A brain tumor removed from a 26-year-old Ph.D. student turned out to be an "evil twin" in the form of a teratoma — a kind of tumor with different types of tissue, including hair, teeth and bone.

The extremely rare tumors are thought to arise when cells of an embryonic twin are absorbed in the body of a developing fetus.

Yamini Karanam, a student at Indiana University's School of Informatics, went to doctors when she suddenly started having trouble in school, was failing to understand basic information and having trouble communicating with friends and school colleagues.

After six months visiting a number of neurologists and neurosurgeons across the country in search of an answer, specialists told Karanam they had detected what they thought was a cyst or tumor on her pineal gland, located deep within the center of the brain.

Surgery to remove it would be risky, she was told, but she located a surgeon in Los Angeles, Dr. Hrayr Shahinian, who specializes in extracting tumors from deep within the brain using minimally invasive "keyhole" techniques.

The surgery was a success, but things turned strange when Shahinian realized that what he had removed wasn't any kind of usual tumor; it was a teratoma, a clump of bone, hair and teeth tissue.

After surgery, when she awoke, Karanam was shocked when she learned of the tumor and the "evil twin sister who's been torturing me for the past 26 years," she said in an interview.

Such teratomas have puzzled medical experts for more than 100 years; the consensus is that they are probably one of a pair of twins that stops developing while still an embryo and is absorbed into the surviving sibling's body in the womb.

While they are sometimes detected after birth — sometimes teratomas can be quite large and attached to the newborn like a portion of a conjoined twin — smaller teratomas can escape detection well into adulthood.

While it can't be positively confirmed Karanam's tumor was a teratoma of a twin, it would likely have been fatal without the surgery to remove it.

Teratomas remain incredibly rare, says Shahinian.

"This is my second one," he said of Karanam's intracranial teratoma, "and I've probably taken out 7,000 or 8,000 brain tumors."

Tests have confirmed that the tumor was benign, and her doctors have told her she can expect a full recovery, Karanam says on her blog.