Mast Cell Tumours 1

Pet Tales
by Barry B. Burtis D.V.M.

Mast Cell Tumours

Kirk is an eight year old male black Labrador retriever. Strongly built, short-coupled and broad, his body reflects ruggedness and his heritage of athleticism. His calm, intelligent eyes, set firmly in his large, solidly sculpted head, clearly convey his kindly, eager to please nature. His confident, yet non-threatening manner, completes the portrait of handsome, secure masculinity, in its prime. Last fall, when I first laid eyes on him, it seemed beyond question that he was enjoying the best of health.

 A short while earlier, though, his owner had just happened to notice a rather flattened, lump or swelling below the skin over his left kneecap. It measured about 5 cm from top to bottom and about 4 cm from side to side.  The swelling was soft, its edges not well-defined and Kirk seemed totally unaware of it.  He showed no discomfort or objection when she touched it and she never noticed him licking it. The hair appeared to be growing normally over top it. His owner thought somehow he must have bumped or banged his leg in that spot. She kept checking the area and when it remained unchanged for over a week, she thought it would be wise to have her veterinarian have a look.

On a general physical examination performed at that time, Kirk was found to be completely normal except for the lesion described. No cause for its presence could be immediately identified. The veterinarian, in order to learn more about the lump, recommended that a diagnostic procedure called a fine needle aspirate be performed. This involves inserting a small gauge sterile needle, just like one used to give vaccines or similar injections, directly into the lesion. It is attached to a small syringe in order to allow a few cells to be drawn into the needle, from where it is placed. After withdrawing the needle from the lesion being investigated, the syringe can be used to expel those cells collected, onto a microscope slide. After applying an appropriate laboratory stain to the cells on the slide, they can be examined under a microscope in an attempt to identify them and therefore know the nature of the tissue from which they came. Fine needle aspirates do not always provide complete answers, sometimes larger biopsies of the tissue involved are necessary. However, their advantage is that when performed by a trained diagnostician, they can yield valuable information, quickly, inexpensively and painlessly to the patient. The procedure can be performed, in most cases, without sedation or anesthesia being required.

When the slide with the aspirate collected from the lump at Kirk's knee was examined microscopically, indeed, there were cells clearly visible. There were clumps of large round cells with heavily staining granules in the space around their nucleus. The cells were recognized as mast cells. Mast cells are specialized cells found throughout our body, particularly in areas of connective tissue. The deep layer of the skin is a common place to find them. They play a role in an animal's response to inflammation and allergies. Mast cells can release several biologically active chemicals when stimulated. Histamine and heparin are two of these chemicals.

Unfortunately, when we see abnormal numbers of mast cells in circumstances such as were present in the lump on Kirk's leg we know trouble has begun. A mast cell tumour was growing in the skin near his knee.

Mast cell tumours (MCT) can vary greatly in appearance. Most are first noticed as skin nodules or bumps that vary from 1 to 10 cm in diameter. Some cause ulceration or rawness of the over lying skin; others are first seen as single or multiple raised, red, inflamed skin lumps that have lost the hair covering them. Sometimes MCTs are soft and pliable, as Kirk's was, when palpated, other times they may be quite hard and firm. The tumour usually begins in the dermis, the deep layer of  the skin, but, rapidly growing ones often extend into the tissue beneath the skin or muscles even deeper below. It is not possible to estimate their malignancy or predict their behaviour based solely on their clinical appearance. Although diagnosis of the tumour is usually able to be made with the help of a fine needle aspirate, it is less able to help in predicting the seriousness of this form of cancer. Nevertheless, once the diagnosis is made a treatment plan can be formulated.

Surgery is almost always indicated in the treatment of a MCT. In my next column I will outline the plan that was followed to prepare for Kirk's treatment for one of the most common types of cancer to affect dogs.