Melanoma with unknown primary: report and analysis of 24 patients

In the literature, there are some papers reporting on patients with metastatic melanoma from an unknown primary lesion (MUP). The pathogenesis of this phenomenon and the prognosis of these patients are still debatable. Therefore, we reviewed our casistics on MUP patients. We identified 24 MUP patients out of all patients registered into a melanoma database from June 1996 to June 2011. The incidence was 1.4%. We compared the survival rate of all patients with MUP stage III-IV with all patients with metastatic melanoma known primary (MMKP) stage III-IV observing a clear survival improvement for MUP patients in front of MMKP patients (p<0.01). In a second instance, we compared stage III MUP patients with only lymph nodal involvement with stage III MMKP patients with only lymph nodal involvement, and again we found statistically significant better survival for MUP patients (p<0.05). In this retrospective study, the number of lymph nodes involved (p=0.8), the sex (p=0.9), and S100 value (p=0.2) were not statistically relevant for prognosis. The better prognosis for these patients is very similar to better survival rate for metastatic melanoma patients and vitiligo. This correlation may be in accord with the hypothesis of a regression of primary lesion by immunological system of the host and also the median age of patients at the time of diagnosis, commonly older than melanoma patients, may correspond to a long period of immunological interferences between the host and the melanoma disease.