Canis ISSN: 2398-2942

Skin: mastocytoma

Synonym(s): Mast cell tumor

Contributor(s): Laura Garrett, David Scarff, Suzanne Murphy


  • Most common malignant skin tumor of dog (20% of all canine cutaneous tumors).
  • Can be subcutaneous (also common). Note the grading system and associated prognoses are not applicable to subcutaneous tumors. 
  • Can occur elsewhere in the body, again the grading systems below only apply to cutaneous tumors.
  • Signs: mass on skin: trunk and perineal region (50%), skin of extremities (40%). Multiple masses may be seen.
  • Behavior: benign (most) to highly malignant (significant minority).
  • Diagnosis: cytology or histopathology.
  • Treatment: surgical resection with or without radiotherapy/medical therapy depending on whether margins are achievable/grade.
  • Prognosis: good for low-grade tumors; guarded to poor for high-grade.
    Print off the owner factsheet on Canine cutaneous mast cell tumours to give to your client.


Predisposing factors


  • Breed.


  • Although often feel like discrete masses, they invade microscopically beyond palpable margins.
  • Mast cells may degranulate → histamine, heparin and other vasoactive amine release → local acute inflammation or systemic signs.
  • Sytemic effects:
    • Prolonged bleeding time Hematology: activated clotting time → heparin, action on clotting cascade.
    • Delayed wound healing → proteolytic enzymes released from tumor at surgery and release of fibroblast suppressor factor due to histamine binding to macrophage H1 and H2 receptors.
    • Anaphylaxis → sudden release of large amounts of histamine.
    • Gastrointestinal ulceration → histamine action on H2 receptors in stomach → increased gastric acid secretion and gastric hypermotility.
    • Above results in vomiting, melena, collapse.


  • Vary from benign to highly malignant, identified most successfully by grade.
  • Can be classified according to aggressiveness (graded) commonly in two ways - referred to as the Patnaik or Kuipel grading system Skin: mast cell tumor - prognostic tests. Grade is closely associated with outcome.
  • Patnaik grading system  - a three grade system:
    • Well-differentiated (Grade I) Cytology well differentiated mast cell tumor :
      • Round cells with distinct abundant purple cytoplasm.
      • Medium sized intracytoplasmic granules.
      • No mitotic figures noted.
      • Compact groups or rows of neoplastic cells confined to dermis.
    • Intermediate (Grade II):
      • Some pleomorphic cells.
      • Some cells having less distinct cytoplasm.
      • Areas of edema or necrosis are noted.
      • Mitotic figures are 0-2 per 10 high power fields.
      • Tumor infiltrating lower dermis/subcutaneous tissue.
    • Poorly-differentiated (Grade III) Cytology intermediate - poorly differentiated mast cell tumor :
      • Dense sheets of pleomorphic cells with indistinct cytoplasm with fine or not obvious intracytoplasmic granules.
      • Mitotic figures 3-6 per high power field.
      • Edema, hemorrhage, necrosis and ulceration common.
      • Tumor infiltrating lower dermis/subcutaneous tissue.
  • Kuipel grading system  - a two grade system (high or low grade):
    • High grade:
      • 7 mitotic figures or more in 10 high power fields (hpf).
      • 3 or more multinucleated (3 or more nuclei) cells in 10 hpf.
      • At least 3 bizarre nuclei in 10 hpf.
      • Karyomegaly (defined as nuclear diameters of at least 10% of neoplastic cells varying by at least two-fold).


  • Weeks to years (depending on grade).


