Is there a correlation between skin cancer survival and birthmarks?

The objective of this study was to find out if number of birthmarks/moles is associated with survival in skin cancer.

Melanoma is a type of skin cancer in the color producing cells of the skins.  Increased number of nevi (birthmarks/ moles) is generally associated with the risk of developing melanoma. The number of nevi usually reduces with age but in melanoma patients, this process is delayed.

Melanoma can grow deep into the inner layers of skin and spread into lymph nodes (tiny, bean-shaped organs that help fight infections). The first lymph node to which melanoma is thought to have spread is called sentinel lymph node (SLN). It is not yet clear whether the number of nevi has an effect on whether cancer is present or absent on examination of the SLN. 

The authors aimed to evaluate if the number of nevi influences the outcome of SLN biopsy (removal of tissue from the lymph node to evaluate for the presence of cancer) and survival in melanoma.

Data from a group of 2,184 melanoma patients from three different studies were analyzed. The average thickness of melanoma (depth of melanoma in skin layer) was 2.27 mm.

Out of the total number of patients, 684 had high nevus counts (>50). The rate of survival for both a 5- and 10-year period was higher with a high number of nevi compared to low nevus counts. Overall, there was 57% reduced risk of death from melanoma when the nevus numbers were >50.  The status of SLN was not dependent on number of nevi. High nevus counts were even associated with better survival (78% reduced risk of death) in melanoma patients with positive SLN (i.e. cancer has spread to SLN), despite the fact that outlook is normally worse in these patients. 

The authors concluded that better survival was achieved in melanoma patients who had high nevus counts. 

Higher risk of melanoma with high nevus counts and improved melanoma survival with increased nevus numbers can seem contradictory. However the results imply that the genetic factors influencing the high nevus count and progression of tumors in melanoma may not be linked.