The short answer
Mast cell tumors are common cancers of the canine skin and subcutis, and their behavior ranges from locally confined to widely metastatic. Fine-needle cytology usually confirms the diagnosis, but grading depends on histopathology. The Vienna working conference consensus (published 2021) treats the two-tier Kiupel grade as standard, and the 2022 Brazilian consensus recommends reporting it alongside the three-tier Patnaik grade. Staging is built around assessment of the sentinel or regional lymph node. Surgery is the main treatment, with radiation, electrochemotherapy, tigilanol tiglate, chemotherapy and tyrosine kinase inhibitors described for incompletely excised, unresectable or high-risk disease, and prognosis rests on a combination of grade, mitotic count, node status, recurrence history and molecular markers.
Key takeaways
- The 2022 Brazilian consensus describes mast cell tumors as the third most common tumor subtype in dogs and the most common malignant skin tumor, at 11% of skin cancer cases [1].
- The Brazilian panel recommends reporting both the Patnaik and Kiupel grades for each tumor. The 2019 Vienna working conference (published 2021) called the Kiupel system "standard in the grading of MCT" [1][4].
- In a lymphatic mapping study cited by the Brazilian panel, the sentinel node did not correspond to the regional node in 42% of cases of dogs with mast cell tumors [1].
- A study of 19 dogs summarized by Garrett found that ultrasound detected 43% of cytologically confirmed splenic infiltration and none of the liver infiltration [3].
- In a 149-dog placebo-controlled trial described by Garrett, 37% of dogs with nonresectable grade II or III tumors responded to toceranib, against 8% on placebo [3].
- Merck reports that up to 65% of incompletely excised mast cell tumors do not recur [2].
Doses on this pageEvery dose below is quoted from the cited source. Verify against the current label/formulary and the patient's full clinical picture.
Who gets it and how it presents
The Brazilian panel lists Boxer, Bull Terrier, French Bulldog, Golden Retriever, Labrador Retriever, Shar-pei and Dachshund as predisposed. It notes that Shar-peis tend toward more aggressive tumors [1]. Merck adds Pugs, Rhodesian Ridgebacks and Boston Terriers and gives a typical age of 8 to 10 years [2]. Garrett cites one study in which 56% of affected Pugs had multiple tumors at diagnosis, though those tumors rarely led to death [3].
Per the Brazilian panel, the distribution of tumors is:
- about half on the trunk, perineum and inguino-genital region
- 40% on the limbs
- 10% on the head and neck [1]
Lesions vary in size and texture, may ulcerate, and some resemble lipomas. Cutaneous and subcutaneous forms can only be told apart on histopathology [1]. Merck notes that although these tumors often seem encapsulated, they are "seldom discrete," with a halo of mast cells that palpates as normal skin [2].
According to the Brazilian panel, about half of affected dogs show secondary signs, including those from mast cell degranulation [1]. These include:
- Darier's sign (redness and swelling after the tumor is handled)
- gastric and duodenal ulceration
- delayed wound healing
- bleeding
- rarely, circulatory collapse
Reported ulcer rates differ. Necropsy data cited by the Brazilian panel put them at 35 to 83%, while Merck says up to 25% of cases [1][2].
Diagnosis and grading
The Brazilian panel says fine-needle cytology reaches the correct diagnosis in 92 to 96% of cases [1]. Both the panel and Garrett flag a stain pitfall: Diff-Quik may leave granules unstained [1][3]. Garrett advises that eosinophils seen with large agranular round cells should raise suspicion and prompt submission for a different stain [3].
Cytologic grading remains unsettled. The Brazilian panel recommends the Camus scheme. The Vienna group said recently proposed cytologic grading schemes "either over- or underdiagnose high grade MCTs" and treats histopathology as the grading reference [1][4].
Patnaik grading has three tiers. The Brazilian panel stresses that it should not be applied to subcutaneous tumors [1]. Merck says up to 80% of canine tumors fall into grade II [2]. Grading consistency is a known problem: Garrett describes a study in which ten pathologists graded the same 60 tumors and agreed unanimously on only four [3].
The Kiupel system assigns high grade if any one of the criteria below is met, per the Vienna consensus table [4].
| Kiupel criterion | High-grade threshold [4] |
|---|
| Mitotic figures | 7 or more per 10 high-power fields |
| Multinucleated cells | 3 or more per 10 high-power fields |
| Bizarre nuclei | 3 or more per 10 high-power fields |
| Karyomegaly | 10% or more of neoplastic cells |
For mitotic count, the Brazilian panel recommends either of two schemes [1]:
- a cutoff of 5 (5 or fewer versus more than 5)
- three bands of 0, 1 to 7 and more than 7
The panel also recommends Ki-67 and KIT immunohistochemistry. It links Ki-67 above 23 positive cells to recurrence and metastasis [1]. Both consensus groups report that internal tandem duplications in KIT exon 11 are associated with more aggressive behavior, while exon 8 duplications have not been associated with a poor prognosis [1][4]. For subcutaneous tumors, the Brazilian panel names three validated adverse factors: mitotic count above 4, multinucleation and an infiltrative growth pattern [1].
