Signet Ring type malignant melanoma in a cat**
History: Axillary mass from an 11 year-old cat.
This specimen consists of sheets of neoplastic cells, with many lymphocytes dispersed throughout the population of neoplastic cells. These lymphocytes sometimes form nodules. These neoplastic cells have abundant eosinophilic cytoplasm with peripherally-located nuclei. The cytoplasm is homogeneous, glassy, and occasionally abundant pigment typical of melanin is present with the cytoplasm of neoplastic cells. There is moderate to marked variation in nuclear size and occasional very large and bizarre nuclei are present.
The location of the biopsy and the presence of lymphocytes forming nodules could indicate that the neoplasm has invaded a lymph node, indicating malignancy.
Based on the histologic appearance of cells with abundant glassy cytoplasm and peripheralized nuclei the Signet ring type of melanoma is the best diagnosis. Differential diagnoses for cells resembling this case include: 1) Granular cell tumor, which should have granular cytoplasm, and would stain positive with PAS; 2) Renal Oncocytoma ; 3) Anaplastic mammary carcinoma; 4) Plasma cell myeloma, in which cells can be filled with immunoglobulins.; 5) Poorly differentiated mast cell tumor.
Malignant melanomas can have a variety of histologic appearances ranging from round cells to spindyloid cells arranged in fascicles. Amelanotic melanomas have no recognizable melanin by light microscopy, but may be identified by the presence of melanosomes or pre-melanosomes by electron microscopy.
**Correction: This case was originally erroneously called a Balloon cell melanoma. The difference between these two types of melanoma is that the balloon cell type often has large clear cytoplasmic vacuoles, whereas the signet ring type of melanoma has eosinophilic cytoplasm and peripheralized nuclei.
Melinda J. Wilkerson, Karen Dolce, Brad M. DeBey, Heather Heeb, Harriet Davidson. Metastatic Balloon Cell Melanoma in a Dog. Veterinary Clinical Pathology Vol. 32 / No. 1 / 2003.
J. S. van der Linde-Sipman, M. M. L. de Wit, E. van Garderen, R. F. Molenbeek, D. van der Velde-Zimmermann and R. A. de Weger. Cutaneous Malignant Melanomas in 57 Cats: Identification of (Amelanotic) Signet-ring and Balloon Cell Types and Verification of Their Origin by Immunohistochemistry, Electron Microscopy, and In Situ Hybridization. Vet Pathol 1997 34: 31