A teratoma is a type of tumor that grows from germ cells, the same early-stage cells that normally develop into eggs or sperm. What makes teratomas unique, and sometimes unsettling, is that they can contain recognizable body tissues like hair, teeth, bone, fat, and even brain-like tissue. This happens because germ cells have the ability to transform into any tissue type in the body, and a teratoma is essentially that process gone wrong in the wrong place.
How Teratomas Form
Every teratoma traces back to a germ cell that started differentiating on its own. Germ cells are pluripotent, meaning they can become any of the three foundational tissue layers that make up the human body: the layer that forms skin and nerves, the layer that forms muscle and bone, and the layer that forms organs like the gut and thyroid. A teratoma typically contains a chaotic mix of tissues from two or all three of these layers, which is why surgeons sometimes open one up to find a clump containing hair follicles, cartilage, and thyroid tissue all packed together.
Where Teratomas Grow
The ovaries are the most common location. Mature cystic teratomas (often called dermoid cysts) account for about 11% of all ovarian tumors, 69% of all ovarian germ cell tumors, and 95% of all ovarian teratomas. They average about 7 cm in diameter and frequently contain well-differentiated cells from all three germ layers.
The testicles are the second most common site in adults, and teratomas can also appear in the brain, the chest cavity near the heart, and the abdomen. In newborns, the most notable type is the sacrococcygeal teratoma, a growth at the base of the tailbone that occurs in roughly 1 in 35,000 to 1 in 27,000 live births. These are classified into four types depending on how much of the tumor sits outside versus inside the pelvis, ranging from mostly external (Type I) to entirely internal with no visible lump (Type IV).
Mature vs. Immature Teratomas
The most important distinction is whether a teratoma is mature or immature, because this determines whether it’s benign or malignant.
Mature teratomas are benign. Their tissues are fully differentiated, meaning they look and behave like normal adult tissues, just growing in the wrong place. The overwhelming majority of teratomas fall into this category. Malignant transformation is rare, occurring in roughly 1 to 3% of mature cystic teratomas, though some studies have reported rates as high as 5 to 6%. When transformation does happen, it most often becomes squamous cell carcinoma.
Immature teratomas are malignant. They contain primitive, underdeveloped tissue mixed in with the mature components, most commonly immature neural tissue that appears under the microscope as small tube-like structures called neurotubules. The proportion of immature tissue determines the tumor’s grade. A higher grade means more immature tissue and a more aggressive tumor. The WHO classification defines four grades, with Grade 0 being a fully mature (benign) teratoma.
Age Matters for Testicular Teratomas
In children before puberty, testicular teratomas are almost always benign, even when they contain immature tissue. After puberty, the picture changes significantly. Testicular teratomas in adolescents and adults are considered malignant regardless of how mature the tissue appears, because they carry a real risk of spreading. This age-dependent behavior is unique to testicular teratomas and influences how aggressively they’re treated.
Symptoms and Complications
Many teratomas, especially smaller ovarian ones, cause no symptoms at all and are discovered incidentally during imaging for something else. When symptoms do appear, the most common is abdominal pain, reported in about 48% of symptomatic cases.
Ovarian teratomas can cause complications as they grow. Torsion, where the weight of the tumor causes the ovary to twist on its blood supply, occurs in about 9.2% of cases and typically causes sudden, severe pain that requires emergency surgery. Spontaneous rupture is much rarer, happening in only 0.3 to 0.7% of cases, because the cyst walls tend to be thick. A ruptured teratoma can spill its oily, hair-filled contents into the abdominal cavity and mimic other conditions like pelvic inflammatory disease, making diagnosis tricky.
Sacrococcygeal teratomas in newborns are often visible at birth as a mass near the tailbone, though the entirely internal types (Type IV) may not be detected until the child develops constipation, difficulty urinating, or other symptoms from the tumor pressing on pelvic structures.
How Teratomas Are Diagnosed
Teratomas have distinctive features on imaging that often allow a confident diagnosis before surgery. On ultrasound, mature cystic teratomas show up as bright, echogenic masses because of the sebaceous (oily, waxy) material and calcifications inside them, things like bits of teeth or bone.
CT scans are particularly useful because fat within a cyst is essentially diagnostic of a teratoma. Few other tumors contain actual fat tissue. MRI can specifically identify the sebaceous component using fat-suppression techniques, which makes the fatty areas “disappear” on certain sequences, confirming what’s inside. Immature teratomas look different: they tend to be larger, more solid, and more irregular, with coarse calcifications and only small scattered pockets of fat.
Blood-based tumor markers are less helpful for pure teratomas than for other germ cell tumors. Most patients with mature or immature teratomas have normal blood marker levels. Elevated markers become significant mainly when a teratoma has undergone malignant transformation or contains elements of other germ cell tumor types mixed in.
Surgical Treatment
Surgery is the standard treatment for teratomas, and the specific approach depends on the tumor’s size, location, and the patient’s age and reproductive goals.
For ovarian mature cystic teratomas, minimally invasive (laparoscopic) surgery is considered the gold standard. It results in less blood loss, less pain afterward, shorter hospital stays, fewer adhesions, and better cosmetic outcomes compared to open surgery. For younger patients who want to preserve fertility, surgeons typically remove just the cyst while saving the rest of the ovary. In postmenopausal patients, or when the cyst is large and there isn’t much healthy ovarian tissue left, removing the entire ovary is the standard approach.
Open surgery becomes more appropriate for very large teratomas. Some guidelines suggest considering it for tumors over 10 cm, though the exact cutoff remains debated. The decision factors in the tumor’s size, whether it has solid components, the surgeon’s laparoscopic experience, and the patient’s individual situation.
For immature teratomas, surgery is also the primary treatment, but it may be followed by chemotherapy depending on the tumor grade and stage. Testicular teratomas in adults are treated with removal of the affected testicle, and further treatment depends on whether the cancer has spread.