Mast Cell Tumours: What the Grade Means
Your dog's lump is a mast cell tumour, and the report gives a grade and margins. What the two grading systems mean, and why the grade isn't the whole story.
Written by thecanineledger
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In a 2005 study, 10 veterinary pathologists at one institution graded the same 60 mast cell tumours independently. The authors found “significant variation among pathologists in grading the MCTs” (Northrup et al., Journal of Veterinary Diagnostic Investigation 2005, abstract).
While the grade on a pathology report looks like a fixed verdict, it is one person’s reading of a sample, using one of two systems. It matters a great deal, but it is not the whole story. This page explains what the grade means, what the other parts of the report mean, and what to ask your vet.
1. What a mast cell tumour is
The Merck Veterinary Manual’s owner page says “Mast cell tumors are the most common malignant tumor seen in dogs.” Mast cells are immune cells “involved in allergic reactions” (Merck, Tumors of the Skin in Dogs).
They can look like anything. “Most commonly, a mast cell tumor appears as a raised lump or mass that may be soft to solid to the touch.” That is why any new lump needs a needle sample.
Breeds. Merck says “Many breeds appear to be prone to the disease, especially Boxers and Pugs (in which tumors are often multiple), Rhodesian Ridgebacks, and Boston Terriers.”
Why they are tricky. Merck: they “appear as a large central tumor but are in fact surrounded by a halo of smaller, microscopic nests of mast cells that infiltrate normal-looking skin.”
→ Found a Lump? Why Your Vet Should Put a Needle in It
2. The two grading systems
A grade comes from looking at the removed tumour under a microscope. Your report may use one system or both.
The three-tier (Patnaik) system. A 2021 expert consensus explains that it “divides cutaneous MCT into three grades”: grade 1 for well-differentiated cells, grade 2 for intermediate, and grade 3 for poorly differentiated (Willmann et al., Frontiers in Veterinary Science 2021).
The two-tier (Kiupel) system. Tumours are either low grade or high grade, based on counts of dividing cells and abnormal nuclei. The consensus panel’s view is that this system “is standard in the grading of MCT”.
Why both are used. Grade 2 in the three-tier system is the middle group, and its behaviour varies. The consensus table lists grade 2 as uncertain for both recurrence and spread, while high grade is linked to both.
| On the report | Generally means |
|---|---|
| Grade 1, or low grade | Less likely to come back or spread |
| Grade 2 | Uncertain. Other tests can help |
| Grade 3, or high grade | More likely to come back and to spread |
Our note: The table is our summary of the consensus’s risk table. We haven’t read the original grading papers, and we don’t give survival times by grade.
3. Why the grade isn’t the whole story
Low grade isn’t always low risk. The consensus says “around 15% of Kiupel low-grade MCTs may have a more aggressive biological behavior”.
Where the tumour is matters. It lists “perineal-perianal region, head and neck, inguinal area, scrotum, digit, and axilla” as sites linked to “a particularly poor prognosis”. Merck adds that tumours “near mucous membranes, feet, prepuce, or on the lower surface of the body are more likely to spread”.
So does size. Merck: “tumors larger than 1¼ inches (3 cm) are associated with decreased survival time.”
Extra tests. The consensus describes further tests on the sample, including proliferation markers and the KIT gene. These “strongly correlate with survival of canine MCT patients and local recurrence”. Your vet may suggest them, especially for a grade 2 tumour.
Signs beyond the lump. Merck notes that “up to a quarter of dogs with mast cell tumors also have stomach ulcers due to histamine release.”
4. What “margins” mean
The margins on a report describe whether tumour cells reach the edge of the tissue removed.
Why surgeons take a wide margin. Because of those microscopic nests, Merck says “Most veterinarians will remove at least 1¼ inches (3 centimeters) of healthy tissue surrounding all borders of these tumors”. Tumours “not removed completely during surgery are also more likely to spread.”
“Dirty” margins aren’t always a disaster. The consensus says “Low-grade cMCTs with a low proliferation index determined by Ki67 and AgNORs have been shown to have a low recurrence rate despite histopathologically dirty margins”. Ki67 and AgNORs are markers of how fast cells are dividing.
If margins aren’t clean. Merck lists further surgery, radiation or chemotherapy as options. For more advanced tumours, “there is no agreed upon treatment” and your vet may refer you to an oncologist.
Our note: Merck’s 3 cm margin is a surgical measurement. Surgeons may now use smaller margins for small tumours.
5. Questions to ask your vet
- Which grading system was used, and what grade did the tumour get in each?
- Were the margins clean, and by how much?
- Has the tumour spread to the lymph nodes, and should they be checked?
- Would further tests on the sample, such as proliferation markers or KIT, change the plan?
- Should we see an oncologist?
Our note: The question list is ours.
→ How Risky Is Anaesthesia for My Dog?
6. What we could not verify
| Commonly said | What we found |
|---|---|
| “A low-grade tumour is cured once removed” | Usually low risk, but not always. About 15% of Kiupel low-grade tumours may behave more aggressively. |
| “Every pathologist would give the same grade” | No. Ten pathologists varied significantly when grading the same 60 tumours. |
| “Dirty margins always mean it will come back” | Not always. Low-grade tumours with low proliferation markers had low recurrence despite dirty margins. |
| “It’s only a skin lump” | No. Merck calls these the most common malignant tumour in dogs. |
| “Survival by grade is well known” | We haven’t verified a figure. |
Sources
- Willmann M, Yuzbasiyan-Gurkan V, Marconato L, et al. Proposed Diagnostic Criteria and Classification of Canine Mast Cell Neoplasms: A Consensus Proposal. Front Vet Sci 2021: expert consensus proposal
- Northrup NC, Harmon BG, Gieger TL, et al. Variation among pathologists in histologic grading of canine cutaneous mast cell tumors. J Vet Diagn Invest 2005. ⚠️ abstract read. 60 tumours, 10 pathologists
- Merck Veterinary Manual: Tumors of the Skin in Dogs (pet owner version; Villalobos, DVM; last updated Apr 2026)
- Berlato D, et al. Value, Limitations, and Recommendations for Grading of Canine Cutaneous Mast Cell Tumors: A Consensus of the Oncology-Pathology Working Group. Vet Pathol 2021. ⚠️ abstract read; it gives no findings. Not quoted
Always confirm with your vet before acting on anything on this page.