Undifferentiated Melanoma: A Rare Phenomenon and a Challenging Diagnosis

Abstract

Abstract Introduction/Objective Undifferentiated melanoma is defined as a primary or metastatic melanoma lacking histologic and immunophenotypic features of melanoma. Dedifferentiated melanoma shows transition between undifferentiated and conventional melanoma components. Dedifferentiation and transdifferentiation (implying dedifferentiation with heterologous elements) in melanoma are exceedingly rare and occur mostly in metastatic deposits. The purpose of this report is to describe a case of undifferentiated melanoma that proved to be a diagnostic challenge and to highlight the importance of the multimodality approach and utilization of advanced diagnostic tools in reaching an accurate diagnosis. Methods/Case Report Methods: A 43-year-old white male who was diagnosed with metastatic melanoma on the right axillary biopsy at a local hospital presented to our institution with a rapidly enlarging 30 cm peritoneal mass. He underwent debulking surgery to lower his tumor burden. On pathologic evaluation, the tumor showed a heterogeneous appearance ranging from highly cellular fascicles of spindle cells to sheets of primitive round cells and epithelioid cells with abundant eosinophilic cytoplasm. One metastatic focus showed heterologous cartilaginous differentiation. This morphologic appearance elicited the differential diagnosis of metastatic melanoma, primitive neuroectodermal tumor, synovial sarcoma, and metastatic germ cell tumor with somatic transformation. Immunochemical staining showed the tumor cells to be positive for SALL4, CD99 (patchy), and TLE1 (patchy) and negative for pan-cytokeratin, S100, SOX- 10, HMB-45, desmin, myogenin, DOG1, CD 117, CDX2, TTF-1, PAX-8, CK7, CK20, chromogranin, CD30, and OCT4. Fluorescence in situ hybridization for EWSR1, SS18(SYT), and isochromosome 12p were negative. BRAF gene mutation analysis detected p.V600E / p.V600D. Subsequently, a diagnosis of undifferentiated melanoma was rendered after expert consultation. Unfortunately, the patient passed away 1 month after surgery. Results (if a Case Study enter NA) NA. Conclusion Diagnosis of undifferentiated melanoma by the conventional H&E and immunohistochemical profile is extremely challenging. Multimodality approach and utilization of advanced diagnostic tools such as molecular studies are helpful in this regard.