A cutaneous plasmacytoma is a skin tumor made of plasma cells, a type of white blood cell. Vets also call it an extramedullary plasma cell tumor, because it grows from plasma cells outside the bone marrow. The skin is one of the places where these tumors turn up most often in dogs, along with the mouth and the lower bowel [1].
Have your vet check any new lump on your dog’s skin. A lump cannot be identified by how it looks, so the vet takes cells or tissue from it and has them examined [2][3].
For other lumps on dogs, see lumps and bumps on dogs.
Other names
You may see the terms plasma cell tumor, extramedullary plasmacytoma or EMP on a lab report [1]. All of them describe the same kind of tumor. It belongs to the round cell tumors of the skin, a group that also includes histiocytomas and mast cell tumors [4]. A histiocytoma is a different tumor; see histiocytoma in dogs.
Single and multiple tumors
- A single lump: the usual picture. Most plasma cell tumors of the skin and mouth behave in a benign way [1].
- Many lumps: rarely, a dog grows several plasma cell tumors in the skin, sometimes a great many, without any cancer in the bone marrow. This is called cutaneous plasmacytosis. In one case series, golden retrievers and Labrador retrievers were the breeds seen most often [5].
Where they grow and what they look like
In a large tumor registry from Spain, plasmacytomas of the skin were found mainly on the face, head and neck, much like histiocytomas [4].
In dogs with many skin plasma cell tumors, the lumps were usually round and raised, pink to red in color, and some had lost their hair or had an open sore on top [5].
How vets diagnose them
Plasma cell tumors are among the common dog tumors that vets and lab pathologists recognize on a cytology slide, which is a smear of cells taken from the lump, often with a thin needle [2]. A biopsy takes a small piece of tissue for a closer look under the microscope.
Some round cell tumors look alike. In one study, several skin tumors first thought to be histiocytomas turned out to be plasmacytomas once extra lab stains were run [3]. So your vet may ask the lab for more tests before giving a final answer. For another skin tumor that is checked the same way, see mast cell tumors in dogs and cats.
Treatment
Your vet will advise on treatment. For most single plasma cell tumors in the skin or mouth, removing the whole lump with surgery is usually enough [1].
Dogs with many skin tumors are a different case. Surgery for each lump is often not practical, and these dogs are usually treated with medicines chosen by a vet, as in the case series above [5].
At home, as directed by a vet:
- note the size, place and date of any lump, so you can show your vet any change;
- go back for every recheck.
Outlook
Outcomes vary with the number of tumors and where they grow:
- Single skin tumors: most are benign and surgery that takes out the whole lump is typically curative [1].
- Tumors in the mouth: some can push into nearby tissue, grow back after removal or keep progressing [1].
- Many skin tumors: this form needs longer treatment and follow-up with your vet [5].
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:
- a new lump on the skin, especially on the face, head or neck [4];
- a lump in the mouth [1];
- more than one lump [5];
- a lump that comes back after it was removed [1].
FAQ
Is a plasmacytoma cancer?
It is a tumor. Most single plasma cell tumors of the skin are benign, and removing them fully is typically curative [1]. Your vet will explain what the lab result means for your dog.
Is it the same as a histiocytoma?
No. Both are round cell tumors that often grow on the head [4], and they can look alike under the microscope, so the lab sometimes needs extra stains to tell them apart [3].
Can it come back?
It can, especially when it grows in the mouth [1]. Your vet will plan rechecks.
The bottom line
Cutaneous plasmacytomas are skin tumors made of plasma cells. In dogs they are found mostly on the face, head and neck. A needle sample or biopsy shows what the lump is, sometimes with extra lab stains. Most single skin tumors are benign and full removal is usually enough, while tumors in the mouth and the rare form with many skin lumps need closer follow-up.
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
- Mulka KR, Gillette D, Durham AC, et al. Canine laryngotracheal plasma cell tumors: Ten cases and literature review. Veterinary Pathology, 2025 (CC BY 4.0). https://pmc.ncbi.nlm.nih.gov/articles/PMC12314212/
- Attini M, Newing E, Oikonomidis IL, et al. Application of a convolutional neural network for automatic cytological diagnosis of 7 common canine tumors on whole slide images: a proof-of-concept. Frontiers in Veterinary Science, 2026 (CC BY). https://pmc.ncbi.nlm.nih.gov/articles/PMC13711538/
- Paździor-Czapula K, Rotkiewicz T, Otrocka-Domagała I, et al. Morphology and immunophenotype of canine cutaneous histiocytic tumours with particular emphasis on diagnostic application. Veterinary Research Communications, 2015 (CC BY). https://pmc.ncbi.nlm.nih.gov/articles/PMC4330462/
- Rodríguez J, Santana Á, Borzollino MA, et al. Epidemiology of canine cutaneous round cell tumours on the Canary Archipelago in Spain. Veterinary and Comparative Oncology, 2023. PubMed abstract. https://pubmed.ncbi.nlm.nih.gov/37143410/
- Boostrom BO, Moore AS, DeRegis CJ, et al. Canine cutaneous plasmacytosis: 21 cases (2005-2015). Journal of Veterinary Internal Medicine, 2017. PubMed abstract. https://pubmed.ncbi.nlm.nih.gov/28514049/