linebacker Tyrone Jones, whose teratoma was discovered when he blew a tooth out of his nose.
See also 
- Ovarian cyst
- Dermoid sinus, more commonly known as a pilonidal cyst
- Proliferating trichilemmal cyst
- List of cutaneous conditions
- Freedberg, et al. (2003). Fitzpatrick's Dermatology in General Medicine. (6th ed.). McGraw-Hill. ISBN 0-07-138076-0.
- Najjar et al (2005) Dorsal Intramedullary Dermoids. Neurosurgery Review. 28:320-325
- Aalst et al (2009) Intraspinal Dermoid and Epidermoid Tumours: Reports of 18 Cases ad Reapproasal of the Literature. Pediatric Neurosurgery. 45:281-290
- Roth et al (1966) Intramedullary dermoid - Journal of neurology, Neurosurgery and Psychiatry 29:262-264
- Muraszko et al (2000) Intramedullary spinal tumours of disordered embryogensis – Journal of Neuro-oncology 47:271-281
- Walid MS, Boddy MG. (2008). "Bilateral dermoid cysts of the ovary in a pregnant woman: case report and review of the literature". Arch Gynecol Obstet. 279 (2): 105–108. doi:10.1007/s00404-008-0695-3. PMID 18509663.
- Maki: Jones returns to say goodbye Globe and Mail, November 16, 2006