| This group of malignancies includes equine sarcoids, fibromatoses, fibrosarcomas, malignant fibrous histiocytomas, neurofibrosarcomas, leiomyosarcomas, rhabdomyosarcomas, and variants of liposarcomas, angiosarcomas, synovial cell sarcomas, mesotheliomas, and meningiomas. As a group, sarcomas are widely recognized, yet poorly characterized neoplasms. The confusion stems in part from the fact that spindle-cell sarcomas demonstrate much greater morphologic heterogeneity than
carcinomas; often, features of one sarcoma are intermixed with features of another. Consequently, it is widely accepted that the cell of origin of all soft-tissue sarcomas is a primitive mesenchymal cell that can differentiate in many different directions. This makes it difficult to define histopathologic criteria necessary for making an unequivocal diagnosis of specific spindle-cell sarcomas. In addition, comparing neoplastic mesenchymal cells with the normal cell they most closely
resemble does not imply origin from those cells. A second cause for the confusion stems from the difficulty in determining whether these are benign or malignant or what their biologic behavior will be in certain locations or breeds. Most spindle-cell sarcomas of domestic animals are locally infiltrative, difficult to excise, and yet seldom metastasize. Because, by definition, only malignant tumors have metastatic potential, these tumors should be considered benign; however, again by
definition, benign neoplasms are not infiltrative, and those tumors should be considered malignant and treated aggressively from the start. In human pathology, infiltrative but nonmetastasizing mesenchymal spindle-cell tumors have been defined as sarcomas of intermediate malignancy, a concept used below. |
| Clinically, 4 general principles relate to spindle-cell sarcomas and soft-tissue sarcomas: The more superficial the location, the more likely the tumor is to be benign (deep tumors tend to be malignant). The larger the tumor, the more likely it is to be malignant. A rapidly growing tumor is more likely to be malignant than one that develops slowly. Benign tumors are relatively avascular, whereas most malignancies are hypervascular. |
| Excision is the treatment of choice; wide excision or amputation should be performed when anatomically feasible because spindle-cell sarcomas often infiltrate along fascial planes, making it difficult to determine from gross examination the peripheral margins of the tumor. The best, if not only, opportunity to completely remove a spindle-cell sarcoma is during the first surgical attempt. A presurgical biopsy should be performed and a clear surgical plan formulated that includes the
intention of complete removal with biopsy samples submitted for margin determinations. Those sarcomas that recur have a greater potential for metastasis, and the time between recurrence often shortens with each subsequent attempt at excision. In addition, many soft-tissue tumors have a pseudocapsule, which on gross examination gives the impression of complete encapsulation; these tumors should not be “shelled out” because neoplastic cells are usually present in the pericapsular
connective tissues. Many sarcomas are shaped like an octopus, with tentacles that extend deeply into the tumor bed. Except for equine sarcoids, cryosurgery is usually not used for these tumors because some types, most notably fibrosarcomas, are resistant to freezing. Spindle-cell sarcomas generally do not respond well to conventional doses of radiation; however, higher doses have been reported to control ~50% of them for 1 yr. Surgical debulking followed by radiation is also an
option for local control. Chemotherapeutic protocols for sarcomas have become more accepted as a means of treatment. Most involve the use of adriamycin often in combination with other agents, including cyclophosphamide, vincristine, dacarbazine, and methotrexate. Some clinicians use carboplatin and will often rotate it with adriamycin. Although chemotherapy may improve the quality and prolong the life of an affected animal, it is seldom curative. |
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Equine sarcoids are the most frequently recognized neoplasm in horses. A viral etiology is suspected, and both papilloma virus and retrovirus particles have been identified on ultrastructural examination of sarcoids. In addition, cell-free extracts of bovine papilloma virus can induce a transient form of equine sarcoid when injected into horses. Evidence also suggests that sarcoids are transmissible by direct contact or via arthropod vectors or fomites, eg, contaminated
brushes and needles. Sarcoids occur in horses, donkeys, or mules, most commonly in those <4 yr old. Although there is a tendency for sarcoids to develop in families, no breed, sex, or coat color predisposition has been defined. Sarcoids may be found anywhere on the body, and up to 84% of affected horses have multiple lesions. The most common site varies with geographic area; in the UK, the penis is the most commonly reported site, while in the northwest USA, the limbs are affected
most frequently. Sarcoids are highly variable in appearance, and 4 manifestations are recognized: 1) verrucous, which may be confused with squamous papillomas or squamous cell carcinomas; 2) fibroblastic, which may be confused with granulation tissue or fibromas; 3) sessile or flat, which may be confused with flat warts (verruca plana); and 4) mixed verrucous and fibroblastic, which may be confused with fibropapillomas. They should be considered sarcomas of intermediate
malignancy—they do not metastasize but are locally invasive. The fibroblastic and sessile forms are generally the most aggressive. |
