Malignant melanoma with metaplastic cartilaginous transdifferentiation: A case report.

Malignant melanoma is notorious for its remarkable morphological variation and aberrant histopathological patterns. However, melanoma with prominent cartilaginous transdifferentiation simulating chondrosarcoma is extremely rare. A 75-year-old male developed a swelling in his left inguinal region and was diagnosed with a metastatic melanoma, which was found to harbor a BRAF V600E mutation. Later on, the left inguinal lymph node was excised and immunohistochemistry done on the specimen revealed an undifferentiated component negative for S-100 protein, HMB-45 and Melan-A and a cartilaginous component positive for S-100 protein and diffusely positive for BRAF V600E mutation. To our knowledge, there are around 14 cases reported in the literature of malignant melanoma with pure cartilaginous transdifferentiation. In all cases, immunohistochemistry of the cartilaginous component was positive for S-100, which is not an indicator of melanoma because cartilage expresses S-100. BRAF mutational studies support that the tumor arose from a common melanoma cell that harbored the mutation and subsequently transdifferentiated. This case illustrates the importance of an accurate and thorough clinical assessment when it comes to the diagnosis of melanomas as they are notable for their impressive degree of morphologic variability. Moreover, this report helps shed light on the use of immunohistochemical analysis to reach a definitive diagnosis.