Melan-A immunolabeling in canine extramedullary plasmacytomas

Area of Science:

• Veterinary Pathology
• Immunohistochemistry
• Canine Oncology

Background:

• Canine extramedullary plasmacytomas (CEMPs) can be challenging to diagnose histologically, especially less differentiated forms.
• Distinguishing CEMPs from other round cell tumors like lymphoma and amelanotic melanoma often requires immunohistochemical confirmation.
• Previous studies suggested CEMPs are MUM1 and λ light chain positive, while melanocytic markers like melan-A are negative.

Purpose of the Study:

• To evaluate the utility of MUM1 and melan-A immunohistochemistry in differentiating CEMPs from canine melanocytic tumors.
• To determine the diagnostic accuracy of a larger immunopanel for CEMPs versus melanocytic neoplasms.

Main Methods:

• Immunohistochemical analysis of 33 CEMPs, 20 melanocytomas, and 20 malignant melanomas.
• Testing included markers MUM1, melan-A, PNL2, PAX5, E-cadherin, CD3, CD18, CD20, S100, and λ/κ light chains.
• Comparative analysis of immunolabeling patterns between CEMPs and melanocytic tumors.

Main Results:

• All CEMPs showed MUM1 and λ light chain immunoreactivity.
• Notably, 39.4% of CEMPs exhibited melan-A immunolabeling, challenging previous assumptions.
• Melanocytic tumors were positive for melan-A (100%) and PNL2 (85%), with two showing unexpected MUM1 positivity.

Conclusions:

• MUM1 and melan-A alone are insufficient for differentiating CEMPs from amelanotic melanomas in dogs.
• A comprehensive immunohistochemical panel including MUM1, melan-A, PNL2, λ light chain, and CD20 is recommended for accurate diagnosis.
• These findings highlight the importance of a multi-marker approach in veterinary tumor diagnostics.