A teratoma is a type of tumor that grows from germ cells, the early cells responsible for forming eggs or sperm. What makes teratomas unusual is their contents: because germ cells have the potential to develop into any type of body tissue, teratomas can contain hair, teeth, bone, fat, muscle, and even brain tissue. Most teratomas are benign and treatable with surgery, though some contain immature tissue that behaves more aggressively.
Why Teratomas Contain Hair, Teeth, and Bone
During early embryonic development, germ cells can differentiate into any of the body’s three foundational tissue layers: ectoderm (which normally forms skin and brain tissue), mesoderm (muscle, fat, and bone), and endoderm (the lining of internal organs like the lungs and gut). A teratoma forms when germ cells begin growing and differentiating on their own, outside of normal development, producing a disorganized mass of tissues that don’t belong where they end up.
The result can look startling on imaging or during surgery. Fat tissue appears in roughly 67 to 75% of cases. Teeth show up in about 31%. Some teratomas contain patches of skin with functional hair follicles and oil glands. Others hold cartilage, thyroid tissue, or mucus-producing cells. These tissues are real, fully formed human tissues, just growing in the wrong place.
Mature vs. Immature Teratomas
The most important distinction is whether the tissues inside a teratoma are mature or immature, because this determines whether the tumor is likely benign or potentially cancerous.
Mature teratomas are by far the more common type. They contain well-differentiated, adult-type tissues and are almost always benign. In the ovaries, they’re commonly called dermoid cysts. They tend to grow slowly and often go undetected for years.
Immature teratomas contain tissue that resembles fetal or embryonic cells, particularly immature nerve tissue. These tumors are typically larger at diagnosis, with irregular solid areas, coarse calcium deposits, and small amounts of fat. They may also show signs of internal bleeding or tissue death. Immature teratomas are graded on a scale from 1 to 3 based on how much immature neural tissue they contain. This grading, known as the Norris system, is one of the strongest predictors of outcome. A grade 1 tumor caught at an early stage often requires only monitoring after surgery, while higher-grade tumors typically need additional treatment.
Where Teratomas Develop
Teratomas most commonly grow in the ovaries and testes, since these organs house germ cells in adults. Ovarian dermoid cysts are the single most common germ cell tumor, typically found in women in their 20s and 30s. In men, testicular teratomas tend to appear during the same age range.
In newborns, the most common location is the tailbone area. These sacrococcygeal teratomas occur in roughly 1 in 40,000 live births, making them the most common congenital tumor. They’re often visible at birth as a mass near the base of the spine. Surgeons aim to remove them within the first week of life whenever possible.
Less commonly, teratomas can develop in the chest, abdomen, brain, or other midline locations. These tend to form along the path that germ cells travel during embryonic development, which explains the midline pattern.
Symptoms and Complications
Most teratomas produce no symptoms at all. Ovarian dermoid cysts in particular grow slowly and are frequently discovered by accident during imaging for an unrelated issue. When symptoms do occur, they’re usually caused by the tumor’s size or by a complication rather than the tumor itself.
For ovarian teratomas, the most concerning complication is torsion, where the weight of the cyst causes the ovary to twist on its blood supply. This produces sudden, severe pelvic pain, often radiating from one side toward the midline, sometimes with nausea and vomiting. The larger the cyst, the greater the torsion risk. Other possible complications include rupture, infection, abdominal bloating, and vaginal bleeding. In rare cases, the cyst produces hormones.
Malignant transformation of a mature teratoma is uncommon, occurring in roughly 1 to 2% of cases. This risk is highest in postmenopausal women, where one of the tissue types inside the teratoma begins growing uncontrollably.
How Teratomas Are Diagnosed
Imaging is usually the first step. On ultrasound, the mix of fat, calcium, and fluid inside a teratoma creates a distinctive pattern that experienced radiologists can often identify. CT and MRI provide more detail about the tumor’s internal composition and help distinguish mature from immature types. Immature teratomas tend to appear larger, more solid, and more irregular.
Blood tests for tumor markers play a supporting role, though they have limitations. Two proteins, alpha-fetoprotein (AFP) and beta-hCG, are commonly measured when a germ cell tumor is suspected. Pure mature teratomas typically don’t elevate either marker, which means normal blood tests don’t rule out a teratoma. Elevated AFP suggests the presence of more aggressive germ cell elements like yolk sac tumor. Overall, these markers have a sensitivity of only about 50% at initial diagnosis, so imaging and surgical pathology remain the definitive tools.
Surgery and Recurrence
Surgery is the primary treatment for nearly all teratomas. For ovarian dermoid cysts in younger women, surgeons typically perform a cystectomy, removing only the cyst while preserving the rest of the ovary. This approach protects future fertility. In some cases, particularly for larger tumors or in older women, the entire ovary is removed.
A study of 442 young women who underwent surgery for ovarian mature teratomas found a 5-year recurrence rate of 11.2% in women who had cystectomy alone. Interestingly, women who had the entire ovary removed had a higher 5-year recurrence rate of 20.3%, likely because the characteristics that led doctors to choose a more aggressive surgery (larger or more complex tumors) also indicated a higher baseline risk. This means that for most young women with a straightforward dermoid cyst, ovary-sparing surgery offers both good fertility preservation and a low chance of the tumor returning.
For immature teratomas, complete surgical removal is equally important. Staging, the tumor’s grade, and whether it contains mixed elements like yolk sac tissue all factor into whether chemotherapy is recommended afterward. Early-stage, low-grade immature teratomas in adults are increasingly managed with surveillance alone after surgery, avoiding chemotherapy unless the tumor returns or progresses.
Sacrococcygeal teratomas in newborns are treated with early surgical excision, which includes removing the coccyx (tailbone) to reduce the chance of recurrence. Long-term follow-up is standard because late recurrences, while uncommon, can occur years after the initial surgery.