All About Soft Tissue Sarcomas in Dogs

Soft tissue sarcomas (STS) account for approximately 15% of all tumors arising from the skin and underlying tissues in dogs. These tumors are most common in middle-aged to older dogs and arise from connective tissues. Since there are many types of connective tissues, there are subsequently many types of soft tissues sarcomas.

These tumors can arise from muscle, fat, fibrous, neurovascular tissue, etc. Examples of soft tissue sarcomas include peripheral nerve sheath tumor (PNST), liposarcoma, myxosarcoma, undifferentiated sarcoma, rhabdomyosarcoma, etc. If your dog has a "spindle cell tumor" or "mesenchymal tumor" involving the skin, these are different ways of saying 'soft tissue sarcoma'.

How to make a diagnosis

Just as with any other skin mass, we start with a needle aspirate to obtain a diagnosis. Soft tissue sarcomas do not release their cells as readily as other tumor types, so during the aspiration, we typically need to apply suction (negative pressure) to the syringe to encourage more cells to travel from the tumor into the needle.

In most cases, this will allow for a diagnosis of "sarcoma", however, since we are only looking at cells (cytology) we will be unable to pinpoint the type of sarcoma and the grade of sarcoma.

In most cases, just knowing that the tumor is a soft tissue sarcoma is enough to help guide us with the most appropriate type of surgery to perform, however, if the family wants to know specifics on tumor grade (which will help determine prognosis), then a biopsy (taking a piece of tissue instead of cells) would be needed.

We have to remember that even a biopsy is a sample from a small section of the tumor. Because of this, it is very possible that the area sampled just happens to be "less aggressive" than the rest of the tumor. In this case, the biopsy may tell us that the tumor is a grade I (low grade), but when we remove the entire tumor at surgery, we may find out that the tumor is a higher grade, and more aggressive.

The bottom line is that a biopsy will give us more information than an aspirate, but we still have to wait until the entire tumor is removed to learn the whole story.

Tumor behavior

Soft tissue sarcomas (STS) are locally invasive. This means that the tumor that you see in the skin is not the full extent of the tumor. It also means that if a vet narrowly removes the mass at surgery, they're not going to get it all and it will grow right back.

Think of a soft tissue sarcoma like an octopus - but with a bigger head and smaller tentacles. There is a central mass, but there are also tendrils of tumor cells invading into surrounding tissues.

Because of this, it makes it difficult to completely remove the tumor with surgery, unless the tumor is surrounded by a lot of loose skin.

In order to completely remove a soft tissue sarcoma, a surgeon must remove the tumor AND 3 cm of 'normal skin' around the tumor, as well as a "fascial plane" (deep layer of tissue underneath the tumor). If this is achieved, the tumor should not grow back at the surgery site.

Sometimes using a CT scan is the best way to help the surgeon ensure a successful surgery. If the tumor is large and growing in a location where there isn't enough extra skin to achieve a "3 cm margin" (this commonly happens where the tumor is on a leg), then the surgeon usually plans to remove "gross disease" (tumor he can see), with the recommendation for the family to follow up with radiation or electrochemotherapy to kill microscopic tumor cells remaining in the skin after surgery. In these cases, surgery plus radiation (or electrochemotherapy) is the best chance of preventing the tumor from growing back at the surgery site.

Soft tissues sarcomas can metastasize (spread to other organs). Their tendency to metastasize and the organs to which they metastasize depends on the type and grade of soft tissue sarcoma the patient is diagnosed with.

Most grade I or II soft tissue sarcomas have a metastatic rate of less than 15%, while grade III soft tissue sarcomas have metastatic rates of greater than 40%.

What is an appropriate work-up?

Most soft tissue sarcomas will spread to the lungs first, so 3-view chest x-rays (reviewed by a radiologist) should be performed prior to surgery. These patients should also have full blood work (complete blood count, chemistry) and urinalysis prior to surgery. If a lymph node near the tumor is enlarged, it should be sampled using an aspirate to ensure that it too is not affected by cancer. Most soft tissue sarcomas do not commonly spread to lymph nodes.

If the patient is older, an abdominal ultrasound is advisable prior to surgery. For most soft tissue sarcomas, we do not expect spread (metastasis) to the abdominal organs, however, prior to taking an older patient to surgery, we want to rule out any (silent) conditions that could affect their life expectancy such as a tumor in the spleen.

Treatment options for soft tissue sarcoma

Most soft tissue sarcomas are low grade (grade I or II) an have a low chance of metastasis, as mentioned above. For these tumors, we are focused on achieving local control (ridding the patient of all cancer cells in the skin where the tumor originated). This is typically achieved with surgery alone or surgery followed by radiation (or electrochemotherapy) if there isn't an adequate amount of normal skin available around the tumor to achieve wide margins at surgery.

Radiation therapy is an excellent option for incompletely removed soft tissue sarcomas, and is considered the gold standard. As an alternative, electrochemotherapy can be pursued if cost is a factor or if fewer treatments are desired. *This is not currently available in most areas of the US/Canada and there is less evidence for it's efficacy than for radiation.

If both radiation and electrochemotherapy are declined as treatment options, metronomic chemotherapy (low dose oral chemotherapy) can be elected. In one study, this worked quite well to prevent recurrence for incompletely removed soft tissue sarcomas. In practice, I think most oncologists are a bit more guarded about expectations.

Grade factors into our decision when we are making recommendations for whether to pursue follow-up radiation therapy.

A grade I soft tissue sarcoma that is incompletely removed has a 7% chance of recurrence (regrowth at the surgery site), while a grade II soft tissue sarcoma has a 34% chance of recurrence, and a grade III soft tissue sarcoma has a 75% chance of recurrence.

If local control is achieved for a grade I or II tumor, the patient should have long-term survival, which means that he should live for years

If local control is not achieved, the tumor may return as a high grade tumor, which may limit life expectency to six months despite additional surgery and chemotherapy. This is why achieving local control is so incredibly important.

If a dog is diagnosed with a high grade (grade III) soft tissue sarcoma, metastasis (spread to other organs) is expected, even with surgery. Since the expected lifespan after surgery is approximately six months, chemotherapy can be used in an attempt to prolong survival further. Unfortunately, we do not have strong evidence that chemotherapy will help in every case.

How do you know if a 'complete excision' is achieved?

After a soft tissue sarcoma is removed, interpreting the pathology report is one of the main roles of an oncologist. This is a great time to get an oncologist involved. Pathologists will often say that a tumor is completely removed, but the veterinarian needs to look for the actual margin measurement. If the margins are 0.5 mm, that tumor might technically be completely removed (because the margins are not 0 mm), but 34% of grade II and 75% of grade III tumors would grow back in that scenario.

Most patients can have a very good outcome with soft tissue sarcoma. In order to achieve this, it typically involves a well-planned surgery and proper guidance and care after surgery is completed.

Dr. Lori Cesario

Board Certified Veterinary Oncologist

PS: I'm happy to now offer online oncology consultations. Learn more about how this service can help by visiting the Vet Cancer Consultants site.

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