CONDITION

Histiocytoma

Why this matters now

Histiocytomas most commonly appear in dogs under three years of age, with a peak incidence around eighteen months. While they can occur in older dogs, their classic presentation is in young animals. These tumours often appear suddenly, growing rapidly over a few weeks before beginning to regress.

The typical histiocytoma appears quickly, sometimes seemingly overnight, and may continue to grow for two to four weeks before plateauing. Over the following one to three months, the immune system mounts a response that causes the tumour to shrink and disappear. Some histiocytomas may ulcerate during their regression phase before healing completely.

Signals & patterns

Early signals

Rapidly appearing dome-shaped lump

Histiocytomas typically appear as smooth, round, button-like growths that seem to develop quickly over days. They are usually firm and well-defined.

Hairless pink or red surface

The surface often appears hairless and may have a pinkish-red colour, sometimes resembling a small button or mushroom cap.

Location on head, ears, or limbs

While histiocytomas can appear anywhere, they commonly develop on the head (including the ear flap), face, or legs.

Single growth in a young dog

The classic presentation is a solitary lump in a dog under two to three years of age. Multiple histiocytomas can occur but are less typical.

Later signals

Surface ulceration during regression

As the immune system destroys the tumour, the surface may ulcerate and form a small scab before healing. This can look worse before it looks better.

Gradual shrinkage over weeks

After reaching maximum size, histiocytomas typically begin to shrink noticeably, eventually disappearing entirely.

Complete resolution leaving normal skin

Most histiocytomas resolve completely, often leaving no trace or only minimal scarring where the growth was.

Click to read about the biological mechanisms

How this is usually investigated

While histiocytomas have a characteristic appearance, confirming the diagnosis may involve sampling the growth to distinguish it from other skin tumours that can look similar.

Fine needle aspirate and cytology

Purpose: Using a small needle to collect cells from the growth, which are then examined under a microscope to identify the cell type.
Considerations: This is typically the first-line diagnostic approach. Histiocytoma cells have a distinctive appearance that pathologists can often recognise readily on cytology.

Clinical monitoring

Purpose: In typical presentations—a single button-like lump on a young dog—veterinary teams may sometimes recommend monitoring for spontaneous regression.
Considerations: If the growth behaves as expected for a histiocytoma (stabilising then shrinking over weeks), this supports the suspected diagnosis. Any growth that persists or enlarges beyond the expected timeframe warrants further investigation.

Biopsy and histopathology

Purpose: Surgical removal of all or part of the growth for detailed microscopic examination.
Considerations: Provides definitive diagnosis and can distinguish histiocytoma from other tumours. Often combines diagnosis with treatment when the growth is removed.

Immunohistochemistry

Purpose: Special staining techniques that identify specific cell markers to confirm the cell type of origin.
Considerations: May be used in ambiguous cases to differentiate histiocytoma from other round cell tumours that can appear similar on routine staining.

Options & trade-offs

Because histiocytomas typically resolve spontaneously, management approaches range from watchful waiting to surgical removal, depending on individual circumstances.

Watchful waiting

Monitoring the growth over several weeks to allow natural regression, particularly when cytology has confirmed or strongly suggested histiocytoma.

Trade-offs: Avoids unnecessary surgery and its associated costs and recovery. Requires owner patience and regular monitoring. The growth may ulcerate before resolving, which can be visually concerning.

Surgical excision

Removing the growth surgically, which provides both definitive diagnosis and immediate resolution.

Trade-offs: Eliminates the waiting period and any cosmetic or practical concerns about the growth. Involves anaesthesia, surgical cost, and recovery time. Particularly considered when diagnosis is uncertain or the location is problematic.

Cryotherapy

Freezing the growth with liquid nitrogen or similar agents to destroy the tissue.

Trade-offs: A less invasive alternative to surgery that may be appropriate for small, accessible lesions. May not provide tissue for histopathological confirmation.

Removal for diagnostic uncertainty

When the growth does not behave typically or when the diagnosis is not confirmed, removal allows complete examination.

Trade-offs: Provides peace of mind and definitive diagnosis. Some growths that look like histiocytomas prove to be other tumour types requiring different management.

Common misconceptions

Misconception:

"All skin lumps in young dogs are histiocytomas"

Reality:

While histiocytomas are common in young dogs, other tumour types can occur in this age group. Mast cell tumours, in particular, can appear similar and require different management. Confirming the identity of any skin growth helps ensure appropriate care.

Misconception:

"Histiocytomas that ulcerate are becoming cancerous"

Reality:

Ulceration is actually a common part of the regression process as the immune system destroys the tumour. While it may look concerning, surface breakdown during the shrinking phase is often a sign that resolution is progressing.

Misconception:

"Once a dog has had one histiocytoma, they will keep getting them"

Reality:

Most dogs that develop a histiocytoma have only one. While some dogs may develop additional histiocytomas, this is not the typical pattern. Multiple or recurrent histiocytomas warrant additional investigation.

Documenting the growth with photographs over time can help track changes and share information with your veterinary team. Understanding the typical timeline—growth over two to four weeks, then regression over one to three months—can help set expectations. If a suspected histiocytoma does not begin to shrink within the expected timeframe, revisiting the diagnosis may be appropriate.

Last reviewed: 24 April 2026 · Dr Alastair Greenway MRCVS