Mast Cell Tumor in Cats: Everything You Need to Know

Mast cell tumor (MCT or mastocytoma) is a tumor based on so-called mast cells, which are normally part of the cat’s immune system. The tumor can occur in all cats and is more common in Siamese.

Mast cell tumors are not as common in cats as in dogs and cutaneous mast cell tumors, ie those that sit in the skin, are usually benign in cats compared to dogs. However, when mast cell tumors occur elsewhere in the body, such as in the spleen or intestine, they are usually malignant.

There is also a variant of mastocytoma in cats called histiocytic mastocytoma, which is mainly seen in younger individuals. Then one or more mast cell tumors develop all over the body and these can in many cases disappear on their own after a year or so.


Cutaneous mast cell tumors have great variation in appearance and growth pattern. They are often called the large deceivers of tumors as they can look like several other skin changes on the outside. They can look like warts or insect bites, be hairless and unpigmented, or sit in the subcutaneous tissue where they are easily confused with lipomas. Cutaneous mast cell tumors are in some cases itchy and have a sore surface. This is because mast cells can form various substances such as histamine, which trigger itching, redness, swelling and more.

Mast cell tumors that occur internally can cause diffuse symptoms such as poor appetite, lethargy, weight loss and vomiting. If the tumor is in the intestine, the cat can get diarrhea, fever and bloody stools.


The mast cell tumor is often diagnosed by the veterinarian testing the tumor with a fine needle, so-called fine-needle aspirate, and looking at cells from the tumor under a microscope. Tissue samples can be taken by biopsy, or the entire tumor is surgically removed and sent for analysis.

If the cat has a mast cell tumor in an internal organ, with the help of imaging diagnostics, such as an X-ray or ultrasound examination, it can be suspected that it is a tumor. To determine what type of tumor it is, you need to take a fine needle aspirate, tissue sample or remove the entire tumor and send it to the laboratory for analysis.


Mast cell tumors are primarily treated surgically. Surgical treatment is usually sufficient for benign mast cell tumors in the skin. If the cat has a malignant tumor in the spleen, the entire spleen is removed and in the treatment of malignant mast cell tumors, the surgery can be supplemented with, for example, cytotoxic drugs and / or radiation treatment.

The removed tumor is sent in its entirety to the laboratory for examination and to find out if it has been possible to remove the entire tumor.

When should a veterinarian be consulted?

It is always advisable to have the veterinarian examine the lumps that have formed on the cat’s skin as soon as possible after they have been detected. Some tumors – both mast cell tumors and others – can be fast-growing and the longer it takes before the cat receives the correct diagnosis, the larger the operation. The prognosis is also affected by how long it takes before the diagnosis is made, if it is a malignant tumor that may have spread to other parts of the body.


The prognosis is mainly determined by how malignant the tumor is, whether it has spread in the body and how difficult it is to remove. A cat with a single cutaneous mast cell tumor has a good prognosis after surgery. A cat with an aggressive mast cell tumor that has spread far into the body, or with a mast cell tumor in an internal organ, more often has a poor prognosis. In such cases, one may decide to treat the symptoms that the tumor may be causing, rather than trying to get rid of the tumor. The treatment is decided on a case-by-case basis and in consultation with the pet owner.

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