Melanoma of unknown primary origin: a population-based study in the Netherlands.

AIM: Few population-based studies have been published on melanoma of unknown primary origin (MUP). This study's aim is to describe characteristics and survival of MUP patients in the Netherlands, based on nationwide data from the Netherlands Cancer Registry (NCR).
METHODS: Patient and tumour characteristics of MUP patients were retrieved from the NCR. Subgroups were made according to metastatic site: nodal or distant. Survival rates were calculated using the Kaplan-Meier method. To obtain a better insight in the composition and prognosis of the MUP group, the survival was compared to that of patients with melanoma of a known primary origin (MKP), tumour-node-metastasis (TNM) stage III and IV.
RESULTS: Of all 33,181 melanoma patients diagnosed between 2003 and 2009, 2.6% (n=857) were diagnosed with MUP. MUP patients with nodal metastases had a similar survival as MKP stage III with macroscopic nodal involvement. After stratification according to the number of involved lymph nodes, the survival of patients with nodal metastases with one involved lymph node was not significantly different between MUP and MKP. The survival of MUP patients with two or more involved lymph nodes was slightly worse than that of MKP stage III patients with macroscopic nodal involvement with two or more involved lymph nodes. MUP patients with distant metastases had a similar survival as MKP stage IV. After stratification according to number of metastatic sites and metastatic site category, the survival in MKP stage IV patients with (sub)cutaneous metastases was slightly worse than MUP distant patients with (sub)cutaneous metastases.
CONCLUSIONS: The results of this study imply that MUP patients form a heterogeneous group, and that MUP patients with nodal metastases could be classified as stage III melanoma with macroscopic nodal involvement, and MUP patients with distant metastases as stage IV melanoma.