Doctors in California have removed a tumour they have described as an “embryologic twin” deep inside the brain of a young woman.
Yamini Karanam, 26, a PhD student in Indiana, had been experiencing difficulties with drowsiness, reading and concentration.
The discovery was made when doctors performed a newly-developed form of surgery to remove the tumour.
The growth, known as a teratoma, had bone and hair.
Ms Karanam, a student at Indiana University, jokingly described the tumour to KNBC, a California TV station, as her “evil twin sister who’s been torturing me for the past 26 years”.
The tumour was found in Ms Karanam’s corpus callosum.
“This tumour was smack in the middle of that, so extremely deep in the brain,” Dr Hrayr Shahinian told the BBC.
It was not until after she awoke from surgery that she realised the details of her condition.
Ms Karanam told the BBC that her reaction to the news was subdued at first, but later, when the anaesthetics wore off, she became more surprised.
“I just thought: ‘There was a living thing inside my brain!'” she said.
Some medical experts have questioned whether it could be called a twin, but the doctor told the BBC it was “accurate, technically speaking” to call it an embryologic twin.
He added that she would have died if she hadn’t had the surgery.
The rare growth was removed using a minimally-invasive “keyhole” surgery that uses fibre-optics to burrow deep into the brain and perform the operation.
“Traditionally they would have had to cut her from ear-to-ear, bring the scalp down in the back, and then open up the entire back of the skull,” Dr Shahinian said.
Instead, the doctor used a half-inch incision in the back of the skull.
That operation, however, came at high cost, which Ms Karanam’s insurance was not able to fully cover.
A fundraising webpage has been set up to help her cover the costs, and at the time of writing has raised over $34,000 (£22,400).
“I was asked if I had any emotions for the ‘twin’ or something like that, but I’m like ‘No!'” she said. “I really don’t have any feelings attached to it.”