Mastocytoma in Dogs: A Comprehensive Guide

Mastocytoma in dogs is a type of skin tumor that's often mistaken for a mosquito bite or a skin tag.

Mastocytomas are typically found on a dog's abdomen, chest, or head, and can be as small as a pea or as large as a golf ball.

They're usually solitary growths, but in some cases, multiple mastocytomas can appear on a dog's body.

Mastocytomas are made up of mast cells, a type of immune system cell that plays a key role in allergic reactions.

Mastocytoma is a type of malignant tumor consisting of mast cells.

Mast cell tumors, also known as mastocytomas, can form nodules or masses in the skin, as well as in other areas of the body like the spleen, liver, intestine, and bone marrow.

Most dogs with mastocytoma develop only one tumor, with approximately 85% of cases being solitary.

A tumor is a mass of abnormal cells that can be benign or malignant. It's a bit like a big cluster of cells that shouldn't be there.

Mast cell tumors, specifically, are a type of malignant tumor consisting of mast cells. They can form in various parts of the body, including the skin, spleen, liver, intestine, and bone marrow.

Mast cell tumors are the most common skin tumor in dogs, affecting around 7% to 21% of them.

The cause of mastocytomas, or mast cell tumors, is a complex and not fully understood topic.

Some breeds of dogs are predisposed to mast cell tumors, which may suggest a genetic component.

A genetic mutation in the protein c-kit tyrosine kinase receptor, called a c-kit oncogene, is found in 25 to 30 percent of tumors.

Chronic inflammation may also predispose dogs to developing mast cell tumors, particularly in those with a history of allergic skin disease.

Mast cell tumors of the skin can occur anywhere on the body and vary in appearance, often presenting as a raised lump or bump on or just under the skin.

They may be red, ulcerated, or swollen, and some can fluctuate in size, getting larger or smaller, even daily. These size changes can occur spontaneously or when the tumor is agitated, which causes degranulation and subsequent swelling of the surrounding tissue due to the histamine release.

Fine-needle aspiration (FNA) is typically used to diagnose mast cell tumors, which involves taking a small needle with a syringe and suctioning a sample of cells directly from the tumor and placing them on a microscope slide.

A veterinary pathologist then examines the slide under a microscope to make a diagnosis. A tissue biopsy can also be used to determine the best course of action, as it can indicate how aggressive the tumor is.

Mast cell tumors are often called "the great pretenders" because they may resemble an insect bite, wart, allergic reaction, or other less serious skin tumors, making it essential to have any skin abnormalities evaluated by a veterinarian.

Here is a summary of the grading and staging of mast cell tumors:

The disease is also staged according to the WHO system:

Mast cell tumors of the skin can occur anywhere on the body and vary in appearance.

They can be a raised lump or bump on or just under the skin, and may be red, ulcerated, or swollen.

Some mast cell tumors may be present for many months without growing much, while others can appear suddenly and grow very quickly.

Others can suddenly grow quickly after months of no change.

These size changes can occur spontaneously or when the tumor is agitated, which causes degranulation and subsequent swelling of the surrounding tissue due to the histamine release.

Ulcers may form in the stomach or intestines, causing vomiting, loss of appetite, lethargy, and melena (black, tarry stools that are associated with bleeding).

Anaphylaxis, a serious, life-threatening allergic reaction, can occur, although it's less common.

MCTs of the skin can spread to the internal organs, causing enlarged lymph nodes, spleen, and liver, sometimes with peritoneal effusion (fluid build-up) in the abdomen, causing the belly to appear rounded or swollen.

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Diagnosis of mast cell tumors in dogs typically starts with a fine needle aspirate and cytology, which can usually make a diagnosis, but grading of the tumor cannot be done at this point.

A veterinary pathologist examines the cells under a microscope, looking for a large number of mast cells, which is a key indicator of the disease. The granules of the mast cell stain blue to dark purple with a Romanowsky stain, and the cells are medium-sized.

A surgical biopsy is required to determine the grade of the tumor, which depends on how well the mast cells are differentiated, mitotic activity, location within the skin, invasiveness, and the presence of inflammation or necrosis.

The grade of the tumor is crucial in determining the prognosis and treatment plan. Here's a breakdown of the different grades:

The disease is also staged according to the WHO system, which takes into account the spread of the tumor and the presence of metastasis.

