Melanoma is among several forms of cancer on the rise and one of three key malignancies with a propensity for spreading to the brain, doctors said Thursday.

Former President Jimmy Carter told a news conference Thursday he had been diagnosed with melanoma and it was found on his brain.

The cancer is frequently a conundrum for doctors. While it is primarily a skin malignancy, it can begin in an eye, the intestines, fingernails, the soles of the feet or the palms of the hands.

StoryCarter on brain cancer: I thought I had a few weeks left

"Melanoma is a very interesting disease process where we have seen patients present with metastasis at the very onset, yet we can't find the primary tumor," said Dr. Rajiv Datta, director of surgical oncology at South Nassau Communities Hospital in Oceanside. "So either the melanoma was removed years ago, or it undergoes an involution."

An involution of the original tumor means it is no longer present and may have been destroyed by the body's immune forces. In either case, cancerous cells escaped via the bloodstream or lymphatics before the tumor disappeared.

Datta, of the hospital's Gertrude and Louis Feil Cancer Center, said therapies in recent years have changed the treatment landscape. So-called immunotherapies stimulate the immune system to seek out cancer cells and destroy them.

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For some melanoma patients, and possibly Carter, the path to it may be indelibly inscribed in their genome, bearing a link to another difficult form of cancer, said Dr. Craig Devoe, acting chief of hematology and oncology at the North Shore-LIJ Health System's Monter Cancer Center.

"Jimmy Carter is well-known for his family history of pancreatic cancer. There are gene linkages for pancreatic cancer and melanoma," Devoe said. Carter's brother, Billy, died at 51 of pancreatic cancer in 1988.

A mutation in the P16 gene is the reason both cancers can emerge in families, said Devoe, who specializes in treatment and research on both malignancies. When healthy, P16 serves as a tumor suppressor. It is located on chromosome 9.

"When you are deficient in this gene product," Devoe said of the protein produced by the gene, "you are at risk for either cancer."

A test for the P16 mutation is available, Devoe said, but he didn't know if Carter, 90, had taken it. As a type of cancer with a tendency to spread, Devoe said melanoma is one of three forms of cancer likely to metastasize to the brain. The others are breast and lung cancers.

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"Brain metastases are the most common forms of brain tumors in adults in the United States," said Dr. Edward Halperin, chancellor of New York Medical College in Valhalla, Westchester County.

Primary tumors -- those that emerge first in the brain -- are far less common.

Carter said Thursday he has started radiation treatment.

Dr. Keith Black, chairman of neurosurgery at Cedars-Sinai in Los Angeles, said radiation therapy "can destroy a tumor without disturbing eloquent areas of the brain."

Radiation has limits, Black said, noting that "when lesions are more than three-centimeters, we tend to do surgery on those."