CONDITION
Plasmacytoma
A plasmacytoma is a growth that arises from a single population of plasma cells — the specialised immune cells responsible for producing antibodies. In dogs and cats, these growths occur most often in or just beneath the skin, though they can develop in other locations including the mouth, ears, and digestive tract. They are typically solitary and behave in a benign fashion, but the appearance of any new lump raises questions about what it is and what it may become. Owners often notice a raised, smooth, sometimes ulcerated mass that has appeared over weeks to months, commonly on the head, ears, limbs, or trunk. The growth may be pink, red, or the colour of the surrounding skin, and it may bleed if traumatised. Less commonly, a plasmacytoma may develop in the mouth or along the gastrointestinal tract, where it may be discovered during investigation of other signs. This page explores the signals that bring plasmacytomas to attention, the biological behaviour that distinguishes them from other plasma cell disorders, the ways they are investigated and characterised, and the approaches used to address them. The goal is to orient you to what may be unfolding and what the road ahead typically looks like.
Why this matters now
Cutaneous plasmacytomas in dogs tend to appear in older animals, often between eight and twelve years of age, though they can develop earlier or later. Cocker Spaniels and other breeds with active immune systems appear to be represented more frequently, but the condition is seen across breeds and in mixed-breed dogs. In cats, plasmacytomas are less common overall and may present at a younger median age. There is no clear environmental or dietary trigger that has been identified, and most cases arise without obvious prior inflammation or injury at the site.
Most cutaneous plasmacytomas grow slowly over weeks to months, reaching a stable size that may range from a few millimetres to several centimetres. Once established, many remain unchanged for extended periods, though some continue to enlarge gradually. Ulceration or bleeding can occur if the surface becomes traumatised, but rapid growth or spread to other sites is uncommon with solitary cutaneous forms. The behaviour of plasmacytomas in the mouth or digestive tract may differ, and progression can be influenced by location and local factors.
Signals & patterns
Early signals
Small raised lump on skin
A smooth, dome-shaped mass appears on the head, ear, limb, or trunk. It may be noticed during grooming or petting and is often firm to the touch.
Pink or red colouration
The surface of the lump may appear pink, red, or fleshy, sometimes resembling an inflamed or vascular growth. The colour can vary depending on the thickness of the overlying skin.
Hairless or thinning patch
The skin over the growth may lose hair or appear thinner than the surrounding area. This can make the mass more visible and easier to detect during routine handling.
Single isolated mass
The lump typically appears on its own, without other similar growths nearby. This solitary pattern is a common feature of cutaneous plasmacytomas in dogs and cats.
Later signals
Ulcerated or weeping surface
The surface of the mass may break down, forming an open sore that can bleed or ooze. This often occurs after minor trauma or persistent licking.
Enlargement over time
A mass that was stable for months may begin to grow more noticeably. The rate of change can vary, and continued enlargement may prompt further investigation.
Oral or mucosal swelling
In cases where the plasmacytoma develops in the mouth, gums, or throat, owners may notice asymmetry, difficulty eating, or bleeding from the mouth. These signs can overlap with other oral conditions.
Click to read about the biological mechanisms
How this is usually investigated
Options & trade-offs
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