Author: Assoc Prof Patrick Emanuel, Dermatopathologist, Auckland, New Zealand, 2017.
Clear cell mesenchymal neoplasm (CCMN) is a rare dermal tumour of uncertain lineage. The tumours have a benign clinical course but histologically closely resemble malignant tumours including dermal metastases.
Histologically, these tumour arise in the dermis and are usually well circumscribed (figure 1). The cells of the tumour are clear, large cells (approximately 6–8 times the size of a stromal lymphocyte and about half the size of a subcutaneous adipocyte) that were oval to polygonal in shape with abundant clear cytoplasm (figure 2). Closer inspection revealed that, while many of the cells were optically clear, others contained lacy, reticular cytoplasm that had a slightly granular appearance (figure 3).
These tumours are negative for immunohistochemical markers for various keratins, melanoma markers, and markers often seen in renal cell carcinoma (Pax-8, RCC).
Metastatic clear cell carcinoma including renal cell carcinoma: This is probably the most important differential diagnosis. Numerous immunohistochemical studies including PAX-8, AE1/3, RCC,EMA need to be confirmed to be negative to exclude this possibility
Melanoma - CCMN is negative with the immunohistochemical markers for melanoma
PEComa - This tumour usually expresses melanocytic markers
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