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Mast cell tumor in dogs: grade, tests and blood work

Mast cell tumors are among the most common skin tumors in dogs. Most are found in the skin. Blood tests cannot diagnose them, but a needle sample of the lump, the grade and a few targeted tests tell your vet how the tumor is likely to behave.

Written by the AI DiagAnimals editorial teamReviewed by the AI DiagAnimals TeamUpdated Our editorial policy

The essentials

A mast cell tumor is diagnosed on a sample of the lump, usually a fine-needle aspirate first. The grade (low or high) is the main guide to the outlook: in one study, dogs with high-grade tumors lived a median of less than 4 months, against more than 2 years for low grade. Staging checks the nearby lymph node and sometimes the spleen and liver. The blood count may show high eosinophils, basophils or platelets, but a normal count does not rule the tumor out.

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What a mast cell tumor is

Skin mast cell tumors are common in dogs and make up 7 to 21% of all skin tumors [1]. One study calls them the most common malignant skin tumor in dogs [2]. In a German series of 70,966 dog tumors, mast cell tumors made up 9.7% of all tumors [3].

In primary care practices in England, 0.27% of dogs had a mast cell tumor. The Boxer (1.95%), Golden Retriever (1.39%) and Weimaraner (0.85%) had the highest rates. Boxers, Pugs and Staffordshire Bull Terriers had higher odds than crossbreds, and sex made no difference [4].

How it is diagnosed: needle sample and grade

  • Fine-needle aspirate (cytology): the vet draws cells from the lump with a fine needle and looks at them under the microscope. Cytology can also give a grade: a tumor is called high grade if it is poorly granulated or shows at least 2 of 4 features (dividing cells, cells with two or more nuclei, nuclei of varied shape, or nuclei of very varied size) [5].
  • How well cytologic grading works: compared with grading on tissue, it had 88% sensitivity and 94% specificity in 152 tumors. Dogs with a cytologic high grade were 25 times more likely to die within 2 years [5].
  • Histologic grade, 2-tier (Kiupel): on the removed tissue, a tumor is high grade if it shows any one of: at least 7 dividing cells in 10 fields, at least 3 cells with 3 or more nuclei, at least 3 bizarre nuclei, or nuclei that vary a lot in size. Pathologists agreed on this grade in 96.8% of cases [1].
  • Histologic grade, 3-tier (Patnaik): grades 1, 2 and 3, first described in 83 dogs followed for 1,500 days [6]. Agreement between pathologists was lower: 63.1% for grade 1, 63.0% for grade 2 and 74.6% for grade 3 [1].
  • Extra tests on the tissue: Ki-67, c-KIT mutation and KIT staining were each linked with a shorter time before recurrence and shorter survival [7].

Your vet may compare this workup with other skin and lymph node cancers, such as lymphoma in dogs.

What the blood work can show

None of these findings proves a mast cell tumor. They are clues read with the needle sample.
FindingWhat it means
High eosinophils (eosinophilia)A tumor-related eosinophilia is most commonly seen with mast cell tumors [8].
High basophils (basophilia)In dogs, linked with respiratory disease, lymphoma and mast cell tumors [8].
High platelets (reactive thrombocytosis)Seen with cancer, especially lymphoma and mast cell tumors [9].
Eosinophil and neutrophil ratiosIn 92 dogs, the relative eosinophil level and the neutrophil-to-eosinophil ratio before treatment were independent predictors of survival [10].
Widespread disease (systemic mastocytosis)In 16 dogs: eosinophilia in 2, basophilia in 5, mast cells in the blood in 6, and clotting changes in 3 of 5 tested [11].

A normal blood count does not rule out a mast cell tumor. For the basics, see the CBC for dogs and the white blood cell count in dogs.

Reading a high eosinophil count

Eosinophils are counted in the blood count differential. When a tumor raises them, it is most commonly a mast cell tumor [8]. A high count has other causes too: see eosinophils in dogs and cats.

An annotated line from a lab report · Illustrative example
AbbreviationResultUnitLaboratory intervalFlag
EOS Eosinophils2.410³/µL0.1–1.3H
  1. Abbreviation EOS or eosinophils, sometimes followed by “#” for the absolute count and “%” for the percentage.
  2. Result 2.4 × 10³/µL in this illustrative line: an eosinophilia.
  3. Unit 10³/µL, 10⁹/L or G/L: the same number in all three.
  4. Laboratory interval An illustrative dog interval; each laboratory sets its own.
  5. Flag H: above the interval. With a skin lump, your vet will usually sample the lump; the blood count alone cannot name the tumor [8].

Staging: lymph nodes, spleen and liver

  • Nearby lymph node: a grading of mast cells in the node (HN0 to HN3) matched and predicted the outcome in 41 dogs [12].
  • Ultrasound of the abdomen was a poor predictor of spread in 82 dogs with high-risk tumors. For the spleen it had 67% sensitivity and 68% specificity; for the liver, 29% and 93% [13].
  • Needle samples of the spleen and liver: in 205 dogs, 10.7% had spread (spleen 10.2%, liver 6.3%). A negative spleen sample made spread to the liver very unlikely (predictive value 0.99) [14].
  • Buffy coat smear (mast cells in the blood): not recommended. In one study, 95.5% of blood smears with mast cells came from dogs without a mast cell tumor [15]. Mast cells in the blood are more often seen with allergies, inflammation and other cancers [8].

