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Soft Tissue Sarcomas

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STS are a heterogenous group of tumors that originate from connective tissues surrounding, supporting, and bridging anatomical structures or tissues. STS have similar biological behaviors, often displaying both benign and malignant characteristics. Although skin and subcutaneous tumors are the most commonly observed STS, these sarcomas can, in principle, arise from any part of the body (Ehrhart 2005; Ettinger 2003; Kuntz et al. 1997). In general, STS are slow‐growing and locally invasive tumors, composed mainly of spindle‐shaped cells, with a low tendency for metastatic spread. STS are grouped together because of their comparable biological behavior and clinical characteristics while further histologic classification and differentiation are often complicated. The nomenclature follows classification of human STS, based on patterns of cellular proliferation and individual cell morphology without conclusive identification of the cells of origin and is poorly standardized for animals. Some pathologists, therefore, prefer the term spindle cell tumors of canine soft tissue (Williamson and Middleton 1998). Further differentiation of histologic diagnosis can be achieved using immunohistochemistry (Gaitero et al. 2008; Ettinger et al. 2006; Liptak and Forrest 2013). Canine STS display histological and immunohistochemical features similar to their human equivalents (Milovancev et al. 2015).

Tumors typically included in the STS group are FSA, perivascular wall tumors (previously called hemangiopericytoma), liposarcoma, malignant fibrous histiocytoma, mesenchymoma, myxosarcoma, nonplexus derived PNSTs (previously called neurofibrosarcoma or schwannoma), and undifferentiated sarcoma.

There are several mesenchymal tumors that are not considered “soft tissue sarcomas” because their individual biological behavior has a more defined character and they can usually be identified on light microscopy, including hemangiosarcoma, synovial cell sarcoma, gastrointestinal stromal tumors (GISTs), oral FSA, and PNSTs. Rhabdomyosarcoma, lymphangiosarcoma, and leiomyosarcoma are included in this exclusion list by some pathologists, whereas others would group them with STS (Ehrhart 2005; Dennis et al. 2011; Liptak and Forrest 2013). Histiocytic sarcoma is also considered not a typical STS because for one it is not arising from connective tissue and also because it has a differing biologic behavior.

The discussion pertaining to the exact histologic differentiation is not one of major clinical importance because the overall biological behavior of STS is similar. Several important features of biological behavior that are common to all STS are listed in Table 4.3.

Table 4.3 Common features of STS (as described by Liptak and Forrest 2013).

1) An ability to arise from any anatomical site in the body
2) STS often have a pseudo‐capsule: these tumors seem to be encapsulated macroscopically while histologically tumor margins are usually poorly defined
3) A tendency to infiltrate along and through fascial planes
4) Local recurrence is common after conservative excision
5) Metastasis through hematogenous route; most common site of metastasis is the lungs
6) A poor response to chemotherapy and radiation therapy in cases where gross tumor is present
Veterinary Surgical Oncology

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