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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, and bony structures, while contrast‐enhanced CT is indicated in cases of (suspected) bone involvement, intra‐abdominal or retroperitoneal tumors, and for screening of pulmonary metastasis (Hanna and Fletcher 1995; López‐Pousa et al. 2016; Fuerst et al. 2017; Spoldi et al. 2017; Ranganathan et al. 2018).