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Soft Tissue Sarcomas
INCIDENCE AND RISK FACTORS
Soft tissue sarcomas are a diverse group of cancers that collectively comprise 15% of all
"skin" and subcutaneous cancers in the dog and 7% in the cat.' The annual
incidence of soft tissue sarcomas in the dog is about 35 per 100,000 dogs at risk and 17
per 100,000 cats at risk. With the exception of the feline very rare sarcoma virusinduced
neoplasms of the young cat and vaccineinduced sarcomas in cats, the cause of these tumors
is unknown. In dogs, sarcomas have been associated with radiation, trauma, and parasites (Spirocerca
lupi). Most sarcomas are solitary in the older dog or cat and no definite sex or
breed predilection is known except for synovial sarcomas, in which the ratio of males to
females is 3:2.5 Rhabdomyosarcoma may occur in animals as young as 4 months. In the
dog, soft tissue sarcomas tend to be reported in the larger breeds.
PATHOLOGY AND NATURAL HISTORY
Soft tissue sarcomas develop from a variety of mesenchymal tissues, although they are
frequently classified inappropriately as skin or subcutaneous tumors. Both malignant and
benign counterparts exist for each cell type, although this section will only emphasize
the malignant varieties. The term soft tissue sarcoma generally excludes those
tumors of hematopoietic or lymphoid origin. They are often considered collectively because
of their similarity in clinical behavior. Hemangiosarcoma, lymphoma, mast cell sarcoma,
oral sarcoma, osteosarcoma and chondrosarcoma, and feline virus-induced sarcoma are
covered separately in other sections under the heading of Cancer & Tumors.
Soft tissue sarcomas tend to have several important common features with regard to their
biologic behavior.
No other group of tumors present more of a diagnostic cha lenge than those originating
in soft tissues. Many of these tumors present histologic patterns with overlapping
features not only among themselves but also with a variety of other neoplasms with
different histogenesis. The development of immunocytochernical procedures and the
availability of monoclonal antibodies and polyclonal antibodies to various tissue markers
have significantly improved the diagnosis of soft tissue sarcomas in human pathology and
to a limited degree in veterinary pathology. Some of the markers used include intermediate
filaments such as acidic and basic features, vimentin, desmin, and neurofilament proteins.
Actins are a major constituent of the micro filamentous cytoskeleton of virtually all
eukaryotic cells. These markers are useful for both differentiation of smooth and striated
muscle tissue. S-100 represents a group of small Ca++ binding modulator proteins involved
in cell-cycle progression, cell differentiation, and cytoskeletal-membrane interactions.
The S-100 protein is found in glial, Schwarm, and satellite cells in the nervous system.
S-100 is expressed on both benign and malignant melanoma. The use of immunocytochemistry
and ultrastructural features are increasing in veterinary pathology and will enhance the
accuracy of diagnosis and prognosis.
The histologic nomenclature for some sarcomas may vary from pathologist to pathologist.
Before the initiation of the best appropriate therapy for the treatment of soft tissue
sarcomas, it is necessary to know the histologic type of sarcoma, the size, the site, the
histologic grade, and the stage of disease. The majority of soft tissue sarcomas can occur
in both low-grade and high-grade histologic forms. In general, histologic grading (low,
intermediate, high) is assigned after histologic characterization
Fibrosarcoma
Most fibrosarcomas arise from the skin, subcutaneous tissue, or palate, and represent
malignant or transformed fibrocytes. The tumor tissue is very cellular, with closely
packed spindle-shaped fibroblasts showing many mitotic figures. Fibrosarcomas are
malignant tumors with a tendency to grow to quite a large size, invade deeper structures
such as tendons, fascia, and muscles, and ulcerate the epidermis.
Hemangiopericytoma
Hemangiopericytoma are one of the most common tumors seen in dogs (very rare in cats) with
a mean age of onset of 10 years and no sex predilection. This is a malignant tumor of the
subcutis that is considered to originate in adventitial pericytes of small vessels. They
occur as nodular, lobulated, and poorly defined tumors, without histologic encapsulation;
however, clinically they may appear encapsulated and discrete, mimicking a lipoma. Most of
the tumors are adherent to deeper tissues and infiltrate underlying fascia, muscle, and
skin. Although these tumors are considered malignant, they tend to have a relatively
benign behavior; that is, they are locally invasive and rarely metastatic. Local
recurrence is common following conservative surgery. They tend to grow slowly and can
range in size from 0.5 cm to over 10 to 12 cm in diameter. In some cases, they can easily
be confused with lipomas on initial clinical examination.
The tumor occurs most commonly on the extremities. The histologic criteria for making a
diagnosis of hemangiopericytoma include typical whorling and the arrangement of plump
spindle cells, which frequently form a circular pattern around open or collapsed vessels.
Neurofibrosarcoma
The neurofibrosarcomas are malignant tumors of nerve sheath origin and have been referred
to as neurogenic sarcomas; malignant schwannomas, and malignant neurolemmomas. These
tumors can occur anywhere in the body. Close association with the nerve may be necessary
in order to differentiate a neurofibrosarcoma from a fibrosarcoma.
