Soft Tissue Sarcoma: The Lump That Needs More Than a Simple "Lumpectomy"

"Soft tissue sarcoma" isn't a single cancer. It's an umbrella term for a group of tumours that arise from the body's connective tissues: fibrous tissue, fat, muscle, blood-vessel walls and the coverings of nerves (VCA Animal Hospitals). They behave alike and they're treated alike, so vets tend to lump them together and manage them as one, even though under the microscope they carry different names: fibrosarcoma, peripheral nerve sheath tumour (you may hear the older name "schwannoma"), perivascular wall tumour (once called haemangiopericytoma), liposarcoma, myxosarcoma and others (VCA Animal Hospitals).

They're not rare. Soft tissue sarcomas make up roughly 15% of skin and under-skin cancers in dogs, and about 7% in cats (VCA Animal Hospitals). They usually appear as a firm mass on or just beneath the skin, often on a limb or the trunk, and they're typically not painful, which is one reason they get left.

This is the part to slow down on, because everything else follows from it.

To the eye and the fingers, a soft tissue sarcoma feels like a tidy, self-contained ball. At surgery it often looks as though it has a neat shell around it and would lift out cleanly. That shell is the catch. It isn't a true capsule. It's a pseudocapsule of squashed tumour cells, and the tumour's busiest cells sit right at its edge (Cannon & Ryan, 2018).

Past that false shell, the cancer pushes microscopic projections out into the normal tissue around it, fingers of tumour cells you cannot see, cannot feel and cannot tell apart from healthy flesh (Cannon & Ryan, 2018). VCA puts it well for owners: picture the tumour as an octopus, the visible lump being the body and the invisible cells creeping outward being the tentacles (VCA Animal Hospitals). PetMD uses the same image, describing "arm-like structures that extend outward" (PetMD).

The problem more or less explains itself. Shell out only the part you can see, and those hidden tentacles stay behind for the cancer to regrow from.

This is precisely where a routine "just take the lump off" approach goes wrong, and the figures make the case plainly. Local recurrence is around eleven times more likely after an incomplete (marginal) removal than after a complete one, and when these tumours do come back after a shell-out they come back fast: one set of figures puts the median time to first recurrence at about 79 days after a marginal removal, against roughly 325 to 419 days after a proper wide or radical operation (dvm360, Multimodality proceedings).

A study that tracked recurrence by how clean the edges were fills in the larger picture. Three years on, the cancer had returned locally in about 41% of dogs whose margins were "infiltrated" (cancer reaching the cut edge), in about 23% where the margins were clean but close, and in just 7% where the margins were properly tumour-free (Chiti et al., 2021). The operation goes by the same name in each case. The outcomes differ enormously, settled by how much normal tissue came out with the lump.

The aim, then, isn't to remove the lump. It's to remove the lump plus a cuff of normal-looking tissue all around and beneath it, working on the assumption that the invisible roots are hiding in there. That cuff is the "margin", which is why your vet keeps coming back to the word. We cover what clean and dirty margins mean, and why the first surgery is the best chance, in our guide to cancer surgery.

A wide excision for a soft tissue sarcoma means taking a cuff of normal tissue to the sides of the lump, scaled to how aggressive the tumour is (commonly around 2 to 3cm for a low- or intermediate-grade tumour, wider for a high-grade one), and going one full layer of fascia or muscle deep rather than stopping at the fat just beneath it, since fat is a poor barrier to the spreading cells (Cannon & Ryan, 2018). On a leg that's a meaningful amount of tissue, which is why these operations run bigger than owners expect and why awkwardly placed tumours are sometimes sent on to a surgical specialist.

One cat tumour is worth flagging on its own. A feline injection-site sarcoma is a soft tissue sarcoma that behaves far more aggressively than most, so it needs much wider, radical surgery, often with margins of around 5cm, and the first operation counts for even more (Cannon & Ryan, 2018). We cover that one separately, because its message runs differently. For most other soft tissue sarcomas in cats, the approach matches dogs (Cannon & Ryan, 2018).

If recurrence is the local worry, whether the cancer travels to the rest of the body comes down to grade. After surgery the lump goes to a pathologist, who grades it from 1 to 3 (low, intermediate, high) on how aggressive the cells look (VCA Animal Hospitals).

Here's the encouraging part. Most soft tissue sarcomas are low or intermediate grade, and for those the risk of spread is genuinely low, generally under about 15 to 20% (Cannon & Ryan, 2018). High-grade (grade 3) tumours are the minority, somewhere around 7 to 17% of cases, and they're the ones that behave badly, spreading in roughly 40 to 50% of dogs, usually to the lungs (PetMD; Cannon & Ryan, 2018). The lesson is the familiar one in cancer care: the grade, rather than the word "sarcoma", tells you which animal you're actually dealing with.

For a low- or intermediate-grade soft tissue sarcoma that's fully removed with clean margins, often nothing more is needed. The cancer in that spot is gone, and recurrence is uncommon. That's the outcome we're aiming for, and for many pets it's exactly what happens.

When surgery can't get a clean margin, because of where the tumour sits, the next step is usually one of two things: a second, wider operation, or a course of radiation therapy to mop up the microscopic cells left behind (VCA Animal Hospitals). Radiation works well here. In one study of dogs whose sarcomas were incompletely removed and then irradiated, the five-year survival rate was 76%, with most dogs free of regrowth for years (McKnight et al., 2000), so an imperfect surgery is a long way from the end of the road.

Chemotherapy plays a smaller part and is generally held back for high-grade tumours, where the risk of spread is real, rather than used as a first-line treatment for the lump itself (VCA Animal Hospitals; PetMD).

Ask the two questions that decide everything: what grade is it, and were the margins clean. Those two answers carry almost the whole story, the chance it comes back locally and the small chance it spreads, far better than the size or feel of the lump ever could.

The other thing is to resist the urge to "just have it whipped off" at the first opportunity. With a soft tissue sarcoma the first operation is the one most likely to get it all, so it's worth a sample beforehand and a proper plan, even if that means a short wait or a referral. That patience is what turns this from a lump that keeps coming back into one that's dealt with once. If you've found a new lump, our piece on the lump you've found is the place to start, and for the bigger picture, our guide to what grade and stage really mean puts all of this in context.