Abstract:

Background: We present a case of collision metastatic melanoma and chordoma, with sarcomatous dedifferentiation, to superficial inguinal lymph node after surgical resection of primaries, with likely pathologic complete response of the melanoma to neoadjuvant BRAF/MEK inhibitor therapy prior to superficial inguinal node dissection. Case Presentation: Patient is 34 year old male with C7 chordoma resection in 2018. Patient noted skin lesion on lower abdomen in 2019, with biopsy demonstrating pT1a melanoma and no evidence of adenopathy. Excision was performed with adequate margins. In 2020, patient presented with 3.7x2.3cm inguinal lymph node; pathology showed malignant neoplasm diffusely positive for S100, SOX-10, and melanoma cocktail, consistent with metastatic melanoma harboring BRAF V600E mutation. Staging showed no other metastasis. Neoadjuvant pembrolizumab was started, but eventually held due to side effects. BRAF/MEK inhibition was started with initial clinical response. Eventually, adenopathy worsened, so patient underwent superficial inguinal node dissection. Pathology demonstrated 5.5x4.8x3.5cm combined high-grade dedifferentiated sarcoma, immunohistochemistry(IHC) negative for melanoma markers, but positive for myoD1/Desmin, with focal melanin pigments and areas of chordoma by morphology and IHC (S100+/Cytokeratin+/Melan A negative). Characteristics were consistent with collision of de-differentiated chordoma and melanoma. NGS is pending. Conclusion: This unique scenario represents a diagnostic dilemma, emphasizing phenotypic diversity of melanoma and apparent dedifferentiation of chordoma with loss of typical IHC markers as potential escape mechanism to resist immunotherapy combined with BRAF/MEK inhibition. This combination of collision tumor of a metastatic melanoma with chordoma has not been reported.

Financial Disclosure:
No current or relevant financial relationships exist.

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Chloe Dominguez, BS