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 |
Incidence and Predisposing Factors
STS represent 8–15% of all skin and subcutaneous tumors in dogs and 7–18% in cats (Miller et al. 1991; Mukaratirwa et al. 2005; Theilen and Madewell 1979; Dobson et al. 2002). An STS incidence of 142 per 100 000 dogs per year was reported in the UK (Dobson et al. 2002). Boerkamp et al. (2014) estimated an incidence of 114 STS/100.000 Golden retrievers/year in the Netherlands. Median age of dogs affected with STS is reported to be between 10 and 11 years (range, 5–17 years) (Ettinger et al. 2006; McSporran 2009; Liptak and Forest 2013; Bray et al. 2014). Middle to large breed dogs are commonly affected, with Golden retrievers, Setters, Swiss mountain dogs, Rottweiler, Dobermann, and Boxers reported to be at increased risk for developing certain STS (Boerkamp et al. 2014; Grüntzig et al. 2016). Median age at presentation for STS in cats is 8–11 years (range 1–17 years) (Alberdein et al. 2007; Davidson et al. 1997; Dillon et al. 2005).
There are case reports of dogs that developed STS in association with previous trauma, implants, previous injury, and parasitic infection (Spirocera lupi) (Vascellari et al. 2003, 2006; van der Merwe et al. 2008; Rayner et al. 2010).
Clinical Signs
STS commonly present as firm, fixed masses (Liptak and Forrest 2013). STS generally grow slowly, and symptoms are related to the site of involvement and the degree of invasion. They can cause dysfunction of an involved organ or signs can be caused by pressure onto surrounding structures. About 60% of STS are found on the limbs, 35% on the trunk, and in 5% the head is involved (Chase et al. 2009; Liptak and Forrest 2013).
Large or fast‐growing STS can cause skin ulceration. Tumor necrosis can develop due to fast expanding growth (Liptak and Forrest 2013).
Metastasis
Higher histologic grades and mitotic counts are factors associated with increased metastatic potential in STS (Dennis et al. 2011). In the case of metastasis, STS usually spread hematogenously, preferably to the lungs (Dernell et al. 1998; Ehrhart 2005; Ettinger 2003; Liptak and Forest 2013). Regional lymph node metastasis is unusual, except for synovial cell sarcoma and histiocytic sarcoma. Reported overall rate of metastasis in dogs is dependent on tumor grade; up to 13% for grades I and II STS compared to 41–44% for grade III STS. (Baker‐Gabb et al. 2003; Ettinger et al. 2006; Kuntz et al. 1997; Simon et al. 2007). Rate of pulmonary metastasis at initial presentation is 6% for grades I and II and 38% for grade III. The likelihood of having metastasis at initial presentation is higher when the STS has been present for over three months (Villedieu et al. 2021). Reported overall metastatic rate in cats is 14–20% (Davidson et al. 1997; Dillon et al. 2005). Interestingly, metastases can be slow growing and may not affect survival (Dennis et al. 2011).
Diagnostic Strategies and Preoperative Planning
Cytology can support a clinical suspicion of STS but rarely confirms the final diagnosis of the type of STS because they tend not to exfoliate well. A cytological diagnosis of “suspect sarcoma” or “mesenchymal proliferation” is a relatively common finding (Ghisleni et al. 2006; Shelly 2003). A correct diagnosis of STS by cytological examination is generally made in around 60% of cases (Baker‐Gabb et al. 2003). The main reason to perform FNA is to rule out benign or nonneoplastic differential diagnoses including lipoma, seroma, and inflammatory processes, or well‐exfoliating tumors such as lymphoma, histiocytoma, carcinoma, and MCTs.
A generally accepted shortcoming of cytology is the inability to provide details on the exact diagnosis and the biological aggressiveness (i.e. histological grade) that may be important to plan treatment. A recent study found that in cytologically diagnosed STS, cytological scoring could accurately predict histological grade of these tumors in 60% of the cases in dogs and in 85% in cats (Millanta et al. 2020). Cytology can also be used to investigate enlarged regional lymph nodes for metastasis, albeit rare for STSs.
Definitive diagnosis of STS is most accurately achieved by histology. Excisional biopsy is only performed when adequate margins can be obtained. Incisional biopsy is preferred if surgical margins cannot be expected and to facilitate the planning of curative surgery.
Incisional biopsies can be taken with a scalpel, biopsy punches, needle core biopsy instruments, or trephines (Ettinger 2003; Ehrhart 2005; Liptak and Forrest 2013; Bray 2016). It is however important to bear in mind that preoperative incisional biopsies are not always accurate in terms of predicting histological tumor grade. Tumor grade was overestimated in 12% and underestimated in 29% of preoperative incisional biopsies compared to postexcision whole‐tumor specimens of 68 dogs with STS (Perry et al. 2014), stressing the need to always consider all available clinical data such as growth rate and behavior before planning definitive treatment and always perform histological evaluation of the complete excised tumor afterward for definitive diagnosis, grading, and surgical margins evaluation.
Tumor seeding from a biopsy is a reported risk in people (Robertson and Baxter 2011), ranging from 1% for human bone neoplasms up to 22% for mesothelioma. By removing the biopsy site at tumor resection this risk can be controlled.
The presence of distant metastases is an important prognostic indicator, rendering radiographic or CT evaluation of the thorax a routine diagnostic step before treatment.
STS appear to be encapsulated but often show an invasive growth pattern. The macroscopic capsule is in fact a pseudocapsule, composed of compressed tumor cells and reactive fibrovascular tissue. The tumor can infiltrate along and through fascial planes with finger‐like micro‐extensions. Marginal excision will leave microscopic tumor behind, often resulting in local recurrence compromising the optimal treatment plan (Ehrhart 2005). Additionally, measurement of tumor boundaries by physical examination commonly underestimates the actual tumor dimensions (McEntee and Samii 2000); therefore, advanced imaging techniques are recommended for surgical planning.
Contrast‐enhanced CT and magnetic resonance imaging (MRI) are useful for surgical planning and identification of metastatic disease. Considering the limited value of manual determination of subcutaneous tumor size (Ranganathan et al. 2018), advanced imaging is often necessary to properly evaluate tumor extension in surrounding tissues of large STS, STS at locations where wide excision is difficult due to proximity of critical tissue structures such as in the head and neck, or an STS within a body cavity. In humans, MRI is the preferred modality for assessment of STS in limbs, delineating muscle groups, and separating tumor from adjacent vascular, muscular,