A histiocytoma is a skin lump that grows from immune cells in the skin. Researchers describe it as a common, benign tumour in dogs that starts from Langerhans cells in the outer layer of the skin [1]. Langerhans cells are immune cells in the top layer of the skin.
Benign means the tumour is not expected to spread. Histiocytomas can grow fast and look worrying under the microscope, yet they rarely spread to other parts of the body [1].
Any new lump on your dog should be checked by your vet. Under the microscope a histiocytoma can look much like other “round cell” tumours, including mast cell tumours, so telling them apart can take extra lab tests [2].
For an overview of other lumps, see lumps and bumps on dogs.
Which dogs get them
- Age: most cases are in young dogs. One review puts 60 to 70% of cases in dogs under four years old, but dogs of any age can get one [1]. A 2026 study of 38 cases found them in older dogs too, with a median age of 7 years in that group [4].
- Breeds: in a Portuguese study of 93 tumours, Boxers were the breed seen most often, followed by Cocker Spaniels [1].
- One larger study: in 359 confirmed cases from Portugal, about half the dogs were 2 years old or younger, and French Bulldogs and Boxers had the highest risk compared with mixed-breed dogs [3]. These are findings from one region.
This article is about dogs. If your cat has a lump, see your vet.
What they look like
- Where: the head (above all the ears, lips and nose area) and the legs, often the lower legs and toes [1].
- Look: usually a single raised, red, hairless lump shaped a bit like a button, often small, that grows quickly and may have a broken, open surface [1]. A broken surface is called an ulcer.
A histiocytoma cannot be told apart from other lumps by sight. Other tumours, including mast cell tumours, can look similar, even to a pathologist [2].
How vets diagnose them
Your vet takes a sample from the lump and sends it to a lab. This may be a few cells drawn out with a thin needle (fine needle aspiration) or a small piece of tissue (biopsy). The lab looks at it under the microscope and may use special stains to confirm the type of cell [2].
More than one lump
Most histiocytomas appear as a single lump, but some dogs get several, at the same time or one after another. This has been described more often in Shar-Peis [1].
Other conditions can also cause several bumps at once, so more than one lump always needs a vet check.
Treatment
Your vet will decide whether a lump should be removed or watched. In the Portuguese study above, none of the dogs whose tumours were removed had the lump come back at the same place or spread within two years of follow-up [1].
At home, as directed by a vet:
- do not squeeze, pick or put anything on the lump unless your vet tells you to;
- stop your dog from licking or scratching it in the way your vet advises;
- note the size of the lump and the date, so you can show your vet any change;
- go back for any recheck your vet suggests.
Outlook
What researchers report:
- A histiocytoma tends to shrink away on its own (regress), usually some weeks or months after it first appears [1][2].
- The dog’s own immune system drives this: immune cells move into the lump as it shrinks [2][3].
These are statements about groups of dogs. They cannot predict how one dog will do. Your vet can explain what this means for your dog.
When to see your vet
See your vet if your dog has:
- any new lump [2];
- a lump that is ulcerated or broken open [1];
- a lump that has not gone away;
- several bumps rather than a single lump [1].
FAQ
Is a histiocytoma cancer?
It is a tumour, but a benign one that rarely spreads [1]. Only a lab test can tell it apart from a cancerous lump [2].
Will the lump go away on its own?
It varies, and no one can promise it. See the Outlook section above.
Can older dogs get histiocytomas?
Yes. They are typical of young dogs, but they can occur at any age [1][4].
Do cats get histiocytomas?
This article covers dogs only. See your vet about any lump on a cat.
The bottom line
A histiocytoma is a common skin lump of young dogs, though older dogs can get one too. It is usually single, raised, hairless and sometimes broken open, and it is found on the head, ears and legs more than elsewhere. Researchers describe it as benign and note that it tends to shrink away on its own. Because other lumps, including mast cell tumours, can look the same, have your vet test any new lump.
This article is for general information only and does not replace advice from your veterinarian. If your dog has a new lump, see your vet.
Sources
- Pires I, Rodrigues P, Alves A, Silva F, Lopes C. Histopathological and Ultrastructural Study of a Canine Langerhans Cell Tumour (Canine Cutaneous Histiocytoma). Cells. 2024;13(15):1263. Open access, CC BY 4.0. https://pmc.ncbi.nlm.nih.gov/articles/PMC11312119/
- Paździor-Czapula K, Rotkiewicz T, Otrocka-Domagała I, Gesek M, Śmiech A. Morphology and immunophenotype of canine cutaneous histiocytic tumours with particular emphasis on diagnostic application. Vet Res Commun. 2015;39(1):7-17. Open access, CC BY 4.0. https://pmc.ncbi.nlm.nih.gov/articles/PMC4330462/
- De Moura C, Pinello K, Catarino J, Niza-Ribeiro J, Santos J, Faísca P. Epidemiology of Canine Cutaneous Histiocytoma: Insights From Age and Topographic Clustering. Vet Dermatol. 2026;37(5):780-787. https://pubmed.ncbi.nlm.nih.gov/41937610/
- Chen YM, Liao ZW, Kuo CL, Kuo CC, Huang WH. Age distribution and regression-associated immunophenotypic features in canine cutaneous histiocytoma. J Vet Med Sci. 2026. https://pubmed.ncbi.nlm.nih.gov/42816321/