At 10 years old, Hannah Powell-Auslam should be enjoying her last few weeks of fifth grade, playing with her friends without a care in the world.

Instead, the Fullerton, Calif., girl is battling stage IIA breast cancer, also known as invasive ductal carcinoma, KCAL-TV reported.

Young children and adolescents have just a 0.1 percent chance of developing breast cancer, said Dr. Cynara Coomer, a breast surgeon at Mount Sinai Hospital in New York City and FOX News Health contributor. It's even rarer for a child to develop invasive ductal carcinoma, which is considered to be an adult cancer.

“Most of the cancers that adolescents would get are different, and it’s known as juvenile secreting neoplasm,” Coomer told FOXNews.com Thursday. “(Invasive ductal carcinoma) is the type we see in adults because it’s from estrogen stimulation.”

Last month, Hannah’s father Jeremy Auslam told the San Gabriel Valley Tribune in California that the situation felt “surreal.”

“We can’t be the only ones,” he told the newspaper.

Hannah was diagnosed in April after she complained of itching in her left breast and her mother noticed a lump, according to the Tribune.

Her parents took her to the doctor, and a small portion of the lump was sent away for tests. No one suspected breast cancer.

"They told me it was not breast cancer, because breast cancer does not happen to children," Hannah’s mother, Carrie Auslam, told the Tribune.

But the tests revealed otherwise: The tumor had grown around the blood vessels in Hannah’s breast.

“I didn’t really know what cancer was,” Hannah said. “I just kept crying and couldn’t stop.”

Coomer, who has not treated Hannah, said the girl’s prognosis is excellent: Her chance of a five-year disease-free survival is 85 percent.

On May 7, Hannah had a "simple left breast mastectomy," according to her Web site, OurLittleSweetPea.com.

So how did Hannah get breast cancer at such a young age?

“Usually there is a genetic disposition,” Coomer said. “That’s usually the case for young woman who develop breast cancer. She may carry the breast cancer gene.”

If she doesn’t carry the mutated BRCA gene, Coomer said there is another scenario that could have led to the diagnosis at such a young age.

When girls go through puberty and are developing breast tissue, the cells in their breasts are undergoing rapid divisions, she said.

Therefore, the cells are more likely to be damaged by cancer-causing agents and environmental exposures that can lead cells to change into cancer cells, she added.

“In regards to not creating hysteria, cancer in adolescents is small, but if you find a lump you should have it checked out,” Coomer said. “Most of the time it will be benign. And one of the ways young girls can lower their risk is for parents to promote healthy lifestyles — avoiding cigarettes and alcohol, exercising and eating a healthy diet.”

Click here for more from KCAL-TV.

FOXNews.com's Jessica Doyle contributed to this report.