CONDITION
Plasmacytoma
A plasmacytoma is a tumour that arises from plasma cells — the specialised white blood cells responsible for producing antibodies. These tumours can develop in the skin, in the mouth or throat, or less commonly in other locations. In dogs, they tend to occur in older animals and are most often found as solitary growths rather than part of widespread disease. Owners typically notice a raised lump on the skin, often on a paw, ear, or lip, or a growth inside the mouth that may bleed, interfere with eating, or simply be visible during routine care. The appearance alone does not reveal whether the growth is a plasmacytoma or another type of tumour, and many owners arrive at this page after a biopsy has been discussed or completed. This page explores what plasmacytomas can look like when they first appear, what investigations help clarify the diagnosis and extent, the biological behaviour of these tumours in different locations, and the range of approaches used to manage them. The goal is to orient you to the landscape of possibilities, not to predict what will happen in any individual case.
Why this matters now
Plasmacytomas tend to appear in middle-aged to older dogs, with most cases diagnosed between eight and ten years of age. Certain breeds appear overrepresented, most notably cocker spaniels, who accounted for close to a quarter of cases in one case series; other breeds are reported only occasionally, and the growth can occur in any dog. In cats, plasmacytomas are rare, with too few reported cases to establish a reliable age pattern distinct from dogs.
Most cutaneous plasmacytomas in dogs behave in a benign fashion, growing slowly over weeks to months as a solitary mass. Surgical removal is typically curative, with local recurrence occurring in fewer than one in ten cases and distant spread remaining uncommon. A small proportion may behave more aggressively, and in rare instances the presence of a plasmacytoma can signal the early stages of a more widespread plasma cell disorder, which is why staging investigations are often part of the initial assessment.
Signals & patterns
Early signals
A new raised lump
Owners often notice a smooth, dome-shaped or slightly irregular mass that has developed on the skin, commonly on the head, ears, limbs, or trunk. The growth may be firm, sometimes pedunculated, and may range from a few millimetres to several centimetres across.
Pink or red surface colour
The overlying skin may appear pink, red, or hairless, sometimes with a smooth or slightly shiny surface. The colour can resemble inflamed tissue, though the mass itself may not feel warm or painful.
Bleeding after trauma
The surface of the growth can be fragile and may ulcerate or bleed if scratched, licked, or knocked. This can lead to intermittent spotting of blood on the fur or bedding.
Gradual enlargement
The mass tends to grow steadily over weeks to months rather than appearing overnight. Owners may notice the lump increasing in size when comparing photographs or recalling earlier observations.
No obvious discomfort
Many plasmacytomas do not cause pain or irritation unless they are located in areas subject to friction or trauma. The animal may show no behavioural change despite the visible growth.
Later signals
Ulceration or crusting
As the mass enlarges or is repeatedly traumatised, the surface may break down, forming an ulcer with crusting, weeping, or scabbing. Secondary infection can develop in these areas.
Multiple lumps
In uncommon cases, more than one plasmacytoma may develop over time, either in the skin or in other tissues. This pattern may prompt broader investigation to distinguish isolated growths from more widespread disease.
Masses in the mouth or digestive tract
Plasmacytomas can arise in less visible locations, including the gums, palate, or gastrointestinal lining. These may be discovered during investigation of bleeding from the mouth, difficulty eating, or other digestive signs.
Click to read about the biological mechanisms
How this is usually investigated
Investigation begins with a physical examination to document the location, size, texture, and appearance of the mass, along with palpation of nearby lymph nodes and a general health assessment. Because any new growth raises questions about its nature, tissue sampling is typically the next step, allowing the cells to be examined directly. Additional tests may be considered to assess overall health and to determine whether the mass is truly solitary or part of a more widespread process.
Cytology
Histopathology
Serum protein electrophoresis
Radiography
Complete blood count
Options & trade-offs
Management is shaped by the location, size, and behaviour of the mass, along with the owner's preferences and the animal's overall health. For many solitary cutaneous plasmacytomas, local treatment is sufficient, though the approach varies depending on whether complete removal is straightforward or whether the site makes surgery more complex. Different combinations of intervention and observation suit different situations, and there is often more than one reasonable path forward.
Surgical excision
The mass is removed with a margin of surrounding normal tissue, aiming to take the entire population of abnormal cells. This is often curative for solitary cutaneous plasmacytomas, particularly when the margins are confirmed to be free of tumour on histopathology. Healing is typically straightforward, and recurrence at the same site is uncommon when excision is complete.
