By the Pethocyn editorial team. Published October 31, 2026. Updated November 1, 2026. Sources are listed at the end of the article.

Malignant fibrous histiocytoma is a cancer of the skin and the soft tissues under it. Pathologists now usually call it pleomorphic sarcoma (or undifferentiated pleomorphic sarcoma), and the old name is still widely used [1][2]. A common form in cats is the giant-cell type, named for the very large cells with many nuclei seen under the microscope [2][3].

It is a rare tumor in cats [1][4]. In a group of 13 affected cats, the average age was about nine and a half years [2], so it is mostly a problem of adult and older cats. It has also been reported in dogs, horses and people [4].

Have your vet check any new lump on your cat.

Where they grow

In a review of 16 reported feline cases, most tumors were on the legs [3]. In the group of 13 cats, the lumps were found mainly on the flank, the side of the chest, the legs and the area between the shoulder blades, and most were larger than 2 cm across [2]. The area between the shoulder blades is a common vaccination site, and one report describes a sarcoma with features of this tumor that grew where a cat had been vaccinated [5]. For lumps at vaccination sites, see injection-site sarcoma in cats.

How vets diagnose them

A lump cannot be identified by how it looks or feels. Your vet may collect cells with a fine needle for a first look, and a tissue sample (biopsy) is usually needed to confirm the type [2][4]. Under the microscope this tumor can look like several other cancers, so the lab may run extra stains on the tissue before it gives a final answer [1]. For other lumps on cats, see lumps and bumps on cats.

Treatment

Your vet will advise on treatment for your cat. For soft tissue sarcomas in general, the main treatment is surgery that removes the whole tumor along with a wide border of normal tissue around it [1]. This matters because these tumors can grow back if some tumor cells are left behind [3][5]. Your vet may discuss other options when a full removal is not possible [1].

At home, as directed by a vet:

  • note the size, place and date of any lump, so you can show your vet any change;
  • do not squeeze or try to drain a lump;
  • go back for every recheck.

Outlook

The main concern with this tumor is local: it can return at the same spot after surgery, sometimes within weeks or months [3][5]. Spread to other parts of the body seems less common. In the review of 16 cats it was proven in only one case [3], although single cases with spread to a nearby lymph node have been reported [4].

These are statements about groups of animals. They cannot predict how one cat will do. Your vet can explain what this means for your cat.

When to see your vet

See your vet if your cat has:

  • a new lump anywhere, including on the legs, flank or chest [2][3];
  • a lump at a place where your cat has had a vaccination or other injection [5];
  • a lump that comes back where one was removed before [3].

FAQ

Is this tumor linked to vaccinations?
Some sarcomas that grow at vaccination sites in cats show features of this tumor [5]. Lumps at vaccination sites are covered in the injection-site sarcoma post linked above. Talk to your vet about your cat’s vaccination plan.

The bottom line

Malignant fibrous histiocytoma, now often called pleomorphic sarcoma, is a rare cancer of the skin and soft tissues, seen mostly in adult and older cats, often on the legs or body and sometimes at vaccination sites. Only a lab exam can tell it from other lumps, and your vet will advise on treatment. Have your vet check any new lump.

This article is for general information only and does not replace advice from your veterinarian. If your cat has a new lump, see your vet.

Sources

  1. Kim K.E. et al. Primary Sublingual S100-Positive Undifferentiated Pleomorphic Sarcoma in a Cat: A Case Report with Comprehensive Exclusionary Immunohistochemistry. Veterinary Sciences, 2026;13(10):1036 (CC BY 4.0). https://pmc.ncbi.nlm.nih.gov/articles/PMC13709510/
  2. de Cecco BS et al. Feline giant-cell pleomorphic sarcoma: cytologic, histologic and immunohistochemical characterization. Journal of Feline Medicine and Surgery, 2021;23(8):738-744. Abstract on PubMed (US National Library of Medicine). https://pubmed.ncbi.nlm.nih.gov/33206030/
  3. Garma-Aviña A. Malignant fibrous histiocytoma of the giant cell type in a cat. Journal of Comparative Pathology, 1987;97(5):551-557. Abstract on PubMed. https://pubmed.ncbi.nlm.nih.gov/2824576/
  4. Gibson KL et al. Malignant fibrous histiocytoma in a cat. Journal of the American Veterinary Medical Association, 1989;194(10):1443-1445. Abstract on PubMed. https://pubmed.ncbi.nlm.nih.gov/2542201/
  5. Burton G, Mason KV. Do postvaccinal sarcomas occur in Australian cats? Australian Veterinary Journal, 1997;75(2):102-106. Abstract on PubMed. https://pubmed.ncbi.nlm.nih.gov/9066965/