CONDITION
Mast Cell Tumours in Cats
Mast cell tumours are growths that develop from mast cells, a type of immune cell that normally lives in the skin and other tissues. In cats, these tumours most often appear as lumps or nodules in the skin, though they can occasionally develop in internal organs. The behaviour of mast cell tumours in cats tends to differ from that in dogs—many in cats are less aggressive, though patterns vary depending on where the tumour is located and what it looks like under the microscope. Owners typically notice a lump on the skin, often on the head, neck, or body. The lump may have been present for weeks or months, may be growing slowly or quickly, and may be single or multiple. Some cats show no other signs; others may have changes in the skin around the lump, or occasionally signs related to substances released by the mast cells themselves. This page explores what signs may be observed, what is happening at the cellular level, how these tumours are investigated and classified, and what approaches exist for managing them. The goal is to help you understand the landscape of possibilities and the questions that shape decision-making.
Why this matters now
Mast cell tumours in cats can appear at any age, though they are most often identified in middle-aged to older cats, typically between eight and twelve years. There is no strong breed predisposition in the way seen in some other species, though Siamese cats may develop a specific form involving multiple small skin nodules, often at a younger age. The condition is not thought to be strongly linked to environmental exposures or diet, and many cases arise without an obvious trigger.
The way these tumours change over time varies considerably depending on their location and microscopic appearance. Many skin-based mast cell tumours in cats remain stable or grow slowly over months, and some may even resolve without intervention. Others may enlarge more quickly, spread to nearby lymph nodes, or involve deeper tissues. Tumours affecting internal organs, such as the spleen or intestines, tend to follow a different course and may be associated with more systemic changes.
Signals & patterns
Early signals
Single skin lump or nodule
The most common early observation is a firm or slightly soft swelling under the skin, often on the head, neck, or trunk. The lump may be noticed during grooming or handling and can range from a few millimetres to a centimetre or more in diameter.
Multiple small raised bumps
Some cats develop several small nodules scattered over the skin, which may appear over weeks to months. This pattern is more often seen in younger cats, particularly Siamese, and the nodules may vary in size and persistence.
Skin colour change around lump
The skin overlying or surrounding the tumour may appear slightly pink, red, or darker than usual. This can reflect local inflammation or the effects of substances released by the mast cells themselves.
Occasional itching or licking
A cat may lick, scratch, or pay attention to the area where the tumour is located. This is not always present but may occur if the lump causes irritation or if the mast cells release histamine locally.
Later signals
Enlargement or ulceration of lump
A tumour that has been stable may begin to grow more noticeably, and the overlying skin may become thin, hairless, or break down into an open sore. This can occur with more aggressive tumours or those in areas subject to trauma.
Swelling near lymph nodes
Lumps may appear under the jaw, in the armpit, or behind the knees if the tumour has spread to local lymph nodes. These swellings are usually firm and may grow independently of the original site.
Vomiting or reduced appetite
Cats with mast cell tumours in the intestines or other internal organs may develop digestive signs, including intermittent vomiting, changes in appetite, or weight loss. These signs can also occur if circulating mast cell substances affect the stomach lining.
Lethargy or abdominal swelling
When tumours involve the spleen or liver, cats may become quieter, less interested in activity, or develop a visibly distended abdomen. These patterns suggest internal involvement and are less common than skin-based disease.
Click to read about the biological mechanisms
How this is usually investigated
Options & trade-offs
Last reviewed: Invalid Date ·