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

"Soft tissue sarcoma" isn't one cancer. It's an umbrella term for a family of tumours that all arise from the body's connective tissues: the fibrous tissue, fat, muscle, blood-vessel walls and the coverings of nerves (VCA Animal Hospitals). Because they behave similarly and are treated similarly, vets tend to group them together and manage them as one, even though under the microscope they have 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). Typically they show up as a firm mass on or just beneath the skin, often on a limb or the trunk, and they're usually not painful, which is part of why they get left.

This is the heart of the whole article, so it's worth slowing down on.

A soft tissue sarcoma looks, to the eye and the fingers, like a tidy, self-contained ball. At surgery it often appears to have a neat shell around it and seems like it would "pop out" cleanly. That shell is the trap. It isn't a true capsule, it's a pseudocapsule, made of squashed tumour cells, and the tumour's most active cells sit right at its edge (Cannon & Ryan, 2018).

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

You can probably already see the problem. If a surgeon simply shells out the part you can see, those hidden tentacles get left behind, and the cancer regrows from them.

This is exactly where a routine "just remove the lump" 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, with one set of figures putting 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 larger picture comes from a study that tracked recurrence by how clean the edges were. Three years on, the cancer had come back 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). Same operation in name. Very different outcomes, decided by how much normal tissue came out with the lump.

So the goal isn't to remove the lump, it's to remove the lump plus a cuff of normal-looking tissue all around and beneath it, on the assumption that the invisible roots are hiding in there. That cuff is the "margin", and it's why your vet keeps using 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.

In practice, 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, because 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 are bigger than owners expect and why awkwardly placed tumours are sometimes referred to a surgical specialist.

One particular cat tumour deserves a flag here. 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 matters even more (Cannon & Ryan, 2018). We cover that one separately, because the message around it is different. For most other soft tissue sarcomas in cats, the approach is the same as in dogs (Cannon & Ryan, 2018).

If recurrence is the local worry, the question of whether the cancer spreads 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) based on how aggressive the cells look (VCA Animal Hospitals).

The reassuring part is that 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, but they're the ones that behave badly, spreading in roughly 40 to 50% of dogs, usually to the lungs (PetMD; Cannon & Ryan, 2018). It's the same lesson as everywhere in cancer care: the grade, not the word "sarcoma", tells you which animal you're dealing with.

For a low- or intermediate-grade soft tissue sarcoma that's completely 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 very far from the end of the road.

Chemotherapy plays a smaller role 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).

The single most useful thing you can do is ask the two questions that decide everything: what grade is it, and were the margins clean. Those two answers tell you 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 instinct 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 exactly 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 if you want the bigger picture, our guide to what grade and stage really mean puts all of this in context.