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 The punch biopsy instrument is then withdrawn from the mass to expose the tissue sample.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.
Excisional Biopsy
The approach to an excisional biopsy is variable based on location, goal of surgery, and predetermined adjuvant therapy. An excisional biopsy has the advantage of being both a diagnostic technique as well as a treatment modality. A great deal of caution should be exercised in cases where the diagnosis is unclear. At a minimum, an FNA should be obtained to discern if a given mass is inflammatory or neoplastic and, if neoplastic, whether benign or malignant. This information is imperative in order to determine surgical dose.
There are cases where an excisional biopsy may be a reasonable option, if doubt or absence of knowledge of the tumor type remains after fine needle aspiration (e.g. nondiagnostic results from cytology), depending on the size and location of the tumor. In these instances, the surgeon must contemplate if an excisional biopsy will compromise the ability to enact a cure by wide excision. If it is deemed that an excisional biopsy can be performed while leaving this option, an excisional biopsy can be considered. For example, a 1 cm in diameter mass on the trunk of a large breed dog can be interrogated by excisional biopsy, whereas a 1 cm in diameter mass on the distal extremity of a dog should be interrogated by incisional biopsy (wedge or punch).
Once an excision is performed, the local anatomy is forever altered, tissue planes both deep and wide to the tumor are invaded, providing an opportunity for the tumor cells to extend and seed deeper and wider into tissues. For this reason, the best chance for complete excision is at the time of the first surgical excision. In order to perform a curative surgery, the surgeon must take the appropriate margin of tissue for the tumor type. In some cases (lipoma), this margin is minimal or even intralesional. In other cases (soft tissue sarcoma), the margin should be more extensive. Unless the tumor type is known at the time of excision, the surgeon may compromise the patient by doing too little or too much surgery.
Specific Biopsy Techniques
Bone Biopsy
The clinician performing the bone biopsy procedure should consider the eventual definitive treatment that is likely to be pursued for each case. The biopsy tract or incision needs to be in a location that can be removed during the definitive treatment. A reactive zone of bone exists in the periphery of most bone tumors, and samples taken from this region are more likely to result in an incorrect diagnosis (Wykes et al. 1985;, Liptak et al. 2004). The surgeon should target the anatomic center of the bony lesion. Two radiographic views of the involved bone should be available during the procedure as this will aid in optimal sampling. The majority of bone biopsies are performed utilizing either a Michele trephine or a Jamshidi needle (Wykes et al. 1985; Powers et al. 1988; Liptak et al. 2004). A trephine instrument provides a large sample and has been associated with 93.8% diagnostic accuracy (Wykes et al. 1985). The disadvantages of the trephine technique include increased likelihood of fracture as compared to other techniques, requirement of a surgical approach, and a more lengthy decalcification time prior to sectioning (Wykes et al. 1985; Ehrhart 1998).
Michele trephines are available in variable diameters. As a small surgical approach is required, a simple surgical pack is needed for the procedure. The biopsy site is clipped free of fur, and the patient is prepared with aseptic technique and draped. A 1–3 cm incision is made over the bony lesion, and the soft tissues are dissected from the surface of the tumor. The trephine is then seated into the tumor using a twisting motion. The trephine is advanced through the cis cortex. An effort should be made to not penetrate both the cis and trans cortex as fracture of the bone is more likely (Liptak et al. 2004). Once the trephine is within the medullary cavity, the trephine is rocked backed and forth to loosen the sample and then removed. A stylet is introduced into the trephine to push the sample out of the trephine onto a gauze square.
The Jamshidi needle technique is considered a less invasive means of obtaining a bone biopsy as compared to a Michele trephine. A small stab incision is necessary to introduce this device and fractures are unlikely. Although a more recent study suggests bone