Staging
The Vienna group's staging workup includes [4]:
- physical examination
- blood count and chemistry
- aspiration of regional nodes "even if normal in size"
- abdominal ultrasound with or without liver and spleen aspiration
- thoracic radiographs
Node size is a poor guide. Garrett cites a study in which 16 of 35 dogs (46%) with normal-sized nodes had metastasis [3]. The Brazilian panel puts node cytology sensitivity at 68 to 75% and favors histopathology of the node [1]. Nodes are classed HN0 to HN3 under the Weishaar system, though the Brazilian authors have reservations about calling HN1 "pre-metastatic" [1][4].
Garrett describes a 52-dog study in which dogs with cytologic liver or spleen infiltration had a survival time of 34 days, against 733 days for dogs without it. Ultrasound appearance alone did not predict survival [3]. Citing the low sensitivity and positive predictive value of ultrasound, the Vienna group recommends ultrasound-guided aspiration of spleen and liver in high-risk dogs regardless of how the organs look [4]. It also says full staging is not indicated after resection of a low-grade tumor unless signs of spread appear, though the sentinel node should still be examined [4]. Garrett notes that lung metastasis is rare and that bone marrow involvement appears in under 5% of cases [3].
The Brazilian panel describes three staging systems [1]:
- the WHO scheme
- a 2008 Southern European Veterinary Conference scheme
- a modified scheme from Horta and colleagues
The Brazilian panel concluded that a single standard system is not yet possible [1].
Treatment approaches described in the sources
Surgery and local therapy
The Brazilian panel calls surgical excision the main treatment [1]. For grade 1 or 2 tumors up to 4 cm, it cites a lateral margin of 2 cm with one fascial plane deep, with recurrence of 0 to 4%. It also notes 36% recurrence for high-grade tumors regardless of margin in one study [1]. Merck still describes 3 cm lateral margins for stage I disease [2]. Garrett summarizes a study of 23 tumors in 21 dogs that were completely excised at 2 cm, and notes that only grade I and II tumors were included [3]. The Brazilian panel recommends lymphadenectomy during the initial surgery [1].
For incomplete or narrow margins, Garrett describes a retrospective study with these recurrence rates [3]:
- 38% with no further local treatment
- 13% after re-excision
- 8% after radiation
The Brazilian panel cites a grade 2 radiation study in which 94% of dogs were in remission at 1 year and 88% at 5 years. It notes that radiotherapy is unavailable in Brazil [1].
The panel recommends electrochemotherapy in three settings [1]:
- small tumors under 3 cm that are hard to resect
- intraoperatively, when free margins are unlikely
- compromised margins
Eligibility limits for tigilanol tiglate differ: the Brazilian panel gives a volume of 10 cm3 or less, and Merck gives under 8 cm3 [1][2].
Systemic therapy
The Brazilian panel notes that most studies consider vinblastine first-line, at "2.0 mg/m2 every 1 or 2 weeks, with or without prednisone," and lomustine a second option [1]. Toceranib and masitinib are licensed for unresectable or relapsed grade II and III tumors [1]. The panel reports a commercial toceranib dosage of 3.25 mg per kg on Mondays, Wednesdays and Fridays. It says 2.5 to 2.75 mg per kg every 48 h "can be advantageous" for safety [1]. Merck warns that the package insert dosage "may lead to unacceptable toxicity" [2].
In the toceranib trial Garrett describes, dogs with KIT mutations responded more often (82%), but dogs without mutations still responded (54%) [3]. The study was unblinded after 6 weeks, after which placebo dogs became eligible for toceranib. Garrett discloses membership of a Zoetis consulting panel for Palladia (toceranib) [3].
In a 202-dog randomized masitinib trial, time to progression was 178 days against 75 days on placebo, but overall survival improved only in dogs with KIT mutations [3]. The Vienna group notes conflicting later data showing worse outcomes in KIT-mutant dogs on toceranib [4]. Garrett describes monitoring blood counts, chemistry and urinalysis on these drugs because of protein-losing kidney effects [3].
Prognosis
Garrett writes: "The majority of MCTs will be cured with surgical excision" [3]. The Brazilian panel gives these metastasis rates [1]:
- under 10% for well-differentiated tumors
- 5 to 22% for moderately differentiated tumors
- 55 to 95% for poorly differentiated tumors
In the 95-dog surgery-only study behind the Kiupel system, high-grade dogs survived a median 3.6 months. Low-grade dogs had not reached a median at over 2 years. Garrett says further validation is warranted [3].
In a 149-case study cited by the Brazilian panel, MCT-related death risk was:
- 62% with recurrence and node metastasis
- 14% with other adverse markers
- 4% without them [1]
Merck links tumors over 3 cm to shorter survival [2]. Garrett reports node metastasis rates of 55 to 59% for muzzle and oral tumors against under 10% elsewhere, though median survivals in those studies were still long [3]. Subcutaneous tumors fared well in a 306-dog study, with 8% local recurrence and 4% metastasis [3].
Client communication
Garrett describes margin decisions as a topic for "further discussion with pet owners" [3]. Garrett also lists visit frequency, travel, side effects, cost and drug approval status as factors in choosing systemic therapy [3]. The Brazilian panel notes that delayed healing and dehiscence are often observed after surgical resection of these tumors [1].
