But as the television icon, 73, reveals in this week's PEOPLE, she is now fighting cancer again – and this time it is an even more formidable foe.
On Jan. 15 she received a diagnosis of leptomeningeal carcinomatosis, a rare condition that occurs when cancer cells spread into the fluid-filled membrane – known as the meninges – surrounding the brain.
The incurable disease, which accounts for less than 2% of all cancers, "comes on quickly and progresses quickly," says Harper's oncologist, Dr. Ronald Natale, director of the Lung Cancer Research Program at Cedars-Sinai Medical Center in Los Angeles. It can prove fatal in as few as three months.
"This is a really complicated condition," says Dr. Jeremy Rudnick, Harper's neuro-oncologist. "The spinal fluid is a collection of fluid that's being circulated [through the brain] kind of like a sink. The fluid itself is growing cancer cells so they are multiplying in there. Those cells start to coat the brain."
Because of the difficulty of getting chemotherapy drugs into the meninges, there are "limitations" in treating the condition, says Dr. Rudnick.
"You have a train that's moving 100 miles per hour, and what we're doing is slowing down the train to five or 10 miles per hour [using chemotherapy drugs]," says Dr. Rudnick.
The first symptoms of the disease often include seizures, blurry vision and severe headaches, along with "a belt-like sensation" across the midsection, says Dr. Rudnick. Harper first started experiencing the strange sensation last August, but an initial round of tests did not detect the condition.
After experiencing numbness in her jaw during rehearsals for her one-woman show Looped on Jan. 11, Harper was hospitalized for more tests. A third spinal tap finally revealed that cancer cells had spread into her spinal fluid.
By sharing her diagnosis, Harper hopes to shed light on the rare condition.
Receiving the news initially, "I was stunned," she says. "And in the next minute I thought, 'This could draw more attention to cancer research.' I think there's an opportunity to help people."