The Appearance of a Candidate Site for a Primary Melanoma: A 5 Year-gap with a Melanoma of an Unknown Site

Although more than 90% of melanomas have cutaneous origins, melanomas sometimes present metastatically with no apparent primary lesion. A 62-year-old female presented with black pigmentation on her left thumbnail that had begun 2 years earlier and after the biopsy, she was diagnosed with malignant melanoma. Interestingly, 7 years earlier, a 4 cm palpable mass on her left axilla had been diagnosed as melanoma from an unknown primary site (MUP) with the involvement of an axillary lymph node. We speculate that the melanoma of the left thumb was the primary site and the melanoma in the axilla was a metastasis from the left thumb, and suggest several hypotheses explaining the appearance of the primary lesion as acral lentiginous melanoma after detecting a metastatic site. We consider this case interesting because it helps us to understand the pathogenesis of MUP and reminds physicians to conduct careful periodical work-ups of melanoma patients, and highlights the importance of continued long-term follow-up, especially for patients with MUP.