You’ll never believe what doctors found inside this woman’s ovary

Here’s what you need to know.

Discovering that you have a tumor is bad enough—but what if doctors told you that your tumor was growing a brain? (Cue freakout.)

A new case report published in the journal Neuropathology details how a 16-year-old girl had a four-inch tumor removed from her ovary that contained “matted hair,” brain matter-like tissue, and a bony shell that resembled a skull. According to the doctors who removed the tumor, the tissue that resembled a brain was “well-differentiated and organized to an exceptionally high degree.”

Even freakier: Doctors discovered the tumor by chance when they were performing a routine appendectomy. After they removed the tumor, doctors cut it open and found that it resembled a small cerebellum and brain stem. (BTW: Doctors said the girl was fine after her operation.)

It sounds crazy and kind of gross, but women’s health expert Jennifer Wider, M.D., says it happens more often than you’d think. Known as a teratoma, these tumors contain cells not usually found in the area where the tumor exists, and happen in roughly 20 percent of ovarian tumors, Wider says.

“They are formed when a mass of cells inside a body grows into different tissue types, including bone, nerves, hair, and even teeth,” she explains. “They’re typically a benign [i.e. not cancerous] tumor surrounded by a capsule, making them relatively easy to remove.”

It’s not unusual to find nerve cells in a teratoma, Wider says, but it is “very rare” to find them organized into a brain-like structure.

While the idea of a tumor containing teeth and bones randomly sprouting up on your body is slightly terrifying, Wider says you shouldn’t stress about it: “These tumors, while more common than people may have thought, are still rare.”