Abstract

Metastatic melanoma of unknown primary site (MUP) is a scenario in which patients present with metastatic melanoma, but it is unclear where the primary melanoma was located. We examined our institution’s experience from 1996 – 2022 to see whether there has been any change over time as screening efforts and melanoma incidence rates have increased. We also wanted learn about the frequency of MUP presenting as a dermal and/or subcutaneous nodule. We identified 1353 MUP patients. The number of new MUP cases per year ranged from 20 to 67 (median, 51) and accounted for 12% to 19% (median, 16%) of all new melanoma diagnoses with stage III/IV disease. Although the number of MUP fluctuated through the years, there was no meaningful change in trend by count or by percentage. The percentage of MUP patients with first presentation of metastatic disease in the skin and subcutis ranged from 11 – 32% (mean, 22%; median 22%), also remained fairly constant. Of 1401 patients who presented with stage IV disease and lived for only one year, 279 had MUP (21%). Of those 147 had an initial single site metastasis, and 23 of the latter were to the skin and subcutis. Our findings show that the incidence rate of MUP has not significantly changed over 25 years. The findings suggest that the primary neoplasms giving rise to MUP may escape screening efforts and may be different from those melanomas associated with an increased incidence rate.