An undifferentiated malignant tumour posing a diagnostic challenge

A 48-year-old man presented with an enlarging mass on his lower back on a background of previous American Joint Committee on Cancer 8th edition stage III melanoma excised from his chest. Biopsy showed a tumour with negative immunohistochemistry melanoma markers. The diagnosis was made using extensive immunohistochemistry staining, expert histopathology opinion, and molecular genetics that identified the same BRAF mutation as the patient’s original melanoma, allowing prompt management.