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Biasoli D, Compston-Garnett L, Ricketts S L et al (2019) A synonymous germline variant in a gene encoding a cell adhesion molecule is associated with cutaneous mast cell tumour development in Labrador and Golden Retrievers. PLoS Genet 15(3), e1007967 PubMed.
  • Smiech A et al (2018) Epidemiological assessment of the risk of canine mast cell tumours based on the Kuipel two-grade malignancy classification. Acta Vet Scand 60 (1), 70 PubMed Full Article.
  • Blackwood L, Murphy S, Buracco P et al (2012) European consensus document on mast cell tumors in dogs and cats. Vet Comp Oncol 10(3), e1-e29 PubMed Full Article. 
  • Kiupel M, Webster J D, Bailey K L et al (2011) Proposal of a 2-tier histologic grading system for canine cutaneous mast cell tumors to more accurately predict biological behavior. Vet Pathol 48(1), 147-155 PubMed.
  • Rassnick K M, Bailey D B, Russell D S et al (2010) A phase II study to evaluate the toxicity and efficacy of alternating CCNU and high-dose vinblastine and prednisone (CVP) for treatment of dogs with high-grade, metastatic or nonresectable mast cell tumors. Vet Comp Oncol 8(2), 138-152 PubMed.
  • London C A et al (2009) 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 15 (11), 3856-65 PubMed.
  • Hahn K A, Oglivie G, Rusk T et al (2008) Mastinib is safe and effective for the treatment of canine mast cell tumors. JVIM 22, 1301-1309 PubMed Full Article.
  • Stanclift R M, Gilson S D (2008) Evaluation of neoadjuvant prednisone administration and surgical excision in treatment of cutaneous mast cell tumours in dogs. JAVMA 232, 53-62 PubMed.
  • Murphy S, Sparkes A H, Blunden A S, Brearley M J & Smith K C (2006) Effects of stage and number of tumours on prognosis of dogs with cutaneous mast cell tumours. Vet Rec 158 (9), 287-291 PubMed.
  • Scase T J, Edwards D, Miller J, Henley W, Smith K, Blunden A & Murphy S (2006) Canine mast cell tumous: correlation of apoptosis and proliferation markers with prognosis. J Vet Intern Med 20 (1), 151-158 PubMed.
  • Sfiligoi G, Rassnick K M, Scarlett J M, Northrup N C & Gieger T L (2005) Outcome of dogs with mast cell tumors in the inguinal or perineal region versus other cutaneous locations: 124 cases (1990-2001). JAVMA 226 (8), 1368-1374 PubMed.
  • Murphy S, Sparkes A H, Smith K C, Blunden A S & Brearley M J (2004) Relationships between the histological grade of cutaneous mast cell tumours in dogs, their survival and the efficacy of surgical resection. Vet Rec 154 (24), 743 -746 PubMed.
  • Simpson A M, Ludwig L L, Newman S K et al (2004) Evaluation of surgical margins required for complete excision of cutaneous mast cell tumors in dogs. JAVMA 224, 236-240 PubMed.
  • Weisse C, Shofer F S & Sorenmo (2002) Recurrence rates and sites for grade II canine cutaneous mast cell tumors following complete surgical excision. JAAHA 38 (1), 71-73 PubMed.
  • Séguin B, Leibman N F, Bregazzi V S et al (2000) Clinical outcome of dogs with grade II mast cell tumors treated with surgery alone - 55 cases (1996-1999). JAVMA 218 (7), 1120-1123 PubMed.
  • Rassnick K M et al (1999) Treatment of canine mast cell tumors with CCNU (Lomustine). J Vet Intern Med 13 (6), 601-605 PubMed.
  • Thamm D H, Mauldin E A, Vail D M (1999) Prednisone and vinblastine chemotherapy for canine mast cell tumor - 41 cases (1992-1997). J Vet Intern Med 13 (5), 491-497 PubMed.
  • LaDue T et al (1998) Radiation therapy for incomplete resected canine mast cell tumors. Veterinary Radiology and Ultrasound 39 (1), 57-62 Wiley Online Library.
  • Frimberger A E et al (1997) Radiotherapy of incompletely resected, moderately differentiated mast cell tumors in the dog - 37 cases (1989-1993). JAAHA 33 (4), 320-324 PubMed.
  • al-Sarraf R et al (1996) A prospective study of radiation therapy for the treatment of grade 2 mast cell tumors in 32 dogs. J Vet Intern Med 10 (6), 376-378 PubMed.
  • O'Keefe D A et al (1987) Systemic mastocytosis in 16 dogs. J Vet Intern Med 1 (2), 75-80 PubMed.
  • Patnaik A K et al (1984) Canine cutaneous mast cell tumor morphologic grading and survival in 83 dogs. Vet Pathol 21 (5), 469-474 PubMed.