Soft tissue sarcomas (STS) represent a group of
tumors that arise from mesenchymal tissues (including fibrous connective tissue,
cartilage, blood vessels, muscles, nerves, or fat). They are histologically
distinct, but share many common features; particularly the propensity for local
invasion. In the majority of these cases the cause of the tumor is unknown.
Histologic subtypes include fibrosarcoma, peripheral nerve sheath tumors,
malignant fibrous histiocytoma, hemangiopericytoma, and synovial cell sarcoma.
Soft tissue sarcomas are relatively common, making up approximately 15% of all
skin and subcutaneous cancers in dogs. Soft tissue sarcomas are characterized by
having a “pseudocapsule,” poorly defined margins, and fingerlike projections
that infiltrate tissue planes (i.e. between muscles and layers of connective
tissues). As a result, they tend to be difficult to completely excise, making
local recurrence common. In addition to local invasion, they can also
metastasize through blood or lymphatic routes. Metastatic rates have been
reported from 8-17% in dogs (primarily to the lungs). Histologic grading of STS
is based on mitotic rate and necrosis and provides important information with
regards to the metastatic potential and survival. For example, grade I and II
tumors appear to metastasize in less than 20% of the cases and grade III STS
develop metastatic disease in up to 50% of cases. However, biologic behavior
can be difficult to predict.
How is STS diagnosed? Patients most often have a
history of a non-painful “lump.” A fine needle aspirate (FNA) is usually
performed as a cursory test, but caution must be taken in the interpretation.
Soft tissue sarcomas do not exfoliate well (release cells from the mass), so a
negative aspirate does not always rule out STS. If an aspirate is suggestive of
a mesenchymal tumor, the next step is a biopsy.
For small, easily moveable
tumors, an excisional biopsy may be done. For larger tumors, an incisional
biopsy is preferred. Wide lateral margins of 3 cm and deep margins of one
fascial plane should always be attempted. In one recent study, 100% of local
disease control was achieved when deep and lateral margins
microscopically were 10 mm (1 cm). Fascial planes tend to act as biologic barriers to local
tumor invasion, but this protective effect may be overcome with high-grade
Staging of STS: Once a tentative or confirmed diagnosis of STS is made, further
testing is necessary to determine if the disease has spread. Thoracic (chest)
radiographs, evaluation (FNA) of draining lymph nodes if palpable, and abdominal
ultrasound (especially for higher grade tumors) are considered appropriate for
routine staging. Immunohistochemistry is recommended for those patients whose
tumors are difficult to classify on routine biopsy. In some patients, CT
scanning may be necessary to better delineate tumors when attempting to make a
decision regarding surgery. Cats with suspected vaccine related sarcomas are
routinely scanned prior to treatment due to the locally invasive nature of this
disease. If surgery is not considered possible based on the scan, then the scan
is used to plan radiation therapy.
A CT scan from a cat diagnosed with a vaccine
associated sarcoma. This patient presented to ACIC after an attempted
curative surgery. The red arrow points to contrast agent placed over
the surgery scar. The white arrows point to the true extent of disease.
Radiation was pursued
Surgery: For localized disease, wide surgical excision is considered the
treatment of choice for STS. Surgery alone can potentially be curative.
However, as is the case with most STS that occur on the extremities or where
microscopically clean margins are not achieved, further treatment is necessary
to prevent local recurrence. If a conservative surgery was initially performed
and clean margins not achieved, a second surgery should be performed. If a
second surgery is not possible due to the location of the tumor, radiation
therapy should be pursued.
Radiation Therapy: Radiation is a very effective
treatment for certain soft tissue sarcomas. It can be used after surgery when
margins are considered “dirty” (not tumor free) or can be used to shrink a
larger tumor to a point where surgery can be more successful. A combination of
surgery and radiation therapy has proven to be effective in keeping patients
disease free for extended periods of time. In one study, 76% of patients were
alive and disease free 5 years following radiation therapy of an incompletely
excised STS. If radiation is used without surgical intervention (i.e.
inoperable), 1 year control rates of 48-67% and 2 year control rates of 33% have
This patient underwent radiation therapy for a vaccine associated
sarcoma. Cats tolerate radiation with minimal side effects. Hair color
changes following radiation therapy are common, but represent only a
Chemotherapy: Most soft tissue sarcomas are locally aggressive and late to
metastasize. Chemotherapy is usually reserved for those more aggressive, higher
grade tumors, with a known high potential for metastasis. For patients with
high grade STS, chemotherapy is recommended, but response rates reported in the
literature have been variable. Chemotherapy is most appropriate in the adjunct
setting (i.e. used in addition to surgery and/or radiation) rather than as the
only form of therapy.
Metronomic Chemotherapy: This form of chemotherapy
utilizes constant exposure to very low dosages of chemotherapy in combination
with non-steroidal anti-inflammatory drugs. The goal of this form of therapy is
to make the tumors therapeutically controllable by stopping the essential blood
supply and nutrients to the tumor. Cancer cells have a higher metabolic rate
than ordinary cells and require a continuous supply of oxygen and nutrients.
The tumor must build a network of new blood vessels (a process called
angiogenesis) in order to obtain the necessary nutrients and oxygen it needs to
survive. In human medical research, a lot of emphasis has been placed on the
study of angiogenesis and novel therapies to target it (i.e. stop angiogenesis
of tumors). In a recent study in veterinary medicine, patients with STS that
were incompletely excised, but did not undergo radiation, and were treated with
metronomic therapy alone showed significantly improved survival rates when
compared to patients that received no further treatment after surgery.
Therefore, the use of metronomic therapy alone or in combination with other
forms of therapy may gain more popularity in the future.
STS arising from the lateral aspect of the forelimb of a dog. The mass
was too large to obtain tumor-free margins with surgery alone, so a
combination of surgery and radiation therapy was used.
Surgery was performed after a full course of radiation was completed.
The patient was cured.