Trade-offs: The ease of excision depends on the location; masses on the limbs or trunk are often more accessible than those on the face, ear, or in the mouth. Incomplete excision may leave residual cells, and a second procedure or alternative approach may then be considered.
Radiation therapy
Focused radiation is delivered to the tumour and surrounding tissues over several sessions, aiming to damage the DNA of the proliferating plasma cells and prevent further growth. This approach is often considered when the location makes complete surgical removal difficult—such as masses in the mouth or close to important structures—or when residual disease remains after incomplete excision. Many plasmacytomas respond well to radiation, with shrinkage or resolution of the mass over weeks to months.
Trade-offs: Radiation requires referral to a specialist centre, involves multiple visits, and may cause temporary inflammation or hair loss in the treated area. It does not provide tissue for histopathology if used as the sole treatment, and long-term control depends on the radiosensitivity of the individual tumour.
Active monitoring
Some small, stable plasmacytomas that are not causing discomfort, ulceration, or functional problems may be observed over time without immediate intervention. The mass is measured and photographed at intervals, and any change in size, colour, or surface integrity prompts reassessment. This approach suits owners who prefer to avoid surgery or anaesthesia when the growth is not currently causing harm.
Trade-offs: Monitoring requires regular follow-up and carries the risk that the mass may enlarge, ulcerate, or become more difficult to manage if intervention is deferred. It does not provide histopathological confirmation unless sampling has already been performed, and the benign nature of the growth remains presumptive until proven.
Cryotherapy or electrosurgery
Freezing or electrocautery techniques can be used to destroy small, superficial plasmacytomas without formal excision. The treated tissue is ablated and sloughs away over days to weeks, and healing occurs by second intention. These methods are less invasive than traditional surgery and may be suitable for very small or pedunculated masses.
Trade-offs: Tissue destruction means that no sample is available for histopathology, so the diagnosis must be established beforehand by cytology or prior biopsy. Local inflammation and delayed healing are common, and recurrence may occur if the entire tumour is not eliminated.
Common misconceptions
"A plasmacytoma is the same as multiple myeloma and will spread throughout the body."
A solitary plasmacytoma arises in one location and typically behaves in a benign, localised fashion, whereas multiple myeloma involves widespread proliferation of plasma cells in the bone marrow and other sites. Most cutaneous plasmacytomas do not progress to myeloma, though the distinction is confirmed through tissue examination and blood tests.
"Once removed, a plasmacytoma will always come back in the same place."
Recurrence at the original site is uncommon when the mass is excised completely with clear margins. The majority of solitary cutaneous plasmacytomas do not return after successful removal, though a small proportion of animals may develop new, unrelated growths elsewhere over time.
"If the mass is not causing any problems, it does not need to be investigated."
While some plasmacytomas remain stable and asymptomatic for extended periods, any new lump warrants characterisation to distinguish it from more aggressive tumours that may require different management. Sampling or removal provides information that observation alone cannot deliver, and this shapes the approach going forward.
Related conditions
Multiple Myeloma
Multiple myeloma represents a systemic form of plasma cell proliferation, in which abnormal plasma cells accumulate throughout the bone marrow rather than forming a single localised growth. A solitary plasmacytoma can, in some cases, progress to multiple myeloma over time, though many remain isolated throughout the animal's life.
Mast Cell Tumours
Mast cell tumours share with plasmacytomas the challenge of distinguishing benign from malignant behaviour based on appearance alone, and both arise from distinct immune cell populations. Owners discovering a new skin mass may be directed toward similar diagnostic pathways—cytology, histopathology, and margin assessment—regardless of which tumour type is ultimately identified.
Histiocytoma
Histiocytomas, like plasmacytomas, are typically solitary, dome-shaped skin masses arising from immune cells, and the two can appear similar on initial examination. Both tend to behave in a benign fashion in most cases, though histiocytomas are more common in young dogs and often regress spontaneously, whereas plasmacytomas occur in older animals and typically require removal.
Soft Tissue Sarcomas
Soft tissue sarcomas and plasmacytomas may both present as firm masses beneath or within the skin, and distinguishing between them often requires biopsy. The two conditions differ in their cellular origin and behaviour, but both raise questions about the need for complete surgical excision and the risk of local recurrence if margins are incomplete.
Once a plasmacytoma has been characterised and addressed, the focus often turns to monitoring for new growths and maintaining overall health as the animal ages. Questions about immune function, the relationship between skin masses and systemic disease, and the patterns of ageing in the individual may become relevant over time. These are themes that sit comfortably within routine wellness conversations and longer-term planning for healthspan.