The chances of dying from skin cancer depend strongly on how thick the tumor is, and, unexpectedly, survival odds are better if there is more than one primary tumor, according to a new study that was recently published in JAMA Dermatology (2013; doi: 10.1001/jamadermatol.2013.4581).

“First, we wanted to know if people with a single primary tumor were more likely to die from melanoma than people with multiple primary tumors,” said lead author Marianne Berwick, PhD, of the University of New Mexico Cancer Center in Albuquerque. “They’re not. But, if you match the thickness of the tumor, people with multiple primary tumors survive better.”

The research team, assembled from Australia, Italy, Canada, and elsewhere in the United States, asked 3,578 participants, newly diagnosed with skin cancer, to answer a lengthy set of questions about their personal history, family history, and lifestyle. The team also fully analyzed tissue and DNA samples from each person to obtain information about how many tumors and what mutations each person had, how thick the tumors were, how actively each tumor was growing, and whether or not the tumor surface had broken. Because of the large number of people, Berwick and her team could draw meaningful relationships between tumor qualities and people’s health.

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Rather than trying to match patients with a healthy person from the general population, the researchers chose instead to look at how different aspects of skin cancer affect survival.

Survival was less likely if the tumor created an ulcer in the inner layer of skin, called the dermis, or if the tissue-sample analysis showed that the tumor was rapidly growing. Tumor thickness was the most significant factor affecting survival. People whose tumors had grown 4 millimeters or more into the dermis were 7.7 times more likely to die than someone whose tumor had penetrated only 1 millimeter into their dermis.

When the research team compared tumor thickness with the number of tumors at initial diagnosis, they got a surprise. They found that for people with multiple primary tumors, those whose tumors were 4 millimeters or deeper were almost three times more likely to die than those whose tumors were only 1 millimeter. But for people who had a single primary tumor, those whose tumor was 4 millimeters or deeper were 13.6 times more likely to die than those whose single tumor was only 1 millimeter.

“Many people would think the opposite,” says Dr. Berwick, “because they think having more tumors is worse. It seems that those people with multiple melanoma have some sort of native immune factor that’s helping them. It’s keeping the melanoma in check.”