Surgery is the preferred treatment for mast cell tumors in dogs. If the tumor is completely removed, the dog's chances of recovery are excellent, especially if the tumor is low-grade.

Antihistamines like diphenhydramine are often given before surgery to protect against the effects of histamine released from the tumor. This helps prevent some of the unpleasant symptoms associated with mast cell tumors.

Wide margins of 2 to 3 centimeters are required when removing the tumor to ensure all cancerous cells are removed. If the tumor is too large or in a difficult location, additional treatment like radiation therapy or chemotherapy may be necessary.

Radiation therapy is effective in preventing regrowth of incompletely excised tumors. In fact, dogs with low-grade mast cell tumors that receive radiation therapy have a 90-95% chance of no recurrence within three years.

Chemotherapy may be recommended for dogs with high-grade tumors, those that have metastasized, or those with a positive c-Kit mutation result. Common chemotherapy agents include Lomustine, Vinblastine, and Palladia.

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A dog's prognosis depends on several factors, including the tumor's grade, location, and whether it has spread. Dogs with completely removed low-grade tumors have an excellent prognosis, while those with tumors that have spread have a poor prognosis.

Here are some general guidelines for the prognosis of mast cell tumors in dogs:

Keep in mind that every dog is different, and these are only general guidelines.

Mast cell tumors (MCTs) are graded based on their histopathologic evaluation, which is considered the gold standard for grading MCTs. This system has evolved over the years, with the Patnaik system being used historically to grade MCTs as grade I, II, or III.

The Patnaik system defines grade I tumors as well differentiated with a low metastatic rate, while grade III tumors are poorly differentiated with a high metastatic rate. Grade II tumors tend to be overrepresented, accounting for 42% to 78% of MCTs, but their biological behavior is variable.

The Kiupel grading system, proposed in 2011, defines MCTs as either low-grade or high-grade based on specific criteria. A high-grade MCT is characterized by the presence of ≥7 mitotic figures in 10 high-power fields (HPF), ≥3 multinucleated cells in 10 HPF, ≥3 bizarre nuclei in 10 HPF, or karyomegaly.

The 2013 Oncology-Pathology Working Group (OPWG) consensus statement suggests applying both the Patnaik and Kiupel grading schemes to better predict the behavior of MCTs. The OPWG system categorizes MCTs as:

Cytologic grading of MCTs can also be done using the Kiupel system, with good correlation between cytologic and histologic grades. However, cytologic preparations may have limitations, such as fewer numbers of cells for evaluation and lower occurrence of mitotic figures.

The mitotic index is a test that measures the rate at which malignant mast cells are dividing and populating at the time of biopsy. A tumor with a mitotic index of 5 or less can be treated as a Grade I MCT with a good prognosis, while a tumor with a mitotic index over 5 should be treated as a Grade III tumor.

Here's a summary of the Patnaik system:

Note: The Kiupel system is not included in this table as it is a more recent grading system that categorizes MCTs as either low-grade or high-grade.

Mastocytoma in dogs can be a complex condition, and as with any medical issue, there are potential limitations to consider.

One potential limitation is that nuclear features can be obscured in heavily granulated tumors, making evaluation difficult.

This can potentially mask atypical features, making it harder to diagnose the condition accurately.

Cytologic grading classified more tumors as high grade than histology in some studies, despite this challenge.

A second concern is that cytology cannot distinguish cutaneous from subcutaneous mast cell tumors.

This is because the current histologic grading systems are applied only to cutaneous MCTs.

A recent study showed 85% agreement with histologic grading, even when both cutaneous and subcutaneous tumors were included.

The type of stain used can also be a potential issue, as aqueous Romanowsky stains may not stain mast cell granules as well as methanolic Romanowsky stains.

This can lead to false indications of high-grade tumors, especially if the granulation of mast cells is used in cytologic grading.

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Approximately 18% of cytologic samples from primary MCTs showed less granulation with aqueous stains than with a standard methanolic stain.

However, even with these limitations, cytologic samples stained with both aqueous stains and MGG stain were graded using the Camus scheme and compared with histologic grading, with a diagnostic accuracy of 85%.