Mast cell tumors (MCTs) are tumors composed of white blood cells called mast cells.
The cause is unknown. Many dogs have mutations in a specific gene that may be responsible for the creation and/or progression of MCTs.
All MCTs are considered potentially malignant.
MCTs usually develop in middle-aged or older dogs but can occur in pups as young as 4 months old.
Breeds that are notorious for developing MCTs include boxers, Boston Terriers, bull terriers, bullmastiffs, Staffordshire bull terriers, fox terriers, English bulldogs, dachsunds, Labrador retrievers, golden retrievers, beagles, pugs, Chinese Shar-peis, Rhodesian ridgebacks, and Weinaraners.
Signs
As skin tumors, MCTs occur on or under the skin and often resemble lipomas (fatty tumors. They can have a number of appearances, however, including haired or hairless bumps, red bumps, and swellings without defined edges.
They can occur on the head, trunk and legs.
MCTs can fluctuate in size over time, sometimes so dramatically that they appear to come and go.
Rarely, substances such as heparin and histamine will be released from mast cells into the surrounding tissues and produce an inflammatory reaction. This reaction can result in swelling, poor blood clotting, low blood pressure, and even death.
Diagnostic Tests
A tumor sample collected with a needle (fine needle aspirate, fna) and examined under the microscope (cytology) is very useful because mast cells are unique. Using this technique, a strong tentative diagnosis can be made at the time of an office visit to check a lump; further diagnostics and treatment planning can then procede with confidence.
Surgical biopsy is the only way to definitively diagnose MCTs. MCTs are graded as stage 1, 2, or 3, with stage 3 being the worst (malignant).
Once the diagnosis is strongly suspected or confirmed, other tests are needed to check for spread (metastasis) of the tumor. These tests include checking nearby lymph nodes, examining blood cells and bone marrow for mast cells, abdominal ultrasound to check the liver, spleen and other organs, and chest xrays to evaluate the lymph nodes in the chest.
Treatment
Surgery should be done as early as possible. An area around the tumor as wide as possible is removed. The tumor is then sent to the pathologist for evaluation, including staging.
If complete removal is not possible, we may recommend chemotherapy or radiation therapy. These treatments are usually reserved for some stage 2 and many stage 3 MCTs.
Mast cell tumors often release histamine, which can produce negative effects including stomach ulcers. Sometimes we will prescribe antihistamines to combat this effect.
Follow-up Care and Prognosis
All new lumps and bumps should be evaluated cytologically soon after they are detected.
Complete surgical removal of stage 1 or 2 MCTs is curative, and the only follow-up necessary is monitoring for new lumps. But patients sometimes will develop new, unrelated MCTs in a different location.
Following removal of high stage 2 MCTs dogs should be evaluated every 3 months for 1 year by exam, lymph node aspirate, and abdominal ultrasound.
Following removal of stage 3 MCTs dogs should be evaluated with bloodwork and a biochemistry profile in addition to the above tests every 2 months for 1 year then every 6 months for an additional 2 years.
