Melanoma usually presents as a lump or bump and typically is fairly obvious to owners and veterinarians. The precise diagnosis of this type of cancer requires microscopic evaluation of cells and/or tissue samples. It can be difficult even for a skilled veterinary pathologist to determine whether melanoma is malignant or benign. Radiographs (X-rays) are commonly used to assess whether the disease has metastasized – especially whether it has spread to the lungs.
The first part of any diagnosis is a complete veterinary physical examination and a thorough case history. Normally, the initial data base also will include assessment of blood and urine samples. The results of these tests may reveal other health abnormalities, but unfortunately they are not conclusively diagnostic of melanoma.
A very common procedure used in suspected melanoma cases is taking a fine needle aspirate (FNA) of suspicious skin masses. This procedure involves inserting a small needle into the lump and pulling back on the attached syringe plunger to gather cells from the affected area. The extracted cells are then ejected onto a glass slide and examined under a microscope – a process that is called cytology. The slides can be dyed or stained before this examination to help enhance distinctions between different cell types. Fine needle aspirates are almost painless, much like a vaccine injection or a routine blood draw.
If the results of a FNA are inconclusive, or if they are suggestive of melanoma, the veterinarian probably will recommend taking a biopsy of the suspicious mass. A biopsy involves removing an actual piece of tissue, rather than simply sampling cells. Unlike FNAs, biopsies typically are performed under sedation, and sometimes under general anesthesia. Biopsy samples are sent to specialized pathology laboratories for processing and evaluation through a process called histopathology.
Additional diagnostic tests are also available, including: thoracic radiographs (chest X-rays) to assess whether the cancer has metastasized to the lungs; fine needle aspirates of nearby lymph nodes to determine whether the cancer has infiltrated the lymphatic system; and ultrasonagraphic examination of the abdomen to assess whether other areas of the dog’s body have been affected.
The techniques available to help veterinarians diagnose melanoma in dogs are evolving. It can be quite difficult to identify the precise stages of malignant melanoma or to catch the subtle changes in cells or tissues that happen when melanoma transforms from benign to malignant. Because of these difficulties, most veterinarians opt to surgically remove any identified melanoma mass, whether or not it is known to have metastasized or to be malignant.