CONDITION
Histiocytoma
A histiocytoma is a small, benign growth that arises from a type of immune cell in the skin. It is one of the more common skin lumps seen in young dogs, typically appearing as a single, firm, dome-shaped nodule that can be pink, red, or ulcerated on the surface. Most owners notice it suddenly, and many are concerned by how quickly it appears or by its raw appearance. These growths are not cancerous and tend to resolve on their own over a period of weeks to months, though their behaviour can vary. The pattern of growth, regression, and occasional persistence shapes how they are approached. This page explores what signs may prompt attention, what is happening in the tissue, how histiocytomas are identified and distinguished from other lumps, and the range of approaches that exist when observation, biopsy, or removal may be considered.
Why this matters now
Histiocytomas are typically seen in dogs under about three and a half years of age. They can occur at any age, but their classic presentation is in young animals.
The typical histiocytoma appears quickly, sometimes seemingly overnight, and grows rapidly over the first one to four weeks. The surface often ulcerates during this growth phase. The immune system then causes the tumour to shrink, and most resolve on their own within two to three months.
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 about three and a half years of age. Multiple histiocytomas can occur but are less typical.
Later signals
Surface ulceration and scabbing
The surface often ulcerates, commonly while the lump is still growing, and may 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
Clinical monitoring
Biopsy and histopathology
Immunohistochemistry
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 allows a confirmed diagnosis on histopathology and removes the lump in a single procedure.
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 a confirmed diagnosis. Some growths that look like histiocytomas prove to be other tumour types requiring different management.
Common misconceptions
"All skin lumps in young dogs are histiocytomas"
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.
"Histiocytomas that ulcerate are becoming cancerous"
Ulceration is common in histiocytomas, often during the rapid growth phase, and does not mean the growth has become malignant. Other skin tumours, including mast cell tumours, can also ulcerate, so ulceration alone does not show which type of lump it is.
"Once a dog has had one histiocytoma, they will keep getting them"
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.
Related conditions
Lymphoma in Dogs
Lymphoma can occasionally present as skin nodules, particularly in its cutaneous form, and may be considered in the differential diagnosis when a lump is biopsied. Histiocytomas arise from a different cell line and behave very differently, but both represent proliferations of immune-related cells.
Haemangiosarcoma
Haemangiosarcoma can present as skin masses, particularly in its dermal form, and may be considered when evaluating any lump that appears suddenly or has an unusual appearance. The cell of origin, biological behaviour, and prognosis differ substantially from histiocytoma.
Mast Cell Tumours in Dogs
Mast cell tumours are common skin tumours in dogs and can look very similar to a histiocytoma, including a raised, hairless or ulcerated lump. They can behave aggressively, which is why sampling a lump to identify its cell type matters before it is assumed to be a histiocytoma.
Cutaneous Histiocytosis
Cutaneous histiocytosis is a separate histiocytic disease in which multiple plaques and nodules develop in the skin. It arises from dermal dendritic cells rather than the epidermal Langerhans cells of a histiocytoma, and it is managed differently.
Documenting the growth with photographs over time can help track changes and share information with the veterinary team. Understanding the typical timeline (rapid growth over one to four weeks, with most resolving within two 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: 9 October 2026 · Dr Alastair Greenway MRCVS