Mast Cell Tumours

Mast cell tumors are one of the more common types of skin tumor in dogs. Some are low-grade and can be cured with surgery. Others behave much more aggressively.

The difficult thing about mast cell tumors is that we cannot reliably identify them just by looking at them. A small, innocent-looking bump may turn out to be a mast cell tumor, while a rather alarming lump may be something much less serious. This is why veterinarians often recommend testing new or changing growths rather than simply watching them.

Mast cells are part of the immune system. They contain substances such as histamine, which normally play a role in inflammation and allergic reactions.

A mast cell tumor develops when these cells multiply abnormally and form a growth. Most develop in or underneath the skin, but they can also occur in less common locations such as the mouth or gastrointestinal tract. Mast cell disease may also involve internal organs such as the spleen and liver, particularly when a more aggressive tumor has spread.

There is no typical appearance.

A mast cell tumor may be:

Some mast cell tumors change size from one day to the next. They may suddenly become swollen, red or itchy, particularly after being rubbed or handled. This happens because the abnormal mast cells can release histamine and other inflammatory substances.

The important point is that appearance alone cannot tell us whether a lump is harmless.

The simplest first test is usually a fine-needle aspirate.

A small needle is inserted into the lump and some cells are collected. The cells are then examined under a microscope, either in the clinic or sent to a laboratory for analysis. Mast cell tumors often have a distinctive appearance under the microscope, so this test can frequently provide an answer without the need for a more invasive procedure.

Occasionally, the sample does not contain enough useful cells and needs to be repeated. A biopsy may also be recommended when the tumor is unusually large, difficult to remove or in a challenging location.

A fine-needle aspirate can usually identify a mast cell tumor, but it does not provide the full histological grade. Grading requires a tissue sample examined by a pathologist, usually after a biopsy or surgery.

Two grading systems are commonly used for skin mast cell tumors.

The older Patnaik system divides them into three grades:

The newer Kiupel system places tumors into two groups: low grade or high grade. Many pathology reports include both systems because they provide different but useful information. Histological grade remains an important prognostic factor, but it is not the only one. Tumor size, location, mitotic count, lymph-node involvement, surgical margins and other pathological features may all affect the treatment plan.

For a localised mast cell tumor that can be safely removed, surgery is usually the preferred treatment because it offers the best chance of a cure.

The surgeon removes the visible tumor together with a border of normal-looking tissue around and underneath it. This border is called the surgical margin. The removed tissue is then sent for histopathology to determine the grade and whether the margins are clear.

Margins are not decided by grade alone. In many cases, the final grade is not known until after surgery. The tumor's size, location and suspected behaviour therefore need to be considered before the operation.

Current 2026 AAHA guidance recommends removing most small, low-grade mast cell tumors with either a two-centimetre lateral margin or a proportional margin equal to the widest diameter of the tumor, together with one deeper fascial plane. Some very small Grade 1 tumors may be completely removed with a one-centimetre margin, depending on their location and the surgeon's assessment. For many Grade 2 tumors, a two-centimetre margin is preferred when it can be achieved safely.

Grade 3 or high-grade tumors are more difficult. Wider surgery may be needed for local control, but surgery alone may not be enough because these tumors have a greater tendency to spread.

When mast cell tumors metastasize, they often spread first to the draining or regional lymph node. From there, they may spread to other sites, including the spleen, liver and bone marrow.

Depending on the tumor's grade, size, location and clinical behaviour, your veterinarian may recommend staging tests such as:

Not every small, slow-growing mast cell tumor needs the same staging process. The investigations should be chosen for the individual dog rather than performed automatically.

Surgery may not be possible if the tumor is too large, has spread, or is growing in a location where adequate removal would cause significant loss of function.

Other treatment options may include:

Chemotherapy

Chemotherapy may be recommended for high-grade tumors, metastatic disease or tumors with other high-risk features. The protocol depends on the individual dog and is best discussed with a veterinary oncologist.

Palladia

Palladia is the brand name for toceranib phosphate, an oral targeted medication. It is licensed in the United States for recurrent Grade 2 or Grade 3 skin mast cell tumors, with or without regional lymph-node involvement. It may also be used in other clinical situations based on the oncologist's assessment.

Palladia can help control or shrink some tumors, but it does not work for every dog and requires regular monitoring for adverse effects.

Stelfonta

Stelfonta, or tigilanol tiglate, is injected directly into certain eligible non-metastatic mast cell tumors. It causes the treated tumor to break down and usually creates an open wound that heals afterwards. It should not be thought of as a simple injection that quietly makes the lump disappear.

Careful case selection and follow-up are important. Significant swelling, pain and tissue damage can occur, and the drug is not suitable for every tumor or every location. Availability also varies between countries.

Radiation therapy

Radiation may be useful when a tumor cannot be fully removed, when the surgical margins are incomplete, or when further surgery would be too damaging. Access to veterinary radiation therapy depends on where you live.

If a mast cell tumor is diagnosed at a late stage and has already spread, a cure may no longer be possible. However, this does not mean that there is nothing more we can do.

Depending on your dog's overall health, the extent of the spread and the behaviour of the cancer, Palladia may be considered to help control the disease. Other treatments, such as chemotherapy or radiation therapy, may also be appropriate for some patients.

At this stage, the aim of treatment is usually not to cure the cancer. It is to slow further spread, reduce or stabilise the size of the tumors where possible, and help the patient remain comfortable for as long as possible.

For some dogs, particularly when further cancer treatment is unlikely to provide a meaningful benefit or may cause too many side effects, palliative care may be the kinder option. Palliative care focuses on maintaining quality of life and managing problems such as pain, nausea, reduced appetite, gastrointestinal irritation, swelling and itchiness.

Palladia and palliative care are not necessarily separate choices. Palladia may form part of a palliative treatment plan if it is helping to control the cancer without compromising the dog's quality of life.

These decisions need to be discussed thoroughly with your veterinarian and, where appropriate, a veterinary oncologist. The most suitable plan depends on the extent of the cancer, your dog's general health, the likely benefits and side effects of treatment, and what will give your dog the greatest number of comfortable, good-quality days.

There is no reliable way to decide that a lump is harmless simply because it is small, soft or has been present for a long time.

Ask your veterinarian to examine any new growth, particularly if it:

Dogs with mast cell tumors can occasionally develop vomiting, reduced appetite, abdominal discomfort or black, tarry stools because of histamine-related stomach irritation. These signs need prompt veterinary attention.

Mast cell tumors can be unpredictable, but a diagnosis does not automatically mean the outlook is poor. Many low-grade tumors are successfully treated with surgery and never cause another problem.

The best approach is to test suspicious growths while they are still small. If the tumor is diagnosed early and can be removed completely before it spreads, surgery may provide a cure. If surgery is not possible, there are still several treatment options that may help control the disease and preserve a good quality of life.

Dr Kelly Yeo graduated from the University of Melbourne in 2009 and has extensive experience in companion animal medicine, with a special interest in geriatric care, chronic disease management, palliative care, and canine longevity.

Her work focuses on helping dogs live longer, healthier, and more comfortable lives through early detection, thoughtful nutrition, practical preventive care, and compassionate support for owners.