Soft tissue sarcomas are a group of malignant tumors that start in the body’s connective tissues. A 2026 consensus report from a veterinary oncology association says they are common in dogs, mostly in medium and large breeds and in middle-aged to older animals [1]. They tend to grow into the tissue around them and come back after removal, while spread to distant organs is less common [1].
You cannot tell what a lump is by looking at it or feeling it. The consensus report treats a tissue biopsy, read by a pathologist, as the way to reach a firm diagnosis [1]. If you find a new lump on or under your dog’s skin, have your vet check it.
For other kinds of lumps, see our post on lumps and bumps on dogs. Sarcomas that can develop at injection sites in cats are covered in lumps and bumps on cats.
What these tumors are
The group covers several tumor types, among them fibrosarcomas, myxosarcomas, nerve sheath tumors and liposarcomas [1]. Under the microscope many of them are built from long, thin cells, which is why they are often called “spindle cell” tumors [1].
Their edges are hard to see. The consensus report explains that these tumors have poorly defined borders, and that the thin layer that seems to wrap many of them (a pseudocapsule) can itself contain tumor cells that reach into nearby tissue [1]. That is the main reason they can come back after surgery.
What you may notice
Most of these tumors appear as a lump in or under the skin. According to the consensus report, up to 60% of them are found on the legs, about a third on the trunk and a small share on the head or neck [1]. What else you notice depends on how big the lump is, how deeply it has grown, whether its surface has broken open, and which part of the body it affects [1].
No single feature tells you whether a lump is harmless. Only a biopsy shows what a lump is [1].
How it is diagnosed
- A fine needle sample (cytology) can raise the suspicion of a sarcoma, but the consensus report notes that it is weak at telling the grades apart [1].
- For large lumps, or tumors where clean edges may be hard to get, the consensus report recommends a biopsy before the operation, so the surgery can be planned [1]. The whole tumor should then be sent to the lab again after removal, because a small biopsy piece can give the wrong grade [1].
- The grade (I, II or III) is based on what the pathologist sees: how well the cells resemble normal tissue, how fast they are dividing, and how much of the tumor is dead tissue [1]. Staging adds the tumor’s size and whether it has reached the lymph nodes or other organs [1].
- A 2026 study of 32 dogs compared small samples taken from different parts of each removed tumor with the grade of the whole tumor. Its authors found that two or more samples, one from the edge and one from the center, matched the final grade more often than a single sample [4].
Treatment
- Surgery is the main treatment. The consensus report describes the aim as removing the tumor together with a rim of normal-looking tissue, so the lab can confirm that the edges are clear [1].
- Getting the whole tumor out in the first operation matters. Several studies link adequate margins at the first surgery with a better outlook, and incomplete removal is tied to faster regrowth [1].
- Because the pseudocapsule can contain tumor cells, the surgeon cuts through healthy tissue all the way around the lump instead of peeling the lump out [1].
- Radiation therapy is recommended when the edges are incomplete or narrow, and it can be used on its own when surgery is not possible or is declined [1].
- The consensus report says chemotherapy should not be a routine main treatment for these tumors, as studies have not shown a consistent survival benefit. It is mostly kept for grade III tumors or tumors that have spread [1].
How wide the margins need to be is still debated. A 2016 review points out that no test can yet predict how much healthy tissue must be removed for a given tumor. In the author’s view, the old habit of always taking very wide margins is not fully backed by newer evidence, yet surgery that is too limited can harm dogs whose tumors are more aggressive [3]. Your vet or surgeon will plan the surgery for your dog’s tumor.
Outlook
The outlook depends a lot on the grade. The consensus report says most of these tumors in dogs are low grade (I or II), grow slowly and seldom spread, and that careful surgery with adequate margins may cure many of them [1]. Grade III tumors behave more aggressively, and the report puts their chance of spreading at about 40% [1].
Outcomes vary between studies:
- A 2019 review pooled 10 studies covering 278 removals of sarcomas in or under the skin. Tumors came back in 16 of 164 cases (9.8%) when the edges were clear, against 38 of 114 (33.3%) when they were not. The authors concluded that clear edges under the microscope lower the risk of the tumor returning [2].
- The 2016 review reports that 40% of dogs with high-grade tumors may develop spread to other organs, even when the tumor itself is controlled at its original site [3].
These are figures and statements about groups of dogs in particular studies. They do not prove that a treatment caused an outcome, and they cannot predict how one dog will do. Your vet can explain what the biopsy and surgery results mean for your dog.
When to see your vet
See your vet if your dog has:
- any new lump on or under the skin
- a lump that is growing
- a lump that comes back where one was removed before [1]
- a lump that has broken open or is sore [1]
FAQ
Can my vet just remove the lump and test it afterwards?
For large lumps or lumps where clean edges may be hard to get, the consensus report recommends a biopsy first, so the operation can be planned, because the first surgery is the best chance to remove the whole tumor [1]. Ask your vet whether a biopsy first is right for your dog’s lump.
Is a small or slow-growing lump safe?
Not necessarily. Many sarcomas are low grade and grow slowly, but they are still tumors that can come back after removal [1]. Have every new lump checked.
Why does the surgery need to be so large?
The tumor’s edges are poorly defined and its outer layer can hold tumor cells, so the surgeon removes a rim of normal-looking tissue around it [1]. A review notes that how wide the margins need to be is still debated [3]; your vet will plan this for your dog.
Will the lump come back?
It can. Regrowth at the same site is the typical problem with these tumors, and it is more likely when the edges are not clear [1][2]. See the Outlook section for what studies found, and ask your vet about your dog’s results.
The bottom line
Soft tissue sarcomas are connective tissue tumors seen fairly often in bigger and older dogs, and they tend to grow into the tissue around them [1]. A biopsy gives the diagnosis, and for larger lumps it is best done before surgery so the operation can be planned [1]. Surgery that removes the whole tumor, ideally in the first operation, is the main treatment [1][2]. Outcomes vary between dogs and studies, and your vet can explain what the results mean for your dog. Have any new or growing lump checked.
This article is for general information and does not replace a visit to your veterinarian. If you find a lump on your dog, contact your vet.
Sources
- Fonseca-Alves CE, Biondi LR, Dos Anjos DS, et al. Canine cutaneous and subcutaneous soft tissue sarcoma in dogs: a consensus report from the Brazilian association of veterinary oncology. Front Vet Sci. 2026;13:1750148. Open access, CC BY 4.0. https://pmc.ncbi.nlm.nih.gov/articles/PMC13192336/
- Milovancev M, Tuohy JL, Townsend KL, Irvin VL. Influence of surgical margin completeness on risk of local tumour recurrence in canine cutaneous and subcutaneous soft tissue sarcoma: A systematic review and meta-analysis. Vet Comp Oncol. 2019;17(3):354-364. https://pubmed.ncbi.nlm.nih.gov/30953384/
- Bray JP. Soft tissue sarcoma in the dog – part 1: a current review. J Small Anim Pract. 2016;57(10):510-519. https://pubmed.ncbi.nlm.nih.gov/27624929/
- Ferraris EI, Maniscalco L, Mazzone E, et al. Multiple preoperative biopsies may increase histologic grade accuracy in canine soft tissue sarcoma: a prospective study. Vet J. 2026;316:106596. https://pubmed.ncbi.nlm.nih.gov/41692151/