Coexisting Onychomycosis and Subungual Malignant Melanoma: A Case Series Illustrating Diagnostic Complexity in Podiatric Practice

Background: Subungual malignant melanoma (SMM) is an uncommon but potentially life-threatening neoplasm that frequently mimics benign nail disorders, particularly onychomycosis. Although misdiagnosis of SMM as fungal infection is well documented, laboratory-confirmed coexistence of onychomycosis and melanoma within the same nail unit is exceedingly rare and remains insufficiently characterized in the literature. Methods: A two-patient case series is presented in which both individuals underwent comprehensive clinical evaluation, nail unit biopsy, histopathologic examination with special staining, and molecular and/or mycologic testing. Longitudinal follow-up was obtained to evaluate clinical course and outcomes. Results: Both patients presented with chronic hallux nail dystrophy and laboratory-confirmed onychomycosis. In each case, fungal infection was confirmed, however, persistent symptoms prompted subsequent biopsies, that revealed malignant melanoma. In Case 1, a 71-year-old man with Candida parapsilosis onychomycosis underwent an initial nail unit biopsy demonstrating at least melanoma in situ with ulceration. Repeat biopsy confirmed invasive melanoma with an approximate Breslow thickness of 1.0 mm, while final pathology following partial hallux amputation revealed residual ulcerated melanoma with a Breslow thickness of 2.1 mm, negative margins, and negative sentinel lymph nodes, consistent with Stage IIB disease. In Case 2, an 88-year-old woman with dermatophytic onychomycosis consistent with Trichophyton species had an initial fragmented biopsy demonstrating invasive subungual melanoma with an estimated Breslow thickness of 0.6 mm. Final pathology following distal hallux amputation confirmed residual acral melanoma with a Breslow thickness of 0.6 mm, no ulceration, a mitotic rate of less than 1/mm2, and negative margins, consistent with pT1a disease. Factors contributing to delayed melanoma recognition differed between cases: prior trauma, delayed intervention, and chronic ulceration complicated Case 1, whereas treatment for presumed onychomycosis without improvement delayed suspicion in Case 2. Conclusions: These findings demonstrate that positive mycologic results do not exclude concurrent melanoma and underscore the potential for diagnostic anchoring bias, supporting early biopsy consideration in refractory or atypical nail dystrophy.