BRAF wild-type metastatic melanoma arising in a congenital melanocytic nevus initially reported as melanoma in situ, 4 years after negative sentinel lymph node biopsy: a case report and literature review

Melanoma arising in a congenital melanocytic nevus is uncommon, and distant relapse years after a thin primary tumor and a negative sentinel lymph node biopsy (SLNB) is rarely documented in detail. A 34-year-old man underwent wide local excision of a bleeding congenital hairy nevus of the sacral skin. Histopathology showed melanoma in situ with a thin invasive component (<0.8 mm, nonulcerated, mitotic rate 7/10 high-power fields; recorded as pT1b), and BRAF testing was negative. After referral 2 years later, lymphoscintigraphy-guided SLNB was negative in all eight nodes from both groins and the right axilla. Four years after nodal staging, he presented with back pain, lower-limb weakness, and urinary retention from multiple vertebral metastases with an epidural component at L3, raising concern for cauda equina compression, together with solitary hepatic and pulmonary lesions, a pattern consistent with hematogenous dissemination. Liver and vertebral biopsies confirmed metastatic melanoma. Next-generation sequencing identified no variant of therapeutic or prognostic significance (BRAF wild-type) and a low tumor mutational burden (2.84 mutations/Mb). Palliative radiotherapy to L1-L4 (20 Gy in five fractions) restored ambulation, and denosumab and ipilimumab plus nivolumab, an accepted first-line option for BRAF wild-type melanoma, were started. In a lesion reported as melanoma in situ, the invasive component, not the in-situ component, confers metastatic potential. A negative SLNB does not exclude occult nodal disease or hematogenous dissemination. New skeletal or neurological symptoms in melanoma survivors warrant prompt imaging, even after a prolonged disease-free interval.