Outlook

With the 2-tier system, the median survival was less than 4 months for high-grade tumors and more than 2 years for low-grade tumors [1]. With the 3-tier system, 93% of dogs with grade 1 tumors lived longer than 1,500 days, against 47% with grade 2 and 6% with grade 3 [1].

Spread also matters. In dogs with high-risk tumors, median survival was not reached without spread to the spleen or liver, 322 days with early spread and 81 days with overt spread [13]. If your dog also has a mass on the spleen, the workup differs: see hemangiosarcoma in dogs.

Related articles

Eosinophils in dogs and cats

Mast cell tumors are one cause of a high count.

Read the article

White blood cell count in dogs

Eosinophils, basophils and the other white cells.

Read the article

Lymphoma in dogs

Another common cancer diagnosed with a needle sample.

Read the article

En français : mastocytome du chien.

Questions to ask your veterinarian

  1. Can we sample the lump with a needle before deciding on surgery?
  2. What is the grade, and which grading system did the laboratory use?
  3. Should the nearby lymph node be checked?
  4. Are the spleen and liver worth sampling for my dog?
  5. Does the blood count show high eosinophils, basophils or platelets?
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Frequently asked questions

How do I tell if my dog has a mast cell tumor?

Only a sample of the lump can tell. A fine-needle aspirate is the usual first test, and grading on cytology matched grading on tissue with 88% sensitivity and 94% specificity [5].

How long does a dog have to live with a mast cell tumor?

It depends mainly on the grade. In one study, median survival was less than 4 months for high-grade tumors and more than 2 years for low-grade tumors [1].

What dog breeds get mast cell tumors?

In England, the Boxer, Golden Retriever and Weimaraner had the highest rates, and Boxers, Pugs and Staffordshire Bull Terriers had higher odds than crossbreds [4].

Can blood tests detect a mast cell tumor in dogs?

No. The blood count can give clues, such as high eosinophils, basophils or platelets, but the diagnosis is made on a sample of the lump [8, 9].

Is a buffy coat test useful for mast cell tumors?

Not really. Most blood smears with mast cells come from dogs without a mast cell tumor, often with allergies or inflammation [8, 15].

Sources

  1. Kiupel M et al. Proposal of a 2-tier histologic grading system for canine cutaneous mast cell tumors to more accurately predict biological behavior. Vet Pathol. 2011;48(1):147-55. doi.org/10.1177/0300985810386469
  2. Galietta V et al. Preliminary assessment of tumor-associated tissue eosinophilia (TATE) in canine mast cell tumors. Animals (Basel). 2023;13(2):283. doi.org/10.3390/ani13020283
  3. Aupperle-Lellbach H et al. Tumour incidence in dogs in Germany: a retrospective analysis of 109,616 histopathological diagnoses (2014-2019). J Comp Pathol. 2022;198:33-55. doi.org/10.1016/j.jcpa.2022.07.009
  4. Shoop SJ et al. Prevalence and risk factors for mast cell tumours in dogs in England. Canine Genet Epidemiol. 2015;2:1. doi.org/10.1186/2052-6687-2-1
  5. Camus MS et al. Cytologic criteria for mast cell tumor grading in dogs with evaluation of clinical outcome. Vet Pathol. 2016;53(6):1117-1123. doi.org/10.1177/0300985816638721
  6. Patnaik AK et al. Canine cutaneous mast cell tumor: morphologic grading and survival time in 83 dogs. Vet Pathol. 1984;21(5):469-74. doi.org/10.1177/030098588402100503
  7. Webster JD et al. Evaluation of prognostic markers for canine mast cell tumors treated with vinblastine and prednisone. BMC Vet Res. 2008;4:32. doi.org/10.1186/1746-6148-4-32
  8. eClinPath, Cornell University College of Veterinary Medicine. Leukocytes. eclinpath.com/hematology/leukogram-changes/leukocytes
  9. eClinPath, Cornell University College of Veterinary Medicine. Platelet numbers. eclinpath.com/hemostasis/disorders/platelet-numbers
  10. Skor O et al. Pretreatment leukocyte ratios and concentrations as predictors of outcome in dogs with cutaneous mast cell tumours. Vet Comp Oncol. 2017;15(4):1333-1345. doi.org/10.1111/vco.12274
  11. O’Keefe DA et al. Systemic mastocytosis in 16 dogs. J Vet Intern Med. 1987;1(2):75-80. doi.org/10.1111/j.1939-1676.1987.tb01990.x
  12. Weishaar KM et al. Correlation of nodal mast cells with clinical outcome in dogs with mast cell tumour and a proposed classification system for the evaluation of node metastasis. J Comp Pathol. 2014;151(4):329-38. doi.org/10.1016/j.jcpa.2014.07.004
  13. Pecceu E et al. Ultrasound is a poor predictor of early or overt liver or spleen metastasis in dogs with high-risk mast cell tumours. Vet Comp Oncol. 2020;18(3):389-401. doi.org/10.1111/vco.12563
  14. Brown M et al. Utility of spleen and liver cytology in staging of canine mast cell tumors. J Am Anim Hosp Assoc. 2022;58(4):168-175. doi.org/10.5326/JAAHA-MS-7006
  15. McManus PM. Frequency and severity of mastocytemia in dogs with and without mast cell tumors: 120 cases (1995-1997). J Am Vet Med Assoc. 1999;215(3):355-7. pubmed.ncbi.nlm.nih.gov/10434973

Laboratory methods and reference intervals vary. This article explains; it does not diagnose. For a decision about your pet, use the original report and talk to your veterinarian. Our editorial policy.

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