Multiple primary melanomas among 16,570 patients with melanoma diagnosed at Kaiser Permanente Northern California, 1996 to 2011.

Background: Published rates of cutaneous multiple primary melanoma (MPM) vary widely.

Objective: We examined incidence of and risk factors associated with MPMs among Kaiser Permanente Northern California members.

Methods: We estimated MPM incidence among 16,570 patients with melanoma from 1996 through 2011. We compared characteristics between patients with MPMs and single primary melanomas and estimated crude and adjusted hazard ratios of MPMs using Cox models.

Results: In all, 15,448 patients had a single melanoma and 1122 had MPMs. Patients with MPMs were older and more often male, non-Hispanic white, and partnered. Subsequent primary melanomas were diagnosed after a mean of 3.83 (SD 3.61, median 2.82) years and were more likely in situ and thinner than initial tumors. The risk of a subsequent melanoma decreased from 2% in the first year after diagnosis to a stable approximately 1% rate through 15 years of follow-up.

Limitations: We lacked data on some known melanoma risk factors and had small numbers of non-white patients and certain tumor subtypes.

Conclusions: The risk of MPMs, although highest in the first year after diagnosis, remains stable thereafter. Those at highest risk of MPMs are older, male, white, and partnered. Clinicians should be aware of the rate of MPMs and recognize high-risk subgroups.