FLI-1/Melan-A dual stain is an alternative to PRAME in differentiating metastatic melanoma from nodal nevus: A monocentric retrospective study

Melanocytic nevi existing in lymph nodes create a diagnostic challenge by mimicking metastases. PReferentially expressed Antigen in MElanoma (PRAME) immunohistochemical (IHC) stain can differentiate one from another. FLI-1 IHC expression has been shown in malignant melanoma with variable sensitivity while melanocytic nevi were reported to be negative. We hypothesized that FLI-1/Melan-A dual IHC staining may be used in the distinction of metastatic melanoma from nodal nevi and can be an alternative and/or complementary to PRAME. In this study, we examined 13 lymph nodes with metastatic melanoma and 13 lymph nodes with benign deposits. We stained all of the lymph nodes with FLI-1, FLI-1/Melan-A dual, and PRAME IHC stains. In addition, we stained paired skin samples of the metastatic lymph nodes with FLI-1 and PRAME. In primary cutaneous melanomas, 11 of 13 were positive for FLI-1 and PRAME expression (85%). Malignant cells in 12 and 13 lymph nodes showed positive expression of PRAME and FLI-1, respectively. Only one case with a nevic cell deposit was weakly positive for FLI-1 and the remaining benign cases were negative for both FLI-1 and PRAME. Our results show that FLI-1/Melan-A dual stain is as sensitive and specific as PRAME in distinguishing lymph nodes with metastatic melanoma from nodal nevi. Further studies with larger case numbers are needed to support our significant results.

Automated summary

Melanocytic nevi in lymph nodes pose a diagnostic challenge and can be mistaken for metastases. The use of PRAME immunohistochemical stain can help differentiate between the two. FLI-1 immunohistochemical expression has shown variable sensitivity in malignant melanoma, while melanocytic nevi are reported to be negative. In this study, we examined lymph nodes with metastatic melanoma and benign deposits, and found that FLI-1/Melan-A dual staining is equally sensitive and specific as PRAME in distinguishing between the two. Larger studies are needed to confirm these significant results.