Mast cell tumor

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Well differentiated mast cell tumor on the muzzle of a dog[1]
Mast cell tumor, well differentiated, Wright-Leishman stain. Well differentiated mast cells have numerous, purple, cytoplasmic granules that partially obscure nuclear morphology.

Mast cell tumors (NCTs) are a common cutaneous round cell neoplasm of dogs and is reported to account for approximately 21% of all canine skin tumors[2].

MCTs are one of the most common neoplastic diseases in dogs and have an extremely variable biologic behavior ranging from a benign mass to a fatal metastatic disease[3].

Canine cutaneous MCTs commonly present as a solitary neoplastic mass in the skin and/or subcutaneous tissue of older dogs, with mean age of onset of approximately 9 years of age. There is no reported sex predilection[4].

A breed predilection exists for Boxers and Boston Terriers, but are also observed in Labrador Retrievers, Golden Retrievers, and Shar Peis[5].

These tumors usually originate in the skin but primary tumors have also been reported in the spleen, kidney, gastrointestinal tract, oral cavity, and bone[6]. Most dogs present with a single cutaneous lesion[7]. The tumors can vary greatly in size, shape, and gross appearance. Dogs also may present with other systemic signs of disease resulting from histamine release by mast cells.

The most common are gastrointestinal signs such as vomiting resulting from ulceration, bleeding, delayed wound healing, and hypotensive shock. Systemic disease also may be seen with metastasis to the lymph nodes, spleen, liver, and bone marrow.

Diagnosis may be tentatively ascertained by clinical appearance of the tumor, but histology is required for definitive confirmation.

Histologically, mast cell neoplasms are graded as well differentiated (grade I), intermediate differentiation (grade II), and poorly differentiated (III). Proliferation markers are commonly used for prognostication of these tumours[8].

Grade I and grade II mast cell tumors are readily identified by the abundance of metachromatic (purple) cytoplasmic granules that sometimes obscure nuclear morphology. Poorly differentiated or anaplastic mast cell tumors have fewer granules that are finer in appearance[9]. They may have indistinct cytoplasmic boundaries, anisokaryosis, anisocytosis, and increased mitotic activity. Due to cellular fragility, extracellular granules may be present in the background on the aspirates. Eosinophils may or may not be present.

A differential diagnosis would include sebaceous carcinoma, squamous cell carcinoma, clear cell adnexal carcinoma, balloon cell melanoma, meibomian carcinoma and liposarcoma[10].

Wide surgical resection is the treatment of choice.

Histological grade is the most important prognostic factor[11].

Adjunct radiation therapy should be considered in cases of histologically metastatic tumor[12].

For grade II mast cell tumors, chemotherapy should be considered, with drugs such as toceranib, paclitaxel or vincristine[13] recommended in combination with prednisolone, omeprazole[14] and diphenhydramine.

With non-resectable mast cell tumors, masitinib has shown promise at improving survival rates[15].


  1. Sgugenetics
  2. Lemarie RJ, Lemarie SL, Hedlund CS (1995) Mast cell tumors: Clinical management. Compend Contin Educ Pract Vet 17:1085-1101
  3. Bostock BE (1973) The prognosis following surgical removal of mastocytomas in dogs. J Small Anim Pract 14:27–40
  4. Thamm DH & Vail DM (2001) Mast cell tumors. In: Withrow SJ, MacEwen EG, editors. Small Animal Clinical Oncology. Philadelphia, PA: Saunders. pp:261–282
  5. Meleo KA (1997) Tumors of the skin and associated structures. Vet Clin N Amer 27:73-94
  6. Patnaik AK, MacEwen EG, Black AP, Luckrow S (1982) Extracutaneous mast-cell tumor in the dog. Vet Pathol 19:608-615
  7. Simoes JP, Schoning P, Butine M. (1994) Prognosis of canine mast cell tumors: A comparison of three methods. Vet Pathol 31:637-647
  8. Berlato D et al (2013) Comparison of mitotic index and Ki67 index in the prognostication of canine cutaneous mast cell tumours. Vet Comp Oncol Mar 12
  9. Duncan JR, Prasse KW. (1979) Cytology of canine cutaneous round cell tumors. Vet Pathol 16:673-679
  10. Piviani M et al (2012) Cytologic features of clear cell adnexal carcinoma in 3 dogs. Vet Clin Pathol 41(3):405-411
  11. Thamm DH, Mauldin EA, Vail DM. (1999) Prednisone and vinblastine chemotherapy for canine mast cell tumor--41 cases (1992-1997). J Vet Intern Med 13:491-7
  12. Frimberger AE, Moore AS, LaRue SM, Gliatto JM, Bengtson AE. (1997) Radiotherapy of incompletely resented, moderately differentiated mast cell tumors in the dog: 37 cases (1989-1992). J Am Anim Hosp Assoc 33:320-324
  13. Rassnick KM, et al (1999) Treatment of canine mast cell tumors with CCNU (lomustine). J Vet Intern Med 13:601-605
  14. Carlsten KS et al (2012) Multicenter prospective trial of hypofractionated radiation treatment, toceranib, and prednisone for measurable canine mast cell tumors. J Vet Intern Med 26(1):135-141
  15. Hahn KA et al (2010) Evaluation of 12- and 24-month survival rates after treatment with masitinib in dogs with nonresectable mast cell tumors. Am J Vet Res 71(11):1354-1361