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Fine needle aspiration is often the most minimally invasive technique for obtaining critical information about a newly identified mass prior to surgery. The accuracy of a FNA is dependent on many factors including the tumor type, location, and amount of inflammation. Overall sensitivity and specificity of cytology have been reported to be 89% and 100%, respectively (Eich et al. 2000; Cohen et al. 2003). Imaging tools such as ultrasound and fluoroscopy can increase the chance of obtaining a diagnostic sample.

In most patients, an FNA of cutaneous or subcutaneous lesions can be obtained with no sedation and a minimal amount of discomfort. Fine needle aspiration has been compared to histopathologic samples in several studies. In one study of the correlation between cytology generated from fine needle aspiration and histopathology in cutaneous and subcutaneous masses, the diagnosis was in agreement in close to 91% of cases (Ghisleni et al. 2006). Cytology was 89% sensitive and 98% specific for diagnosing neoplasia, and these numbers varied slightly based on tumor type (Ghisleni et al. 2006). For example, both the sensitivity and specificity were 100% for mast cell tumors (Ghisleni et al. 2006). In one study looking at the accuracy of cytology of lymph nodes in dogs and cats, cytology had a sensitivity of 67%, specificity of 92%, and accuracy of 77% for a diagnosis of neoplasia (Ku et al. 2017). In that study, 31% of metastatic lymph nodes secondary to a mast cell tumor were falsely negative (Ku et al. 2017). In another study evaluating the value of cytology of lymph nodes to detect metastasis of solid tumors, the sensitivity of needle aspirates of the lymph node was 67% for sarcomas, 100% for carcinomas, 63% for melanomas, 75% for mast cell tumors, and 100% for other round cell tumors. The specificity varied between 83 and 96%; also, 20% of nondiagnostic samples were metastatic (Fournier et al. 2018).

Table 1.1 Factors affecting the goal of surgery and consequently the dose of surgery

Tumor type	stage	Size	location	Owner’s goals	prognosis	Overall health of patient	Goal of surgery	Dose of surgery
Benign								Marginal
Malignant	Metastasis present						Palliative	Marginal
No metastasis	Small	Trunk		Good to excellent	Good	Curative	Wide
Significant	Limb	Owners accept amputation	Good to excellent for function	Good	Curative	Radical
Significant	Limb	Owners refuse amputation but accept surgeries with higher morbidity and risks	Good for local control and long term survival	Good	Curative	Wide (with reconstructive surgery)
Significant	Limb	Owners refuse amputation or surgeries with higher morbidity and risks	Good with adjuvant therapy	Good	Cytoreductive	Marginal
				Significant co‐morbidities	Palliative	Marginal

The goal of fine needle aspiration is to differentiate between an inflammatory or neoplastic process and, if neoplastic, whether the tumor is benign or malignant. In some cases, the specific tumor type can be determined (e.g. mast cell tumor). In other cases, the class of tumor may be identified (e.g. sarcoma), but the specific diagnosis requires histopathology (e.g. chondrosarcoma versus osteosarcoma). The overall purpose of obtaining the FNA is to guide the staging diagnostics (where to look for metastasis or paraneoplastic diseases) and surgical dose. For example, an FNA of a mass showing normal adipocytes would indicate the mass is not inflammatory, rather it is a neoplastic process and it is benign (lipoma). Based on the knowledge of the biologic behavior of this tumor, no other staging tests would be performed and minimal surgical dose would be prescribed (marginal resection). Alternatively, if the FNA of a mass indicated carcinoma cells, more advanced staging (three‐view thoracic radiographs, abdominal ultrasound and/or thoracic and abdominal CT, lymph node aspirates) would be indicated and a larger surgical dose would be prescribed.

Fine needle aspiration of internal organs can also be performed and may be helpful in guiding diagnostic and treatment choices. Image guidance should be utilized when obtaining FNAs of masses within a body cavity. Aspirates of lung and other thoracic organs can be performed safely in most cases. In one study, fine needle aspiration of lung masses had a sensitivity of 77% and a specificity of 100% (DeBerry et al. 2002). The aspiration of cranial mediastinal masses is beneficial, as thymomas can be diagnosed by cytology (Rae et al. 1989; Atwater et al. 1994; Lana et al. 2006). Cytologic diagnosis of thymoma requires the presence of a population of unequivocal malignant epithelial cells. The presence of mast cells is also common in thymoma and often supports the diagnosis (Atwater et al. 1994). Flow cytometry is another diagnostic tool that will differentiate thymoma from lymphoma using an FNA sample. Thymomas will contain both CD4+ and CD8+ lymphocytes, whereas lymphoma would typically contain a clonal expansion of one lymphocyte type (Lana et al. 2006).

Fine needle aspiration of hepatic and splenic neoplasia has been described in several studies (Osborne et al. 1974; Hanson et al. 2001; Roth 2001; Wang et al. 2004). Successful diagnosis of hepatic neoplasia with fine needle aspiration is variable. A study has reported diagnostic rates for liver cytology of multiple pathologies (including neoplasia) as high as 80% (Roth 2001); however, another study demonstrated less success with diagnostic rates of 14% in dogs and 33% in cats for fine needle aspiration of hepatic neoplasia (Wang et al. 2004). In cases of suspected splenic hemangiosarcoma, fine needle aspiration is generally not recommended, as an accurate diagnosis is unlikely due to the abundance of blood‐filled cavities. Additionally, complications may include severe bleeding from the aspiration site. Fine needle aspiration of splenic neoplasia such as lymphoma and mast cell tumors is often diagnostic (Hanson et al. 2001).

Other tumors in which fine needle aspiration has been utilized to obtain diagnostic information include gastrointestinal tumors and bony tumors. The accuracy of fine needle aspiration in the diagnosis of gastrointestinal neoplasia is often dependent on the type of neoplasia present. For instance, fine needle aspiration of gastrointestinal lymphoma tends to have a higher sensitivity than aspiration of gastrointestinal carcinoma/adenocarcinoma or leiomyoma/leiomyosarcoma (Bonfanti et al. 2006). The specificity of the diagnosis is similar among these neoplastic diseases with fine needle aspiration (Bonfanti et al. 2006). In one study, ultrasound‐guided fine needle aspiration of osteosarcoma lesions was found to have a sensitivity of 97% and specificity of 100% for the diagnosis of a sarcoma (Britt et al. 2007). Another study found that cytology after fine needle aspiration agreed with incisional and excisional biopsies of bony lesions in 71% of cases (Berzina et al. 2008). In a more recent study, histology of a bone lesion was superior to cytology (Sabattini et al. 2017). Histology of a biopsy had a sensitivity of 72%, specificity of 100%, and accuracy of 82%, whereas cytology had a sensitivity of 83%, specificity of 80%, and accuracy of 83% (Sabattini et al. 2017).

As with any procedure, FNAs are not without risk. In certain cases, bleeding or fluid leakage can be problematic, especially within a closed body cavity where it cannot be easily controlled. Tumor seeding and implantation along the needle tract is a rare occurrence, but in certain tumors has been reported more frequently. Localized tumor implantation following ultrasound‐guided FNA of transitional cell carcinoma of the bladder has been reported (Nyland et al. 2002) and should be a consideration when deciding on methods for diagnosing bladder masses. Fine needle aspiration of mast cell tumors brings the risk to cause degranulation, and clinicians should be prepared to treat untoward systemic effects following aspiration of a suspicious or known mast cell tumor. Despite the risks associated with needle aspiration, it remains an effective, inexpensive, and valuable tool in the preoperative planning process.