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PITTSBURGH (KDKA) – When it comes to skin cancer, melanoma is less common than other types, but it’s much more aggressive.

That’s why it’s important to keep an eye on your skin, especially when you see a new mole form.

Dermatologist Dr. Brian Horvath, with Horvath Dermatology, says new moles are worth paying attention to.

“Most of our time when we’re finding new skin cancers, it is a brand new spot,” he said.

In fact, a study reviewing a collection of 38 previously published studies shows checking the same old moles isn’t where it’s at.

“This study showed that only about 29 percent of melanomas come from moles that have been there before,” Horvath said. “That means that the majority of melanomas are brand new spots that were not there previously.”

And these skin cancers coming from new spots behave badly.

“The melanoma centers that develop from existing moles tend to be a little bit thinner and less aggressive. They were more easily treated,” Horvath said. “The brand new ones tend to be a little bit more aggressive and difficult to treat.”

Pigment cells called melanocytes grow together in a cluster to form a mole. How many moles you have depends on family history and sun exposure. But not every new mole becomes melanoma.

“Most normal moles will appear rapidly,” Horvath said. “Cancerous moles are different. They will change. They’ll keep getting bigger in size. One part becomes much darker. They will evolve and change.”