Amelanotic subungual melanoma becoming clinically apparent within 24 h after minor blunt trauma: a case report.

Amelanotic subungual melanoma (ASM) is a rare variant of nail unit melanoma (NUM) lacking melanin pigmentation, frequently misdiagnosed as a benign post-traumatic or infectious lesion. To our knowledge, ASM becoming clinically apparent within 24 h following minor blunt trauma has not been reported previously. We report the case of a 66-year-old male who presented with a persistent flesh-colored nodule beneath his right thumbnail that was manifested within 24 h after minor blunt trauma caused by nail clippers. The lesion was non-tender, non-pigmented, and unresponsive to 2 weeks of antibiotic therapy. Histopathology revealed diffuse subepidermal proliferation of atypical cells with signet-ring morphology. Immunohistochemistry (IHC) demonstrated positivity for vimentin, HMB45, and Melan-A, and negativity for cytokeratin 5 (CK5) and S-100. Ki-67 was approximately 30%; Breslow thickness was 2.5 mm. Final diagnosis: ASM, AJCC stage IIA (T3aN0M0). Diagnostic excisional biopsy with a 2-mm margin was performed; sentinel lymph node biopsy of two axillary lymph nodes was negative. At 1-year follow-up, no local recurrence or distant metastasis was observed. This case demonstrates that ASM can become clinically evident within 24 h after acute trauma, likely through unmasking of a pre-existing subclinical tumor. For post-traumatic nail lesions unresponsive to antibiotics, early biopsy with a comprehensive IHC panel (including Melan-A, HMB45, and S-100) is essential.