Mast cell tumours in dogs: symptoms, tests and treatment

Overview

Cutaneous mast cell tumours are among the common malignant skin tumours in dogs. The typical finding is a single skin nodule, but one dog can have several tumours at different sites. A cutaneous mast cell tumour starts in the dermis and may extend deeper into subcutaneous tissue. A subcutaneous mast cell tumour instead originates in the tissue beneath the skin.

Most subcutaneous tumours behave less aggressively than typical mast cell tumours arising in the skin. However, a subcutaneous tumour can still recur at the surgical site. Rarely, a tumour can arise in a mucous membrane or internal organ without a visible skin lesion.

The tissue of origin influences the outlook for a mast cell tumour. Tumours within the same tissue also vary in behaviour according to microscopic grade.

Causes

Altered mast cells multiply uncontrollably within the tumour. What triggers a mast cell tumour in an individual dog is usually unknown. Normal mast cells are immune cells involved in allergic reactions, among other functions. They also occur in healthy skin and mucous membranes where the body contacts the environment.

Mast cells contain small granules storing signalling substances such as histamine. Tumour cells can also release histamine, sometimes without external stimulation. Some tumours have KIT gene changes affecting a receptor that regulates cell growth. Certain KIT changes are associated with a more aggressive course, but their independent prognostic value has not been confirmed.

Histamine and heparin are examples of substances within mast cell granules. These substances also participate in the body's inflammatory reactions. Handling the tumour can release granule contents and redden the surrounding area.

Risk factors

The tumour is most often diagnosed in older dogs, with an estimated average age of nine years. However, it has also occurred in dogs under one year old. Neither sex is considered clearly more susceptible.

Mast cell tumours are more common in breeds including Boxers, Pugs and retriever breeds. However, a new skin lesion can be a mast cell tumour in any breed. Some Rhodesian Ridgeback breed clubs also record mast cell tumours in breeding health monitoring. A previously treated cutaneous mast cell tumour increases the need to monitor new lumps, because a separate tumour can develop later.

At 94 veterinary practices in England between 2007 and 2013, mast cell tumours were recorded in 453 dogs, or 0.27% of 168,636 dogs followed. Median age at diagnosis was 8.2 years. At the same practices, mast cell tumours were diagnosed in 1.95% of Boxers. The corresponding proportion in Golden Retrievers was 1.39%.

Typical symptoms

Often, the owner notices only a lump while the dog's behaviour remains unchanged. A slow-growing lesion can remain the same size for months or years. Another tumour may enlarge noticeably within days or weeks.

• A nodule may rise above the skin or feel like a lump just beneath it.
• The lump may have a hairless area over it.
• The lesion may be red or look bruised.
• Its surface can break down and bleed.
• The lump may vary in size as it swells and shrinks again.
• A dog may lick, scratch or bite an irritated area.

Some dogs have vomiting or diarrhoea. Appetite may decrease or the dog may become more tired than usual. More marked general symptoms occur particularly with a large tumour burden or disease spread to internal organs.

Complications and consequences

Histamine released by the tumour can increase stomach acid secretion. This may ulcerate the stomach or intestinal lining. A gastrointestinal ulcer can bleed and cause iron-deficiency anaemia. If an ulcer perforates the intestinal wall, peritonitis can follow. Sometimes released signalling substances cause a severe, body-wide anaphylactic reaction.

Tumour cells most commonly spread to local lymph nodes, the liver or spleen. A high-grade cutaneous tumour is more likely to recur after surgery than a low-grade tumour. Among 306 dogs operated on for subcutaneous mast cell tumours, 24, or 8%, had recurrence at the surgical site. Metastases were found in 13 of these 306 dogs, or 4%.

A tumour can cause a stomach ulcer without obvious digestive symptoms. A severe stomach ulcer can cause abdominal pain.

Diagnosis at the vet

Cell sampling and tissue examination

The vet feels the lump and assesses its size, location and mobility relative to surrounding tissue. Feeling it does not identify the cells present or how a tumour may behave. Fine-needle sampling collects cells from the lump, which are spread onto a slide for microscopic examination. Sedation is often unnecessary for cell sampling. Characteristic mast cell granules help identify the tumour in a cell sample.

A tissue sample can establish whether the tumour starts in the dermis or subcutaneous tissue. The microscopic grade of a cutaneous tumour describes features including cell appearance and division activity. The Kiupel system divides cutaneous tumours into low and high grades. The Patnaik system has three grades. Cutaneous grading systems were developed for tumours arising in the dermis, so subcutaneous tumours are assessed using their own tissue features.

Many dividing cells in a subcutaneous tumour may indicate a greater risk of progression. In some cases, tissue is tested for KIT gene changes when medication targeting this growth signal is being considered.

