Rare mediastinal small round cell melanoma with synovial sarcoma-like immunophenotype: A potential diagnostic pitfall

Pathol Res Pract. 2024 Oct:262:155517. doi: 10.1016/j.prp.2024.155517. Epub 2024 Aug 6.

Abstract

Melanoma can pose a significant diagnostic challenge due to the high variability in histological morphology and expression of non-melanocytic immunomarkers. We present a case of a 47-year-old male with an aggressive mediastinal neoplasm and disseminated disease posing several diagnostic challenges. Multiple biopsies were submitted from different anatomic locations and during multiple time points showing an undifferentiated round cell tumor (URCT) with synovial sarcoma-like immunophenotype. SS18::SSX fusion was sought through NGS study for diagnostic confirmation. NGS results revealed NRAS and CDKN2A mutations and absence of fusions, resulting in a new review of the histologic material with a broader immunohistochemical panel, finding strong positivity to melanic antibodies. This case is an illustrative example of a malignant melanoma with small round cell morphology showing aberrant expression of CD99, BCL2, TLE1 and SS18-SSX antibodies exposing a potentially hazardous pitfall highlighting the importance of a wide differential diagnosis and the role of confirmational studies with molecular tests.

Keywords: Diagnostic pitfall; Immunohistochemistry; Melanoma; SS18-SSX fusion; Small round cell tumor; Synovial sarcoma; TLE1.

Publication types

  • Case Reports

MeSH terms

  • Biomarkers, Tumor* / analysis
  • Biomarkers, Tumor* / genetics
  • Diagnosis, Differential
  • GTP Phosphohydrolases / genetics
  • Humans
  • Immunophenotyping* / methods
  • Male
  • Mediastinal Neoplasms* / diagnosis
  • Mediastinal Neoplasms* / genetics
  • Mediastinal Neoplasms* / immunology
  • Mediastinal Neoplasms* / pathology
  • Melanoma* / diagnosis
  • Melanoma* / genetics
  • Melanoma* / immunology
  • Melanoma* / pathology
  • Membrane Proteins / genetics
  • Membrane Proteins / metabolism
  • Middle Aged
  • Oncogene Proteins, Fusion / genetics
  • Sarcoma, Synovial* / diagnosis
  • Sarcoma, Synovial* / genetics
  • Sarcoma, Synovial* / immunology
  • Sarcoma, Synovial* / pathology

Substances

  • Biomarkers, Tumor
  • Oncogene Proteins, Fusion
  • Membrane Proteins
  • GTP Phosphohydrolases
  • NRAS protein, human