Any lumps or bumps felt on or under a dog’s skin should be assessed by a veterinarian as soon as possible. Mast cell tumors (MCTs) are common in domestic dogs and are especially variable in appearance, size, location and growth pattern. They often are mistaken by owners as a minor insect bite or other unimportant skin reaction.
Canine mast cell tumors can present in any number of ways. They can look like any other skin tumor, lesion or “bump,” including insect bites, skin tags, lipomas (fatty tumors), hives, weeping sores, allergic reactions or other benign or malignant masses. Usually, MCTs are solitary, although they can occur in clusters or in multiple places. Most of these tumors are found incidentally during a routine veterinary physical examination, because most dogs show no clinical signs associated with mast cell tumors.
MCTs normally are not difficult to diagnose. They tend to be readily identifiable by veterinarians when observed on a stained glass slide under a microscope. Specifically, the granules inside mast cells usually stain a characteristic blue-ish purple, which can be seen microscopically on a smear sample taken from the mass. Most veterinarians will perform a complete blood count, serum chemistry panel and urinalysis of dogs suspected of having a mast cell tumor. A fine needle aspirate from the lymph nodes nearest to the mass is also typically taken. The results of each of these tests can increase the relative certainty that the mass is a mast cell tumor. However, the only way to absolutely confirm the diagnosis is by taking and assessing an actual tissue sample of the affected area.
Once a primary mast cell tumor has been diagnosed, the veterinarian will want to determine whether it has spread, or metastasized, to other locations. She also will want to stage the severity of the disease. Further diagnostics may include abdominal ultrasound, bone marrow aspiration and assessment, regional lymph node biopsy and/or tissue biopsy of the primary tumor. If other organs are enlarged or otherwise appear affected, they probably will be assessed by aspiration, biopsy and/or ultrasound as well.
Once they are diagnosed, mast cell tumors normally are staged, or graded, based upon a number of different parameters. The staging can range from I to III or IV. Grade/Stage I tumors are well-differentiated (they are easily recognizable by veterinarians and veterinary pathologists as being distinctly separated from surrounding normal tissue); they carry a good to excellent prognosis with appropriate treatment. Grade/Stage II tumors are moderately differentiated but still carry a very good prognosis with appropriate treatment. Grade/Stage III or IV tumors are considered very late-stage, poorly differentiated tumors, and they carry a guarded to grave prognosis due to the presence of malignancy and metastasis.