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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