Understanding Soft Tissue Sarcoma in Dogs: Signs, Treatment, and Hope

When you find a new lump on your dog while scratching their ears or giving them a belly rub, it’s only natural for your heart to sink a little. While we often associate lumps with “old age,” some of these masses require a closer look. One of the most common types of skin tumors we see in dogs is the Soft Tissue Sarcoma (STS).

If your primary veterinarian has mentioned this term to you, or if you’ve noticed a firm, growing mass on your dog, here is what you need to know from a specialist’s perspective.

What is Soft Tissue Sarcoma in Dogs?

Soft tissue sarcomas aren’t one type of cancer; they are a “family” of tumors that arise from connective tissues. These are the tissues that “glue” the body together—muscles, fat, nerves, blood vessels, and fibrous tissues found in places like skin layers.

Even though these tumors can start in different types of cells, we group them together because they behave very similarly. They tend to be “locally invasive,” meaning they don’t just sit on the surface like a marble; they send out microscopic “tentacles” into the surrounding healthy tissue.

Common types you might see on a pathology report include:

Fibrosarcomas (connective tissue)
Peripheral Nerve Sheath Tumors (nerves)
Myxosarcomas (another type of connective tissue)
Liposarcomas (fatty tissue)

What Should You Look For?

The tricky thing about soft tissue sarcomas is that they often don’t make a dog feel “sick”, until potentially later on in the tumor’s course. In the early stages, your dog will likely eat, play, and wag their tail as if nothing is wrong.

Common signs include:

A Firm or Semi-Soft Lump: These usually appear under the skin on the trunk or limbs. They can feel like they are “fixed” to the tissue underneath rather than sliding around freely.
Slow but Steady Growth: Many of these tumors grow slowly over months, which sometimes leads owners to think they aren’t dangerous.
Lameness or Difficulty Moving: If a tumor develops on a leg or near a joint, it may physically interfere with your dog’s gait, even if it isn’t painful.
Pain (Rare): Most STS are non-painful unless they grow large enough to involve a nerve, press on a nerve or if the skin over the top begins to ulcerate (break open).

How We Diagnose the “Tentacles”

If you bring your dog to see us at Portland Veterinary Emergency and Specialty Care (PVESC) in Portland, Maine, our first goal is to determine exactly what the lump is.

We usually start with a Fine Needle Aspirate (FNA). We use a small needle to collect cells from the mass to look at under a microscope. While this can often tell us it’s a sarcoma, a surgical biopsy is often needed to determine the “Grade” of the tumor.

The Grade (I, II, or III) is the most important piece of the puzzle. It tells us how aggressive the tumor is and how likely it is to spread to other parts of the body or “metastasize”, like the lungs.

Schedule a consultation with our Board-Certified Veterinary Oncologist.

Treatment Options

At PVESC, we believe in a “quality of life first” approach. We work with you to find a path that fits your family and your dog’s specific needs.

Surgery: Because these tumors are so invasive locally, the best chance for a cure is aggressive surgery. We don’t just “shell out” the lump; we aim for “wide margins.” This means removing the tumor along with a significant amount of healthy-looking tissue around it to ensure we catch those microscopic tentacles. This is submitted to the lab to get the confirmation adequate margins were taken.
Radiation Therapy: Sometimes, a tumor is in a difficult spot—like the lower leg or the face—where we simply can’t get wide enough margins without damaging vital structures. In these cases, we use radiation therapy to “clean up” any microscopic cancer cells left behind after surgery.
Chemotherapy: For Grade III (high-grade) tumors, there is a higher risk of the cancer spreading (metastasizing) to the lungs. In these instances, we may discuss chemotherapy to help protect the rest of the body.

Learn more about on Oncology

What to Expect: Living with a Sarcoma Diagnosis

For the majority of dogs with low-to-intermediate grade soft tissue sarcomas, the prognosis is good to excellent if we can achieve “clean” surgical margins. Many of these dogs go on to live completely normal, long lives.

If your dog is diagnosed with an STS, here is what the journey usually looks like:

The Consultation: We sit down and look at the “map” of the tumor (often using a CT scan for larger masses) to plan the best surgical approach.
The Recovery: This can vary depending on the location and extent of tumor removal. Many dogs are back on their feet within a few days.
The Follow-up: We typically recommend “staging” every 3 to 6 months. This involves chest X-rays to ensure the lungs remain clear and a physical exam to make sure the tumor hasn’t returned to the original site.

A Final Thought from a Specialist

If you find a lump, don’t wait. It is much easier (and safer) for us to remove a small mass with wide margins than a large one. At PVESC, our oncology and surgery teams work hand-in-hand to ensure your dog receives the most compassionate and advanced care available in Maine.

We are here to help you navigate the options, answer the “what-ifs,” and most importantly, get your dog back to their favorite spot on the couch.