Valerie Harper's terminal brain cancer: What is leptomeningeal carcinomatosis?

Valerie Harper attends NBC's taping of "Betty White's 90th Birthday: A Tribute to America's Golden Girl" at Millennium Biltmore Hotel on Jan. 8, 2012, in Los Angeles. Getty

Valerie Harper's disclosure that she was diagnosed with a rare, terminal form of brain cancer has people talking about leptomeningeal carcinomatosis.

The actress who played Rhoda Morgenstern on "The Mary Tyler Moore Show" told People Magazine for this week's issue that she has terminal brain cancer, and her doctors say she has as little as three months to live.

"I don't think of dying," the 73-year-old told the magazine. "I think of being here now."

Harper battled lung cancer in 2009.

Dr. Anders Cohen, chief of neurosurgery and spine surgery at The Brooklyn Hospital Center in New York City, explained to CBSNews.com that the cancer is likely a recurrence of her lung cancer showing up in her brain.

Cohen, who has no involvement in Harper's care, said leptomeningeal carcinomatosis is a metastatic disease, meaning cancer from one area of the body may have spread to another, in this case, the brain. Since 20 percent of the body's blood flow goes to the brain, he added, the brain is a very common place for other cancers to go. However, most types of these metastatic brain cancers involve a solid mass, or tumor, whereas leptomeningeal carcinomatosis is different, and occurs in about 5 percent of cancers that spread to the brain.

Unlike other tumors, leptomeningeal carcinomatosis affects the coating of the brain and spinal cord called the meninges. The cancer is "literally sprinkled all over" the brain he said, and doctors sometimes refer to this on an MRI scan as "sugar-coating" the brain.

"It's relatively rare," said Cohen. "Unfortunately, in my line of work I see a couple a year."

People with leptomeningeal carcinomatosis may experience dizziness, numbness on the side the body or trouble speaking. These symptoms may be caused by low-level seizure activity, because the brain is affected by the cancer.

Because the cancer does not present as one or multiple tumors like more common forms of brain cancer, Cohen said patients with leptomeningeal carcinomatosis don't typically respond as well to chemotherapy. Doctors may have to use a catheter to give chemotherapy to the whole brain, but that carries a lot of side effects, and even then, the prognosis is about four to six months, he said.

Without any treatment, a person might only have a few weeks after diagnosis, he added.

"If you don't treat this disease rapidly, it will grow like a weed in weeks," said Cohen.

Dr. Jeremy Rudnick, Harper's neuro-oncologist at Cedars-Sinai Medical Center in Los Angeles, gave a similar assessment to People.

"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]," he said.

He told the magazine that Harper first experienced a strange "belt-like sensation" across the midsection last August, but an initial round of tests found no cancer. After experiencing numbness in her jaw while rehearing for her one-woman show "Looped" this January, she was hospitalized and more tests revealed the cancer had spread into her spinal fluid, People reported.

She told the magazine she was stunned after receiving her diagnosis, but immediately thought about using it to spread awareness for the rare condition.

"And in the next minute I thought, 'This could draw more attention to cancer research.' I think there's an opportunity to help people," said Harper.

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