Soft Tissue Sarcoma
in Pets

Soft tissue sarcomas are a common tumor in the dog and less so in the cat. These tumors can occur anywhere but are most often found in the skin. Usually the tumors start as a mass that grows slowly over time. These masses can vary in how hard or soft they feel. They may be covered in hair or could be bald. In time, they can break open and bleed.  

Early in their development, they are rarely painful but in time, can cause severe discomfort.

These tumors are malignant, meaning they can spread to other locations. The good news is that this tumor, while locally very aggressive, rarely spreads to other locations. Most commonly, sarcomas spread via the bloodstream. When this happens, the tumor cells travel through the bloodstream and can result in spread to the lungs (this is the first place the large tumor cells encounter tiny blood vessels and literally get stuck). These tumors can spread through the lymphatic system as well.  When this happens, the cells may be found in the lymph nodes and cause swelling in those locations. Your primary veterinarian or your oncologist will recommend looking for the spread in these locations as when this occurs, the prognosis may be very different.  To look in the lungs, radiographs can be used. To see malignancy in the lungs with X-rays, the masses must be at least 7-9 millimeters in size. Most patients can be radiographed while they are awake.  A CT scan can also be used to look “inside” the lungs.  With this technology, tumors of only a couple of millimeters can be visualized. A CT requires anesthesia or in some cases, heavy sedation. Arizona Veterinary Oncology offers CT scan at our locations in Gilbert, AZ.

To diagnose a soft tissue sarcoma, fine needle aspirates or surgical biopsies  may be used.

‍Soft tissue sarcomas may not easily give up their cells to sampling. Frequently, the pathologist can report a sarcoma was seen in the sample but may not be able to tell us exactly what type of sarcoma we are dealing with. In most cases, this is academic as with rare exception, our sarcomas tend to behave in the same way and thus are treated the same way.

Treatment for hemangiopericytomas, fibrosarcomas, peripheral nerve sheath tumors and the like (different types of soft tissue sarcomas) focus on the local tumor.

Surgery is our first choice when possible.

If the entire tumor can be removed with surgery, including the tiny tendrils that extend away from the tumor like a sea urchin, then this is the treatment of choice.  When these masses grow on the trunk, this is commonly possible. When these tumors grow on the legs or head, surgery is rarely curative.  In these cases, surgery can be used to debulk the tumor, leaving only microscopic cells so that other treatments can be used more effectively. When conventionally fractionated radiation therapy (CFRT) is used with microscopic soft tissue sarcomas, studies show that 76% of dogs are still disease free at that site up to 5 years later. For these tumors, we typically recommend 19 fractions of radiation delivered over 4 weeks time, Monday through Friday. Each treatment requires a light and brief anesthesia. While not risk free, this anesthesia is very light and fairly short, so even in our older dogs tends to be safe. Local side effects only occur where we give the radiation. “Radiation sickness,” as reported in people, is not seen in dogs. We only see hair loss and blister like lesions. These are the worst the week after the treatments finish and heal quickly after that. The hair usually grows back but usually grows back white.

When a sarcoma cannot be removed surgically, or when 19 fractions of radiation is not desirable, stereotactic radiosurgery (SRS) can be used instead.  While data is still immature at this time, it appears to these tumors respond well to these large doses of radiation but do so very slowly. We are finding that these tumors may shrink for many months following treatment. Side effects still include hair loss but the blister like lesions noted above are less common. Again, the hair usually grows back but usually grows back white. We have also begun to explore using SRS to treat the incision when the tumor is first surgically removed and microscopic cells are left behind. We hope that by giving a large dose of radiation along the incision that we will sterilize the remaining cells just like with CFRT but in only a single treatment and with much less in the way of side effects.

Chemotherapy is not considered effective for soft tissue sarcomas.

Cats can experience a variation of a soft tissue sarcoma often referred to as an Injection Site Sarcoma or a Vaccine Associated Sarcoma. There is still a poorly defined link between things like vaccines or depot type injections (some steroids for example) and the development of these cancers. Unlike our soft tissue sarcomas in dogs, these tumors tend to be much more aggressive and much harder to control. As noted above, surgery is our first choice when possible but it is very rare to obtain “clean” margins following these surgeries. As such, follow-up with radiation is usually recommended. Conventionally fractionated radiation (CFRT) has been used and has shown to increase survival but rarely cure these cats. Less is known about using Stereotactic radiosurgery (SRS) but increasing experience suggests that we can get a response with this treatment as well.