Washington: Scientists claim to have found a clue to why nodular melanoma, a deadliest form of skin cancer, often proves fatal, as they spread vertically and doesn`t grow in diameter like other cancers that can easily be detected.
Nodular melanoma accounts for about 14 percent of all diagnosed skin cancers, but makes up 37 percent of ultimately fatal cases as they grow in depth and become more deadly, the researchers found.
"You can have melanomas that don`t follow the rules. These nodular melanomas tend to be more dangerous than the other melanomas," said study co-author Martin Weinstock, a professor of dermatology at Brown University.
"If you`re just relying on the typical signs of melanoma, there`s the risk that if you get a nodular melanoma, then you won`t notice it," he was quoted as saying by LiveScience.
For their study, published in the journal Archives of Dermatology, the researchers pored through more than 111,000 cases of invasive melanoma reported between 1978 and 2007 in the widely used database called the Surveillance Epidemiology and End Results Program, and found that at least one in five cases of nodular melanoma is ultimately fatal.
Both nodular melanomas and the more typical "spreading" melanomas begin in skin cells called melanocytes, which produce the skin pigment melanin.
When most melanomas develop, they stay in the skin`s top layer, growing no deeper than one-tenth of a millimeter for months or years and are easier to spot. It usually takes only a little novocaine and a quick mole removal to cure it.
But in nodular melanoma, the cancer cells quickly start growing vertically: A bump forms at the skin`s surface, and they send roots down into the fat, blood and other tissue of the body.
This matters, because a cancer`s depth can determine the chances of survival, the researchers said.
"One of the most important aspects of prognosis is how deep the melanoma is when it`s found," said Dr Roy Grekin, a professor of dermatology at the University of California San Francisco.
Grekin said if a cancerous mole grows 3.5 or 4mm deep into the skin before it`s found, then only 60 per cent of such patients will survive in the next five years.
Once the cancer grows deep enough, the subtype of melanoma has no bearing on the chance of survival, Weinstock said, so he suggests the public pay more attention to catching nodular melanoma in earlier stages.
Dermatologists have long promoted looking for the "ABCD" symptoms of moles: asymmetry, borders, color and diameter (a dangerous spot is usually 6 mm wide, or about the size of a pencil eraser).
"In general, we`ve done such a good job of educating the public about melanoma, but it only covers the superficial type," said Dr David Leffell of the Yale School of Medicine.
Finding nodular melanoma requires adding more letters to the ABCD model, he said.
"If you`re just relying on the ABCDs, there`s the risk that if you get a nodular melanoma, then you won`t notice it," Weinstock added.