Most common skin tumor
Mast cell tumors are one of the most common cutaneous tumors in dogs and are reported to make up about 16 to 21% of all skin tumors. Many breeds are overrepresented, especially bulldog descent (Boxers, Pugs, Boston terriers, French bulldogs, English bulldogs), Labrador, golden retrievers, schnauzers, Staffordshire terriers, beagles, and Chinese shar-peis. These tumors are generally seen in older dogs, but any age can develop them. Mast cell tumors can be located anywhere on the body. Dogs most often develop them on the trunk and limbs. 1 These masses can look like anything. Mast cell tumors can be red, raised, and well demarcated too subcutaneous, soft, and ill-defined.
What mast cells do in the body
Mast cell tumors arise from mast cells which are in the dermis and subcutaneous tissues. Mast cells are normal cells in the body and react to allergic reactions and bug bites. This is why mast cell tumors release histamine, heparin, and other vasoactive amines which account for some of the tumor changes seen. 1 Darier’s sign occurs with tumor erythema and wheal formation from local histamine release from tumor manipulation or occur spontaneously. Pet owners will describe the tumor going up and down in size and itching like a bug bit.
When there is a large surge of histamine release, this is called degranulation. This can occur during surgical manipulation or when a more aggressive and very large mass cell tumors, or when there is a very large disease burden of mast cell tumors in the body. A massive release of histamine can cause an anaphylactic like reaction leading to swelling, signs of stomach upset, hypotension, and bleeding. In severe cases degranulation can be fatal. 1
How to diagnose mast cell tumors?
Mast cell tumors are generally easy to diagnose. Fine needle aspirate and cytology often diagnose mast cell tumors with their classic appearance of round cells with cytoplasmic granules. Diff-Quick stain does not always stain the granules and can confuse the appearance in clinics. Many pathology labs also use Wright-Giemsa or toluidine blue stains, which will show the cytoplasmic granules. Cytology can grade mast cell tumors. While this is not as accurate as histologic grading, you can get a suggestion from cytology if the tumor appears aggressive.
Has it spread?
Mast cell tumors can metastasize. Mast cell tumors can spread to the local lymph nodes, and then continue to spread viscerally to the liver and spleen. The lungs are a very rare organ for this disease to metastasize to. Staging is the testing to determine the extent of cancer in the body. With mast cell tumors, it is important to do fine needle, aspirate and cytology of the regional lymph nodes. It can be difficult to determine where the regional lymph nodes could be located, and imaging may be needed to further evaluate this. Studies are also looking into sentinel lymph node mapping to help identify the draining lymph node. Blood work will rarely show spread of mast cell tumor disease. Dogs can have peripheral mastocytosis from multiple conditions, such as skin disease, trauma, and parvovirus. This is why we no longer recommend doing buffy coats of blood. 1
Imaging
Imaging can be done for further staging when you cannot feel the external or peripheral lymph nodes. Imaging is also recommended when mast cell tumors are higher grade and/or more aggressive. Abdominal ultrasound is now recommended by most for high grade tumors or if there is regional lymph node metastasis. We also recommend doing fine needle aspirate and cytology of the liver and spleen even if those organs appear normal on ultrasound. A study showed the sensitivity for detecting mast cell tumor in the spleen is only 43% and 0% in the liver. This study found dogs which had cytologic spread of mast cell disease in the liver or spleen had a survival time of only 100 days versus dogs without spread had a survival of 291 days. All these dogs had high-grade mast cell tumors. Another important note is that no dogs developed complications secondary to aspirating their liver and spleen. 2
Chest x-rays can be done, but we are looking for enlarged lymph nodes. As mentioned earlier, lung metastasis is extremely rare with mast cell tumors. This is why chest x-rays are not routinely recommended with this tumor.
Is the tumor aggressive?
Histology (tumor tissue under the microscope) can give valuable information on mast cell tumors because it can determine the grade. Grade has been associated with how the mast cell tumor may behave and survival time. There are two grading schemes that are often used by pathologists. Patnaik and Kiupel are the two systems. Patnaik is the traditional grading system and uses a scale of one to three. Grade I is well-differentiated while grade III is anaplastic or undifferentiated and grade II is in the middle. Grade I have been associated as a more benign tumor and generally curative with surgery. While grade III is very aggressive and further treatments will be needed and many dogs succumb to the disease. Grade II has been challenging to determine how it will behave, as well as what treatment should be done.
Some pathologists will report high grade II. This came from a combination of the Patnaik and Kiupel grading but also another paper found mitotic index can be prognostic. This study found that a mitotic index less than five had a the survival time of 70 months versus a mitotic index greater than five survival was only two months. 3
Another study found the majority of pathologists agreed on grade III tumors, but there was disagreement with less than 64% of pathologist agreeing on grade I and II designations. 4 This lead to a new grading system, the Kiupel scheme. This grades tumors as either high or low and looks at a specific measurable set of factors which can be seen histologically. This study found that tumors graded high had a survival time of less than four months versus tumors which were graded low had survival times of over two years. 4
Treatment options
There are multiple treatment options for mast cell tumors, including surgery, injectable treatments, radiation, and chemotherapy. The type and extent of treatment depends on the location, stage, and grade of the tumor. We will often offer a mix of multiple therapies to treat these tumors depending on the tumor size, location, grade, and if there is any spread of cancer.
Mast cell tumors which are small, low grade, and completely removed by surgery, do well for a long time. Meaning most dogs do not succumb to the cancer. It is still important to monitor these dogs after surgery to make sure regrowth or spread do not occur. Dogs with high grade mast cell tumors or tumors which cannot be removed, have a more aggressive behavior and do not do as well. Multiple treatment options may be needed for these dogs.
Finally, Pet Cancer Care Consulting is a teleconsulting service to help connect pet owners, veterinarians, and oncology specialists to increase access to veterinary oncology care. Mast cell tumors can be very straight forward to very complicated to test and treat. This is when an oncologist can help guide pet owners and veterinarians to make the best next steps before it is too late. Message Pet Cancer Care Consulting to learn more.
References
| 1. | London C, Thamm D. Mast Cell Tumors. In: Vail D, Thamm D, Liptak J, eds. Withrow & MacEwen’s Small Animal Clinical Oncology, 6th edition. 6th ed. St. Louis: Elsevier; 2020. |
| 2. | Book A, Fidel J, Wills T, Bryan J, Sellon R, Mattoon J. Correlation of ultrasound findings, liver and spleen cytology, and prognosis in the clinical staging of high metastatic risk canine mast cell tumors. Vet Radiol Ultrasound.. Sept-Oct 2011;52(5):548-54. |
| 3. | Romansik E, Reilly C, Kass P, Moore P, London C. Mitotic Index Is Predictive for Survival for Canine Cutaneous Mast Cell Tumors. Veterinary Pathology. 2007;44(3):335-341. |
| 4. | Kiupel M, Webster J, Bailey K, et al. Proposal of a 2-tier histologic grading system for canine cutaneous mast cell tumors to more accurately predict biological behavior.. Vet Pathol. Jan 2011;48(1):147-55. |