| Cryosurgery, after surgical debulking of larger lesions, is the treatment of choice. Two freeze-thaw cycles are generally used. A 1-yr remission rate of ~90% has been reported. Also, nontreated lesions may regress spontaneously, although ≥50% of equine sarcoids recur after surgery. Radiation therapy using iridium may control up to 85% of equine sarcoids. This treatment is most commonly used when sarcoids develop in locations not amenable to cryosurgery or excision. Immunotherapy
with inoculations of bacillus Calmette-Guérin (BCG) or cell wall extract of
Mycobacterium
bovis
remains controversial; some reports indicate ~50% of tumors can be controlled with such immunomodulating therapies. Tumors so treated may take several months to regress. Treatment with flunixin meglumine and prednisolone 30 min before BCG inoculation is recommended. Lastly, BCG therapy should not be used when treated horses could be exposed to cattle because BCG can induce a positive tuberculin reaction in the latter. |
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Fibromatosis
(aggressive fibromatosis,
extra-abdominal desmoids,
desmoid tumors,
low-grade fibrosarcomas, nodular fasciitis) is a sclerosing and infiltrative proliferation of well-differentiated fibroblasts derived from aponeuroses and tendon sheaths. They are generally seen on the heads of dogs, especially Doberman Pinschers and Golden Retrievers, where they are commonly diagnosed as nodular fasciitis. In veterinary medicine, the term nodular
fasciitis is applied to 2 different diseases—one that behaves as a fibromatosis and one that commonly affects the periocular tissues (known as canine fibrous histiocytoma [see
Fibrohistiocytic Tumors]). Fibromatoses are infrequently diagnosed in cats and horses. Grossly, fibromatoses are generally indistinguishable from infiltrative fibrosarcomas; however, they can be differentiated on histologic examination. Focal lymphoid nodules are scattered throughout the tissues. The fibromatoses are locally infiltrative with essentially no metastatic potential. If feasible, excision is the treatment of choice. Recurrence is common, and radiation therapy
may be of value for local control. |
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Fibrosarcomas are aggressive mesenchymal tumors in which fibroblasts are the predominant cell type. They are the most common soft-tissue tumors in cats and are also common in dogs but are rare in other domestic animals. In dogs, these tumors are most common on the trunk and extremities. Gordon Setters, Irish Wolfhounds, Brittany Spaniels, Golden Retrievers, and Doberman Pinschers may be predisposed. Fibrosarcomas vary markedly in their appearance and size. Neoplasms
arising in the dermis may appear nodular. Those arising in the subcutaneous fat or subjacent soft tissues may require palpation to identify. They appear as firm, fleshy lesions involving the dermis and subcutaneous fat and often invade musculature along fascial planes. When tumors are multiple, they are usually found within the same anatomic region. Fibrosarcomas with abundant interstitial proteoglycans (connective tissue mucins) are called
myxosarcomas or
myxofibrosarcomas . Myxosarcomas remain poorly defined in veterinary medicine, and many of them could be characterized as variants of liposarcomas or malignant fibrous histiocytomas. Fibrosarcomas in dogs are invasive tumors; ~10% metastasize. Factors that affect whether a fibrosarcoma can be completely excised include the surgeon’s skill, intent of the surgical plan, rate of growth (as defined by the mitotic index and quantity of necrosis), degree of cellular atypia,
and the tumor’s infiltrative nature, size, and location (which may require imaging to define properly). |
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Three forms of fibrosarcoma are recognized in cats: a multicentric form in the young (generally <4 yr old) caused by the feline sarcoma virus (FSV); a solitary form in the young or old, in which FSV has not been implicated; and a fibrosarcoma that develops in the soft tissues where cats are commonly vaccinated. This latter neoplasm is being recognized with increasing frequency in the USA (see also
Health-management Interaction: Small Animals: Introduction). An association with rabies and feline leukemia virus vaccinations is better defined than with vaccinations for other viral or bacterial diseases. Aluminum (commonly used in adjuvants) has been identified in vaccine-induced fibrosarcomas, and a prolonged proliferation of fibroblasts in response to the adjuvant may predispose them to undergo neoplastic transformation. These tumors appear as nodules or plaques between the shoulder blades, in the
soft tissues of the proximal hindlimbs, or less commonly, over the lumbar areas. Although commonly classified as fibrosarcomas, vaccination-site sarcomas are extremely heterogeneous and may be appropriately called malignant fibrous histiocytomas (giant cell tumors), liposarcomas, osteosarcomas, or chondrosarcomas. |
| Wide and deep surgical excision is the treatment of choice for fibrosarcomas, but because most practitioners underestimate the necessary margin extent, recurrence is common (>70% within 1 yr of the initial surgery). The rate of recurrence is >90% for vaccine-associated sarcomas. Even when surgical excision is clinically and histologically complete, recurrence is still the rule. Chemotherapy with carboplatin, doxorubicin and cyclophosphamide, or dacarbazine has been
recommended for nonresectable tumors. Initial results using a biologic response modifer (used intratumorally before excision and followed by radiation therapy) appears promising. Further work suggests that its effectiveness as an adjunct to surgery and radiation may increase tumor-free intervals up to 20% compared with historical controls. |
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