Page 426 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

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Soft Tissue Sarcomas

JULIUS M. LIPTAK AND NEIL I. CHRISTENSEN

Incidence and Risk Factors is dependent on tumor size, degree of infiltration, completeness of
histologic excision, and histologic grade; the likelihood of metas-
Soft tissue sarcomas (STSs) are a heterogeneous population of tasis is dependent primarily on histologic grade.
mesenchymal tumors that comprise 15% and 7% of all skin and STS is a general term encompassing a heterogenous group of
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subcutaneous tumors in the dog and cat, respectively. The annual tumors, but STSs can be subclassified according to the tissue of
incidence of STSs in companion animals is about 35 per 100,000 origin or phenotype. These include FSA, perivascular wall tumor
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dogs at risk and 17 per 100,000 cats at risk. In dogs, sarcomas (PWT), PNST (nonbrachial plexus), liposarcoma, myxosarcoma,
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have been associated with radiation, trauma, foreign bodies, pleomorphic sarcoma (or malignant fibrous histiocytoma), malig-
orthopedic implants, and the parasite Spirocerca lupi. 3–9 nant mesenchymoma, and undifferentiated sarcoma. 12,14,15 These
Most STSs are solitary tumors in middle-aged to older dogs can be difficult to differentiate histologically because common com-
and cats, except for rhabdomyosarcomas which occur in young ponents include an intercellular collagen matrix and spindle or fusi-
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dogs. 10,11 There is no specific breed or sex predilection for STSs. form mesenchymal cells forming bundles, streams, and whorls.
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STSs tend to be overrepresented in large-breed dogs. STSs may be characterized histologically by areas of mature tissue
or via immunohistochemistry (IHC) by the expression of certain
Pathology and Natural History cellular markers to determine phenotype (Table 22.1). 12,16–21 How-
ever, STSs may display more than one histologic pattern, and his-
STSs are typically regarded as a heterogeneous group of tumors tologic patterns and IHC features may not be exclusive to a single
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whose classification is based on similar pathologic appearance cell type of origin or phenotype. STSs are sometimes referred to
and clinical behavior; however, this may be an overly simplistic by alternate names such as spindle cell tumors of soft tissue because
interpretation. Sarcomas arise from mesenchymal tissues and have of the complexity of phenotypic differentiation, the use of the term
features similar to those of the cell type of origin. These tumors “sarcoma” for tumors that have a low metastatic potential, and the
originate in connective tissues, including muscle, adipose, neuro- difficulty in differentiating benign from low-grade malignant vari-
vascular, fascial, and fibrous tissue, and can give rise to benign and ants of some mesenchymal tumors of soft tissue (Table 22.2). 12,22–25
malignant entities. STSs can arise at any anatomic location, but Histologic distinction of tumor type may not be clinically impor-
they most commonly involve the skin and subcutaneous tissues. tant because most STSs have a similar biologic behavior (i.e., locally
For simplicity and consistency, a number of sarcomas arising from aggressive with a low to moderate risk of distant metastasis); how-
soft tissue are excluded from the umbrella term of cutaneous and ever, this may be an overly simplistic approach as there is increasing
subcutaneous STSs because of differences in anatomic location, evidence that discernible differences in presentations and outcomes
biologic behavior (such as a higher metastatic rate and/or a dif- may exist between different types of STSs.
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ferent distribution of metastasis), and histologic features. These
include histiocytic sarcoma (HS), synovial cell sarcoma (SCS),
hemangiosarcoma (HSA), lymphangiosarcoma, rhabdomyo- Specific Tumor Types
sarcoma, oral fibrosarcoma (FSA), and peripheral nerve sheath
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tumors (PNSTs) of the brachial and lumbar plexi. HS, SCS, Tumors of Fibrous Tissue
HSA, and oral FSA are covered in other chapters, with this chap- Nodular Fasciitis (Fibromatosis, Pseudosarcomatous
ter concentrating primarily on malignant STSs. Fibromatosis)
The majority of cutaneous and subcutaneous STSs have a simi- Nodular fasciitis is a benign nonneoplastic lesion arising from the
lar biologic behavior. This is characterized by a locally expansile subcutaneous fascia or superficial portions of the deep fascia in
mass growing between fascial planes, but STSs can also be infiltra- dogs. These lesions are usually nodular, poorly circumscribed, and
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tive. STSs are often surrounded by a pseudocapsule formed by the very invasive. Histologically, nodular fasciitis is characterized by
compression of peritumoral connective tissue, which may contain large plump or spindle-shaped fibroblasts in a stromal network
or be confluent with neoplastic tissue. 12,13 Overall, cutaneous and of variable amounts of collagen and reticular fibers with scattered
subcutaneous STSs have a low to moderate local recurrence rate lymphocytes, plasma cells, and macrophages. The morphologic
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after surgical excision, with or without adjuvant radiation therapy and pathologic characteristics of nodular fasciitis can result in
(RT), and a low metastatic rate. The likelihood of local recurrence these lesions being misdiagnosed as FSA. Infantile desmoid-type

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