Clinical Rounds: Battling a Labrador's oral malignant melanoma - Veterinary Medicine
Medicine Center
DVM Veterinary Medicine Featuring Information from:


Clinical Rounds: Battling a Labrador's oral malignant melanoma
After a referring veterinarian diagnosed this deadly neoplasm in a senior Labrador retriever, this team of experts stepped in to help extend the dog's life with targeted treatment. What can you learn from their approach to help your patients?



Jennifer Scruggs, DVM

Jennifer Scruggs, DVM
Definitive diagnosis of malignant melanoma based on fine-needle aspiration is often difficult. Although samples are typically of good cellularity, cytomorphology of both benign and malignant lesions can exhibit marked variation and may range from round to epithelioid or spindle-shaped.

1. A fine-needle aspirate from a lymph node in a dog. The sample contains individual and aggregated round to spindle-shaped cells. The cells have varying amounts of pale-grey, nonpigmented cytoplasm and large, round nuclei. Note the nuclear criteria of malignancy, including moderate to marked anisokaryosis, coarse chromatin, and prominent, multiple, and pleomorphic nucleoli. Rare small lymphocytes and a plasma cell are present.
The presence of cytoplasmic pigment (i.e. fine to coarse, brown, black, or green granules) may help identify the cells as melanocytic in origin; however, it is not always possible to distinguish melanin from other pigment (especially hemosiderin) without the benefit of special stains. Pigmentation of melanomas can vary both within and among tumors. Some lesions are amelanotic; therefore, the lack of melanin granules does not exclude melanoma from a differential list. Close inspection of nuclei may provide some insight into the malignant potential of the lesion—increased anisokaryosis, prominent and pleomorphic nucleoli, and coarse chromatin are most consistent with malignancy (Figure 1).

Diagnosis of metastatic disease in a regional lymph node is not difficult if high numbers of neoplastic cells are present. However, it becomes more challenging when low numbers of neoplastic cells are present given that rare melanocytes may be normally found in healthy lymph nodes. Additionally, melanocytes may be difficult to distinguish from pigment-laden macrophages.

Special stains that may be useful diagnostically for cytologic examination of melanocytic tumors include Fontana-Masson, which may help highlight melanin granules in poorly melanotic lesions, and Prussian blue, which highlights hemosiderin.9


Click here