Checking for spread

Staging establishes where tumour tissue is present and whether it extends to lymph nodes or more distant sites. Even a normal-sized lymph node can contain tumour cells. Conversely, inflammation can also enlarge a lymph node. A cell sample or tissue biopsy from a node can be examined microscopically for tumour cells. A sentinel lymph node receives lymph from the tumour area and is not always the node nearest the lump.

When needed, the vet examines the abdomen with ultrasound and takes cell samples from the liver or spleen. In mast cell tumour staging, lymph nodes are generally a more important target for investigation than the lungs. Not all dogs have the same imaging: tests are selected according to whether the result would affect treatment or prognosis. Blood and urine tests assess general health.

A sentinel lymph node can be located by imaging lymph drainage from the tumour area. In a small study of 17 dogs, it was not the nearest node in around 25% of cases.

Treatment

Surgery and additional treatments

The vet prescribes treatment and any medication. Most cutaneous mast cell tumours are treated with the aim of surgical removal. Surgery aims to remove enough surrounding tissue along with the tumour. For a low-grade tumour, an example of a lateral surgical margin is 2 cm or the tumour's greatest diameter. An equivalent example in depth is one fascial plane.

A pathologist assesses the removed tumour's margins to determine whether tumour reaches the cut edge. Specimen edges can be marked with ink so the pathologist can locate the margin nearest the tumour. If tumour tissue remains at the surgical site, options may include further surgery or radiotherapy. A regional lymph node can also be examined or removed during surgery.

Chemotherapy may be used particularly for high-grade or disseminated tumours. In some cases, a tyrosine kinase inhibitor targeting growth signals may be used, for example if the tumour cannot be removed. Gastroprotective medication may be used if a tumour-related gastrointestinal ulcer is suspected. Possible chemotherapy side effects include temporary vomiting and diarrhoea.

Radiotherapy can cause redness and irritation of skin in the treated area. If surgery cannot remove the tumour, radiotherapy can also relieve local symptoms.

The outlook after surgery

Removing a localised, low-grade cutaneous tumour may be curative. Spread beyond a single local lymph node particularly worsens the outlook. In follow-up of 306 dogs operated on for subcutaneous tumours, estimated six-month survival was 95% when deaths of unknown cause were not counted as tumour deaths. Estimated two-year survival in the same dogs was 92%. In follow-up of 43 dogs with subcutaneous mast cell tumours treated at specialist oncology centres, one-year survival was 90%. Two-year survival in this group was 77%. These 43 dogs had regional lymph nodes removed as well as the tumour.

Prevention

Because the exact cause is unknown, no specific preventive measure has been established. Early investigation is useful because some tumours can spread aggressively. Monitoring after treatment checks for both local recurrence and new tumours elsewhere on the skin. A new cutaneous mast cell tumour can arise independently of a previously removed tumour. Follow-up intervals are tailored to the dog's circumstances, tumour type and treatment stage.

A new symptom after treatment may also result from a treatment side effect or another disease, so its cause is investigated during follow-up. Monitoring continues even when the treated tumour can no longer be seen or felt.

Important for dog owners

Record the lump's location and any change in size or shape. Do not squeeze or rub it, as handling can make mast cells release histamine. Tell the vet if your dog has previously had a mast cell tumour removed or you notice several lumps. Monitor appetite and digestive symptoms during treatment: vomiting, for example, may relate to either disease or treatment. Ask for an explanation of tumour grade and surgical margins, as they provide different information for further care.

Also note whether your dog has started licking the area, as an irritated tumour can cause discomfort.

Frequently asked questions

Can a soft lump be a mast cell tumour?

Yes: mast cell tumours vary in appearance and consistency. A benign lipoma often feels soft too. Softness does not identify the type of lump. A fine-needle sample helps the vet determine which cells are present. The lump does not need removal to obtain a cell sample.

Why does the lump sometimes shrink?

A mast cell tumour can change size over time as swelling varies. Local swelling may result from substances released by tumour cells, for example when the dog scratches the lump. Temporary shrinkage does not show that the tumour has disappeared.

Does high grade mean the tumour has spread?

No: grade describes how the cells and tumour tissue look under the microscope. Stage describes how widely tumour tissue is found. A lymph node sample, for example, can establish whether tumour cells have reached it. Grade can influence further treatment even when no metastases have been detected.

Do several lumps mean metastases?

Not necessarily: one dog can have several separate cutaneous mast cell tumours. A tumour appearing later most often arises independently. A new lump is identified by examining its own cells. The same dog may also have a different skin lesion, such as a lipoma.

Does every dog need cancer medication after surgery?

No: removing a low-grade cutaneous tumour is sufficient for some dogs. If a tumour has spread or is high-grade, chemotherapy may form part of the plan. Surgical margin results help assess whether additional local treatment is needed. Any repeat surgery targets the original tumour area.