Melanomas like those suffered by Sen. John McCain are more lethal than other types of skin cancers because the pigment-producing melanocytes that produce them are actually not skin cells at all.
Though the basal cells and squamous cells that are responsible for the most common types of skin cancer are integral parts of the skin from the beginning, melanocytes are visitors -- nerve cells that are produced in the spinal column during infancy before migrating to the skin.
They are not bound to the skin in the same way that other skin cells are, so when they turn cancerous, they find it very easy to invade a blood vessel or lymph channel and travel to other parts of the body, said Dr. David Peng, director of the Pigmented Lesion Clinic at USC's Norris Comprehensive Cancer Center.
And it is not the primary tumor that kills a patient, but the metastasis, said Dr. Glen Justice, director of Orange Coast Memorial Hospital Cancer Center in Fountain Valley. The key to curing melanomas is to catch and excise them before they have spread, he said.
If the tumors are caught early, surgery cures at least two-thirds of patients, experts said. After the tumor has metastasized, chemotherapy and radiation may be attempted, but they are generally palliative, not a cure, Justice said.
Each year, about 50,000 malignant melanomas are diagnosed in the United States, and they account for about three-quarters of all skin cancer deaths. The vast majority of deaths occur in the 20% of patients whose tumors have already metastasized at the time of diagnosis.
McCain has had at least four melanomas. The most common risk factors include fair skin, fair eyes, light hair, excessive sunburns and excessive exposure to sunlight -- all of which he has. Genetics also plays a major role, but McCain has no family history of the disease that would make him more susceptible.
About 14% to 18% of melanoma patients have a second, unrelated tumor later, Justice said. "If [McCain] has had four, he'll probably have a fifth," he said. "But there is a superb chance it will be curable" because it will most likely be caught early.
The physicians from the Mayo Clinic in Scottsdale who cared for McCain emphasized that each of his four melanomas were independent tumors and not recurrences.
Melanocytes sit in the junction between the dermis, the outer layer of the skin, and the epidermis, the middle layer. In a primary melanoma, pathologists can see the tumor growing out from the junction and be confident that it is not a recurrence.
All four of the melanomas McCain suffered displayed this characteristic, including the large tumor that was removed from his temple in August 2000.
But buried in the nearly 1,200 pages of medical records released Friday was the fact that there were apparently two melanomas observed at the time, the large one that was the focus of the surgery and a smaller one at the same site that had very little involvement with the dermis.
"It might have come from underneath, and might have been a micro-metastatic lesion, indicating it was a recurrence from a previous tumor," Peng said. But because McCain has had no recurrence in the eight years since the surgery, Peng added, "he is doing pretty well."