Mast cells originate from specific precursor cells in the bone marrow, are released into the blood, and settle in specific locations in the body. Their physiological function consists of their involvement in immune, allergic, and inflammatory responses.
A mast cell tumor is an accumulation of neoplastic (degenerate) mast cells. Mast cell tumors are one of the most common types of skin cancer in dogs. These tumors are often located on the limbs, trunk, or head. Less commonly, dogs develop a generalized form or a form in which the malignant cells primarily settle in internal organs.
The cause of malignant changes in mast cells is not yet known. However, certain dog breeds are more commonly affected, such as retrievers and boxers. Boxers are particularly susceptible, but in most cases they develop a less malignant form of the tumor disease when examined histologically. This is why affected boxers tend to have a better prognosis (1). In some studies, boxers account for over 25% of mast cell tumor patients. The higher incidence of mast cell tumors in certain breeds suggests a genetic component in the development of mast cell tumors.
How can I tell if my pet is ill?
In principle, any skin lump can be a mast cell tumor, so it is very important to take your dog to a veterinarian for examination as soon as possible if you notice any unusual changes. The skin changes can present as nodular to doughy, hairless or hair-covered lumps, but can also occur as a weeping open wound. In most cases, these are single skin lumps, but in 11% to 14% of affected animals, changes are observed in several areas (2).
Mast cells store various active biological substances (e.g., heparin, histamine, certain enzymes) inside their cells and can release them spontaneously or through manipulation. The release of these substances can have various effects that affect the entire body or parts of the body that are even far away from the actual tumor site (e.g., local swelling, vomiting, shock symptoms as a paraneoplastic syndrome).
Substances released from mast cells can lead to bleeding tendencies, itching, or wound healing disorders, among other things. In many affected dogs, these substances cause stomach or small intestine ulcers (ulcerations), which can manifest as dark-colored feces (due to blood in the stool), vomiting, loss of appetite, or abdominal pain. If large amounts of these substances are released at once, allergy-like reactions can also occur, which in some cases can even be life-threatening.
How is the diagnosis made?
Since the external appearance is so varied, an accurate diagnosis must be made using fine needle aspiration. To do this, a few cells are removed from the tumor using a fine needle, spread on a slide, stained, and examined under a microscope. This can be done on site at the veterinary clinic or practice or in a special laboratory.
If the suspicion is confirmed, the treatment of choice is surgical removal of the tumor. It is extremely important to have the tumor examined by a pathologist using fine tissue analysis, as the further course of treatment and prognosis depend on the histopathological differentiation (grade I–III). Before surgery, however, it is equally important to know whether the tumor cells have already spread (metastasis). To determine this, the surrounding lymph nodes are examined clinically and cytologically, i.e., if possible, a fine needle aspiration is taken from them and examined. Furthermore, in the case of particularly aggressive tumors (grade III), an ultrasound examination of the abdominal cavity should be performed to clarify whether there are already structural changes in, for example, the spleen or liver that are suspicious for metastases.
What treatment options are available?
Grade I and II mast cell tumors are considered to have a very favorable prognosis if they are completely removed from the surrounding tissue. If they are completely removed, the animals are cured of this tumor in many cases. However, if they cannot be completely removed from the surrounding tissue, radical follow-up surgery or radiation therapy is recommended. Radiation therapy involves approximately 15 treatment sessions to kill any remaining tumor cells in the tissue.
Since aggressive grade III mast cell tumors have a significantly higher rate of metastasis and recurrence, chemotherapy is recommended in addition to surgical removal of the primary tumor. If the tumor has already spread, chemotherapy alone can improve quality of life and prolong life in many cases.
What is my pet's life expectancy and quality of life?
The prognosis for grade I and II mast cell tumors is good after surgical removal, and the likelihood of a cure is high. Many animals live for more than two years after the tumor is removed.
If lymph nodes are involved or clinical signs such as loss of appetite, vomiting, or dark-colored feces occur, the prognosis is less favorable.
Grade III mast cell tumors have a very poor prognosis when treated with surgery alone, as this tumor type is much more aggressive. Despite chemotherapy, the tumor usually recurs within a year or metastasizes to lymph nodes and internal organs (3).
What else can be done in addition to chemotherapy and surgical removal?
To prevent the development of stomach ulcers, the patient may be given an antihistamine (H2 blocker, e.g. cetirizine or famotidine). To relieve any additional itching and prevent allergy-like reactions, an antihistamine (H1 blocker, e.g. diphenhydramine) may be used.
However, all of these medications should only be used as prescribed by a veterinarian.
Literature:
1. Bostock DE: Neoplasms of the skin and subcutaneous tissues in dogs and cats, Br Vet J 142:1-19,1986.
2. Mullins MN, Dernell WS, Withrow Sj et al: The syndrome of multiple cutaneous canine mast cell tumors: 54 cases (1998-2004), J Am Vet Med Assoc 228:91-95, 2006.
3. Webster JD, Yuzbasiyan-Gurkan V, Thamm DH, Hamilton E, Kiupel M. Evaluation of prognostic markers for canine mast cell tumors treated with vinblastine and prednisone. BMC Vet Res. 2008;13:32.
4. Vickery KR, Wilson H, Vail DM, Thamm DH. Dose-escalating vinblastine for the treatment of canine mast cell tumor. Vet Comp Oncol. 2008;6:111-9.
5. Rassnick KM, Bailey DB, Flory AB, Balkman CE, Kiselow MA, Intile JL, Autio K. Efficacy of vinblastine for treatment of canine mast cell tumors. J Vet Intern Med. 2008;22:1390-6.
6. Rassnick KM, Williams LE, Kristal O, Al-Sarraf R, Baez JL, Zwahlen CH, Dank G. Lomustine for treatment of mast cell tumors in cats: 38 cases (1999-2005). J Am Vet Med Assoc. 2008 Apr 15;232(8):1200-5.
7. Hahn KA, Ogilvie G, Rusk T, Devauchelle P, Leblanc A, Legendre A, Powers B, Leventhal PS, Kinet JP, Palmerini F, Dubreuil P, Moussy A, Hermine O. Masitinib is safe and effective for the treatment of canine mast cell tumors. J Vet Intern Med. 2008;22:1301-9.
8. London CA, Malpas PB, Wood-Follis SL, Boucher JF, Rusk AW, Rosenberg MP, Henry CJ, Mitchener KL, Klein MK, Hintermeister JG, Bergman PJ, Couto GC, Mauldin GN, Michels GM. Multi-center, Placebo-controlled, Double-blind, Randomized Study of Oral Toceranib Phosphate (SU11654), a Receptor Tyrosine Kinase Inhibitor, for the Treatment of Dogs with Recurrent (Either Local or Distant) Mast Cell Tumor Following Surgical Excision. Clin Cancer Res. 2009;15:3856-3865.