Soft tissue sarcomas, AKA spindle cell tumors – why they just kinda suck

Let’s talk tumors, growths, lumps, cancer, whatever you want to call it. And why certain ones suck, but do not suck as bad as others.

There are two broad categories of tumor, based on the cell type from which they originate. Round cell tumors are your lymphomas, mast cells, other tumors whose cells look, well, round when your vet checks them out under a microscope. The name might end in “carcinoma” but doesn’t always. The tumors are the generous types that readily give up cells when your vet asks (usually by poking a needle into the mass). Histiocytomas fall into this category (click here to read about the coolest kind of growth ever!).

The other category of cell type is spindle cells tumors. These are often of connective tissue, such as bones (osteosarcoma), ligaments, blood vessels, and muscle (tumors of the heart). However, spindle cell tumors can appear on the skin of a dog or cat as well, and be rather frustrating. The name of these often ends in “sarcoma.”

In cats, there is a very specific type of spindle cell tumor that is still being studied and hotly debated among specialists. It was once called a “vaxosarc” for “Vaccine-related Sarcoma.” It was thought to arise at places on the body where vaccines were given. Some researchers even went as far as to single out the leukemia vaccine as a suspected culprit. These tumors are, thankfully, quite rare, but they are incredibly aggressive and almost always fatal. And, there is currently debate as to whether these rare but super aggressive tumors are related to vaccines specifically, or caused by ANY injection of anything given to the cat. There is much more work to be done on this topic, but I wanted to at least bring it up. For the record, these injection-site related sarcomas, as we are currently calling them, totally and completely suck, unlike the other tumors in this article, which just kinda suck.

What we’re focusing on here are those lumps that pop up on dogs (sometimes cats) that can look like anything. They can be hairless, big, little, and be basically anywhere on the body. They are very frustrating for veterinarians because they are not generous at all with their cells. We can stick a needle into these masses 3-4 times, trying to aspirate out cells for a sample, and get nothing but blood. Not helpful. Yet, we know by process of elimination that if the mass is not sharing its cells (clearly they did not watch enough Sesame Street) it most likely is a spindle cell tumor. (The round cell type tumors tend to throw their cells around!)

soft tissue sarcoma, dog cancerMost spindle cell tumors (AKA soft tissue sarcomas) of the skin are not fatal, just annoying. First, we have to surgically remove them. And we can’t just cut around the edge, no no! We have to get nice wide margins, meaning we take the tumor’s outline, and add an inch or so all around it. Why is this important? Spindle cell tumors like to extend tiny finger-like projections around the main mass, and we want to make sure we get all of these suckers cut out!
There are a few different kinds of soft tissue sarcoma. A common one is a hemangiopericytoma, which in very school we called a peripheral nerve sheath tumor. Other times, we send the lump off to the lab for pathologists to tell us what it is, and all they can tell is “spindle cell tumor” or “soft tissue sarcoma.” Yes, these tumors are highly uncooperative.

So after all this, why do they only “kinda suck”? They rarely metastasize, or spread to other organs and cause major problems. They tend to want to come back in the exact same spot, especially if the vet was unable to get the extra skin around the mass (what we call “wide margins”) due to the location (ie – a foot or leg, where there is not a lot of skin to worth with). Many tend to not come back. And if they do, you kinda know where to suspect it – they are not sneaky!

If your pet has a really persistent spindle cell tumor that comes back, the best way to ensure it will not return again is with radiation therapy. This often involves leaving your pet at a facility to get radiation every day for 3 weeks, and most people don’t dig that. If you happen to live very close to a major specialty hospital that offers radiation, then you are one of the lucky few who can drop off and pick up your pet daily.

So yes, soft tissue sarcomas are frustrating to diagnose, and are best treated with surgical removal. Still, as far as cancers go, they aren’t too exciting. Sure, you might not get an actual diagnosis of the tumor, but you know if it comes back, where it will be (the same place it was!) Kinda like the thief who’s not so smart and rings the doorbell before breaking in the front door. The rate of metastasis is low, making it rarely a fatal cancer.

Can it spread? It is possible – it is cancer after all, and cancer does not play by the rules. As far as tumors go, however, soft tissue sarcomas / spindle cell tumors rank low on the scale of “how likely is this to shorten my pet’s life.”

I’ll take it.

Posted in General health, Senior pets.


  1. What is your advice about removal of a spindle cell tumor from a dog who already has cancer?
    Our 10 year old mixed lab was diagnosed with anal sac adenocarcinoma cancer in 2014. Since then she has had 2 surgeries to remove the cancer, 2 rounds of chemotherapy and radiation in January 2017. She currently takes Palladia & Proxicam . Her August CT scan showed the mass treated with radiation, instead of surgery because inoperativ, was non-existent. Which was great news. Now 2 lumps have been diagnosed as spindle cell cancer. Should we put her through surgery again & disrupt her quality of life or leave the lumps alone?

    • Wow! You’ve done a lot, and Anal sac adenocarcinoma is a bad one! Without being her vet, I can’t legally give direct advice. But I can tell you that I myself have asked the question of “how much should I put this dog through?” If I’m not sold on putting them through more, I haven’t. But it never hurts to get the opinion of her oncologist!

  2. Thank you. I appreciate your response. We are always looking at our pets quality of life. We did meet with her Oncologist after I sent in my comment & received more information. She suggests surgery to remove the 2 places because they are very small & on the outside of her body so less invasive. Corona,our dog, has tolerated surgeries very well with no side effects and recover for this will be a week. If we do not remove they could become larger & interfere with other organs. I do dread the day we have to make the decision to not do any more treatments.

  3. Electrochemotherapy is also a good treatment options for soft tissue sarcomas, 9 publications on the subject, please do not hesitate if you want more information.

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