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.
Biopsy
Clinicians often use the term “biopsy” as a nonspecific description of obtaining a tissue sample for histopathologic interpretation. Because of this, two major categories of biopsy have been designated: pretreatment biopsy (tissue obtained before treatment initiation) or posttreatment biopsy (tissue obtained at the time of definitive tumor resection). All biopsy procedures, whether pretreatment or posttreatment, should be carefully planned with several factors in mind. These factors include known patient comorbidities, anatomic location of the mass, differential diagnoses, biopsy technique, eventual definitive treatment, and any neoadjuvant/adjuvant therapies that may need to be incorporated.
Pretreatment Biopsy
Needle Core Biopsy
This technique is commonly used for soft tissue, visceral, and thoracic masses (Osborne et al. 1974; Atwater et al. 1994; deRycke et al. 1999). Image guidance is recommended when using this technique in closed body cavities. Most patients require sedation and local anesthesia but may not need general anesthesia.
Instrumentation includes a needle core biopsy instrument (automated or manual) (Figure 1.3), #11 scalpel blade, local anesthetic, and a 22 g hypodermic needle. To perform the procedure, the area surrounding the mass is clipped free of fur and prepared with aseptic technique. If intact skin is to be penetrated and the animal is not anesthetized, the skin overlying the area to be penetrated is anesthetized with lidocaine or bupivacaine. A 1–2 mm stab incision is made over the mass to allow for placement of the needle core biopsy instrument. The instrument is oriented properly and fired, and the instrument is withdrawn. The 22 g needle can be used to gently remove the biopsy from the trough of the needle core instrument. This identical procedure is performed for masses within a body cavity; however, it is necessary to use image guidance (most commonly ultrasound) for proper placement of the instrument within the desired tissue. Imaging can be used to determine the depth of penetration and to safely avoid nearby vital structures.
Punch Biopsy
This technique is most effective for cutaneous lesions as well as intraoperatively for biopsies of masses within organs such as the liver, spleen, and kidney. Subcutaneous lesions can be biopsied using this method, but it is best to incise the skin overlying the mass and then obtain the sample using the biopsy instrument.
Figure 1.3 (a) Automated needle core biopsy instrument. (b) The tip of the needle has an indentation, which is filled with the tumor tissue when inserted. There is a sleeve with a cutting edge (red arrow), which cuts the piece of tissue in the indentation of the needle.
Instrumentation includes a punch biopsy instrument (Figure 1.4), which typically comes in sizes of 2, 4, 6, and 8 mm; #11 scalpel blade; local anesthetic; Metzenbaum scissors; forceps; and suture. The area containing the mass is clipped free of fur and prepared with aseptic technique. If intact skin will be penetrated and the animal is not anesthetized, the skin overlying the lesion is anesthetized with lidocaine or bupivacaine. For cutaneous masses, an incision is not necessary. For subcutaneous masses, make an incision in the skin over the mass and dissect tissues overlying the mass if present to allow for the procurement of a better sample. The skin incision should be large enough for the punch biopsy instrument to be placed and allow it to be twisted without engaging skin. Twist the punch biopsy instrument until the device is embedded into the mass to the hub. The punch biopsy instrument is then withdrawn from the mass to expose the tissue sample. Gently grasp the sample with forceps, utilize Metzenbaum scissors to sever the deep aspect of the sample from the rest of the tissue, and remove the sample. A single suture is generally sufficient to close the incision. The same procedure can be performed on visceral organs.
Incisional (Wedge) Biopsy
This technique is effective for masses in all locations and generates a larger sample for histopathologic evaluation as compared to the needle core biopsy. The location of the incision should be carefully planned, as the biopsy incision will need to be removed during the definitive treatment. Care should be taken to avoid dissection and prevent hematoma or seroma formation as these may potentially seed tumor cells into the adjacent subcutaneous space. Although the junction of normal and abnormal tissue is often mentioned as the ideal place to obtain a biopsy sample, one should take care to avoid entering uninvolved tissues. The most important principle to consider is to obtain a representative sample of the mass. It is also important to obtain a sample that is deep enough and contains the actual tumor, rather than just the fibrous capsule surrounding the mass. Incisional biopsy has a higher potential for complications such as bleeding, swelling, and infection due to the increase in incision size and dissection.
Figure 1.4 Punch biopsy instrument, 8 mm in diameter.
Instrumentation includes a scalpel blade, local anesthetic, Metzenbaum scissors, forceps, suture, and hemostats. A Gelpi retractor or similar self‐retaining retractor aids in visualization if the mass is covered by skin. If the skin is intact and moveable over the mass, a single incision is made in the skin. Once the tissue layer containing the tumor is exposed, two incisions made in a parallel direction are started superficially and then meet at a deep location to form a wedge. The wedge is then grasped with forceps and removed. If the deep margin of the wedge is still attached, the Metzenbaum scissors can be used to sever the biopsy sample free of the parent tumor. The wedge site is then closed with a suture.