Neurofibrosarcomas may cause nodular enlargement of nerves that results in the compression
of nerves. They often involve the brachial or lumbosacral plexus, resulting in paralysis
and pain. They can invade the spinal cord and metastasis can occur but is considered
uncommon.
Liposarcomas
Liposarcomas are rare malignant lesions of adipose tissue found in older animals. No breed
or sex predilection is known. Liposarcomas tend to be aggressive and locally invasive, and
they may metastasize to the lungs, liver, and bone. They tend to occur in the subcutaneous
ventral aspects of the body as well as the abdominal cavity.' Mitoses are frequent, as are
infiltrations into the veins, lymphatics, and underlying muscle and fascia. On palpation
these tumors tend to have a very firm texture and are poorly defined. Liposarcomas will
not be confused with lipomas either clinically or cytologically.
Infiltrative Lipoma
Infiltrative lipomas are rare tumors composed of well-differentiated adipose cells without
evidence of anaplasia. They are considered benign and do not metastasize. Infiltrative
lipomas commonly invade adjacent muscle, fascia, nerve, myocardium, joint capsule, and
even bone.
Lipoma
Lipomas are benign neoplasms of well-differentiated lipocytes or adipocytes. These tumors
occur in adult animals and are most frequently seen in females. Lipomas may occur as
individual tumors or may be multiple. The most common sites are the ventral abdomen and
the thorax. On rare occasions lipomas may be noted within body cavities.
Lymphangiosarcoma
Lymphangiosarcomas arise from lymphatic endothelial cells and are extremely rare in both
dogs and cats. They are usually soft and cystlike and may appear invasive. In most cases
the clinical signs are associated with drainage of lymph through the skin or a cystic
mass. Aspiration may reveal a fluid-filled mass. Lymphangiosarcomas usually arise from the
subcutaneous tissues but can occur in such sites as the liver, pericardium, and
nasopharynx. They tend to have a moderate metastatic potential.
Rhabdomyosarcoma
Rhabdomyosarcomas are malignant tumors derived from striated muscle cells.
Rhabdomyosarcomas consist of large, pleomorphic, elongated tumor cells, so-called strap
cells, which may show cross striation in their eosinophilic cytoplasm. These tumors tend
to be locally invasive and can metastasize to the lungs, liver, spleen, kidneys, and
adrenal glands. They tend to be diffuse, infiltrative, and poorly circumscribed.
Rhabdomyosarcomas can also be termed "botryoid" because of their grapelike
appearance. Botryoid rhabdomyosarcomas have usually been reported in the bladder of large
young dogs.
Synovial Cell Sarcoma
Synovial sarcomas are malignant neoplasms thought to arise from tenosynovial tissue in
either the joints, bursa, or tendon sheaths and are considered rare in dogs and cats. The
tumor may occur at any age but is more common in middle age (6-8 years). The stifle joint
is most often affected, and the elbow is the second most common site. Histopathologically,
there is one of two cell populations: epithelioid or synovioblastic and spindle or
fibrosarcomatous cell types. Histologic grading has been shown to provide important
prognostic information. The synovial sarcoma tends to be invasive into tendons, muscles,
and bone. The tumor tends to originate from periarticular tissue rather than the synovium
of the joints, tendons, or bursa, and it commonly recurs following local surgical
excision. Extensive bone destruction has been reported and metastasis is grade dependent.
Leiomyoma and Leiomyosarcoma
Leiomyosarcomas are tumors that arise from smooth muscle and are found as firm, white,
lobulated masses in any part of the gastrointestinal tract from the esophagus to the
rectum They also are found to originate in the spleen and genitourinary tract. Benign
lesions are usually small, localized, and encapsulated, with cells resembling normal
intestinal smooth muscle. Many leiomyomas can occur within the gastrointestinal tract and
have been associated with chronic blood loss. Tumors affecting the vagina or vulva are
usually pedunculated and will often protrude from the vulva.
Leiomyosarcomas commonly arise in the retroperitoneal space, the spleen, or the
gastrointestinal tract. They tend to be highly aggressive and can metastasize widely.
Splenic leiomyosarcomas may commonly show leukocytosis (> 17,000 WBC/mm3), anemia, and
a large splenic mass.
Malignant Fibrous Histiocytoma
Malignant fibrous histiocytomas (MFHs) are uncommon tumors and are characterized as
primitive, pleomorphic sarcoma with partial fibroblastic and histiocytic differentiation.
This group of sarcomas have also been labeled as giant cell fascial sarcoma, epithelioid
sarcoma, malignant histiocytoma, reticulum cell sarcoma, and giant cell tumor. They may be
confused with fibrosarcoma or extraskeletal osteosarcoma. This neoplasm should not be
confused with malignant histiocytosis or systemic histiocytosis. MFHs are usually firm and
invasive tumors arising in the subcutis and rarely metastasize. They are most commonly
seen in middle-aged animals, with an average age of 9 years. These tumors seem to have the
same biologic behavior in the cat as the dog, that is, invasive, high recurrence and low
metastatic potential. The dorsal thoracic and scapular areas are the most common sites for
MFH.
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