Mast Cell Tumor

Patty Khuly

Summary

A mast cell tumor is a cancerous mass composed of a white blood cell type called a “mast cell.” These cells function very specifically to thwart invasion by parasites through the release of toxic granules of histamine.

Though mast cells are normally found in connective tissues throughout the mammalian body, mast cell tumors are abnormal collections of these cells which, unfortunately, can release histamine. Histamine can have toxic effects both locally (in the mass itself) and faraway, leading to inflammatory effects throughout a pet’s body.

In dogs, mast cell tumors are considered common in all ages. In fact, one out of five dogs will be affected by them. Luckily they’re found most typically in the skin, which makes them amenable to surgical removal. Theoretically, however, mast cell tumors can be found anywhere in the body they normally reside. These internal tumors are far less forgiving when it comes to surgical intervention.

In cats, mast cell tumors work essentially the same way, though they disproportionally affect older cats. The good news is that mast cell tumors are much less common in felines. The bad news is that cats’ mast cell tumors are more commonly found internally than they are in dogs. This so-called “visceral” form usually affects the spleen, liver, or intestines.

Though highly treatable in general, mast cell tumors are considered very invasive and a significant percentage of them are not as amenable to treatment as the majority. These difficult-to-treat tumors, however, can still be managed, especially with the advent of veterinary science’s newer cancer treatment protocols.

Symptoms and Identification

Mast cell tumors of the skin are considered fairly easy to diagnose via fine needle aspirate. Unfortunately, this requires first that we identify the mass as worthy of aspiration.

Though most commonly visible as a raised, hairless mass, mast cell tumors can look like all kinds of inflammatory lesions, large and small. Which means they often go unnoticed or underappreciated by pet owners so that many of them can remain on an animal for a long time before they’re treated.

In the cat, mast cell tumors of the skin are most likely to affect the head and neck. In dogs, the trunk and limbs are more likely locations.

Cats and dogs with internal mast cell tumors may present in various ways depending on the organs affected. Weight loss, inappetance and vomiting are the most common signs, especially in cats with the visceral form of the disease. Regardless of location, once it’s identified and aspirated, the mass’s characteristic granules will be evident on microscopic evaluation. Thus confirmed, the mass is biopsied to help stage the pet’s disease through the use of a grading system:

Grade I tumors are least likely to lead to significant disease and are most amenable to a complete surgical cure while Grade III tumors are much more likely to lead to serious disease either through local tissue invasion or by spreading to distant organs (metastasis). Grade II masses are somewhere in between the two in terms of severity and metastatic potential.

A pet with confirmed Grade II or III tumors should ideally receive additional tests beyond the basics (complete blood count, serum biochemistry panel, and urinalysis), including …

  • Lymph node aspiration
  • Abdominal ultrasound
  • Liver aspiration
  • Splenic aspiration
  • Bone marrow aspiration

This will help veterinarians outline a treatment plan tailored to a patients’ individual stage of the disease.

Affected Breeds

The Boxer is considered most notoriously mast cell tumor-prone. However, any breed can be affected. English Bulldogs, Boston Terriers, Shar-Peis, Labrador Retrievers, Golden Retrievers, Schnauzers, and Cocker Spaniels have all been found to be at higher risk. No specific mode of inheritance has been identified as of yet.

Treatment

For all but the most aggressive and invasive mast cell tumors of the skin, surgery is the immediate approach to treatment. Removing them by a wide margin whenever possible is considered the gold standard first line treatment.

For Grade I tumors, surgery is considered curative.

For Grade II tumors, either wide surgical excision or surgery coupled with radiation therapy is typically recommended. Pets treated with this combination have been determined to enjoy a 95% chance of tumor control for a year and a 85-95% chance of tumor control for 2-5 years.

For Grade III tumors, surgery and systemic chemotherapy is considered the treatment of choice. Chemotherapy is administered at doses designed to limit noxious side effects while targeting cancer cells that may have already spread to distant sites or that may potentially spread to distant sites. In some cases, chemotherapy alone is attempted. Pets with Grade III tumors should also receive drugs that mitigate the systemic effects of the histamine granules mast cell tumors are wont to release.

Veterinary Cost

The cost of mast cell tumor treatment depends heavily on the grade of tumor and stage of the disease. The cost of surgical intervention can also vary dramatically depending on the affected site. $500 to $1,000 is a fairly typical expense for a mast cell removal. If a board certified surgeon is elected due to difficult access to the site (for internal tumors or for less surgically amenable locations on the skin), costs are likely to increase two- to five-fold.

Should radiation therapy be deemed recommendable, owners should expect typical costs to range anywhere from $4,000 to $10,000. Chemotherapy costs will vary depending on the drugs elected but owners should expect a high price tag associated with the expense of these drugs, the need for specialized delivery, and the costs associated with close monitoring of these patients for weeks to months of therapy.

Prevention

Unfortunately there is no known mode of prevention for mast cell tumors.



References

Al-Sarraf R, Mauldin GN, Patnaik AK , et al. A prospective study of radiation therapy for the treatment of grade 2 mast cell tumors in 32 dogs. J Vet Int Med 1996;10:376-378.

Frimberger AE, Moore AS , LaRue SM, et al. Radiotherapy of incompletely resected, moderately differentiated mast cell tumors in the dog: 37 cases (1989-1993). J Am Anim Hosp Assoc 1997;33:320-324.

LaDue T, Price GS, Dodge R, et al. Radiation therapy for incompletely resected canine mast cell tumors. Vet Radiol Ultrasound 1998;39:57-62.

McCaw DL, Miller MA, Ogilvie GK, et al. Response of canine mast cell tumors to treatment with oral prednisone. J Vet Int Med 1994;8:406-408.

Rassnick KM, Moore AS , Williams LE, et al. Treatment of canine mast cell tumors with CCNU (lomustine). J Vet Int Med 1999;13:601-605.

Thamm DH, Mauldin EA, Vail DM. Prednisone and vinblastine chemotherapy for canine mast cell tumor - 41 cases (1992-1997). J Vet Int Med 1999;13:491-497.