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why and when is a biopsy necessary?
Our success in treating diseases in people and their pets is dependent in large part on our ability to correctly diagnose and stage such diseases. Here, we will review why and when a biopsy may be indicated to diagnose and stage tumors (cancer), but the same principles apply to many other diseases.
First, we should answer the question, what is a biopsy? According to the definition provided by the National Cancer Institute, a biopsy (By-op-see) is the removal of cells or tissues (from a growth, organ, or lesion) for examination by a pathologist. The pathologist may study the tissue under a microscope or perform other tests on the cells or tissue.
There are several different types of biopsies that can be performed: an incisional biopsy is when only a sample of tissue is removed; when an entire lump or suspicious area is removed, the procedure is called an excisional biopsy. Finally, when a sample of tissue or fluid is removed with a needle, the procedure is called a needle biopsy, core biopsy, or fine-needle aspirate. Any of these types of samples may provide the pathologist sufficient information to make a diagnosis of cancer. However, the rule “more is better” generally applies when it is important to accurately classify and stage that cancer.
Generally, physicians and veterinarians follow a deliberate, systematic process to diagnose an ailment. This includes a thorough history and physical exam, followed sequentially by laboratory tests that help eliminate or “rule out” possibilities that can explain the signs and symptoms. In the case of cancer, a high index of suspicion can be achieved using non-invasive tests, such as radiographs, ultrasound, or magnetic resonance imaging, but a definitive diagnosis requires examination of the affected tissues. It is at this point where the pros and cons of each type of biopsy need to be considered.
The least invasive procedure is a fine needle aspirate. For this procedure, the patient is usually awake or only lightly sedated, a needle is inserted into the abnormal tissue, and negative pressure is applied with a syringe to aspirate cells into the needle and the syringe hub. The cells that were aspirated can then be placed on a glass slide for microscopic examination (cytopathology), or they can be placed in a preservative for other tests. The advantages of the procedure are that it is minimally invasive and generally very safe, it is less costly than an incisional or an excisional biopsy, and the results can be obtained rapidly, especially because the attending physician or veterinarian can provide a preliminary assessment of the microscopic appearance of the sample. The disadvantages are that the tissue architecture is destroyed, and so the information that can be gleaned from the sample is limited. Furthermore, extensive training and experience are necessary to be able to provide a definitive cytopathology interpretation from fine needle aspirates, and even the most experienced pathologists can sometimes only provide a tentative diagnosis from aspirate samples that do not have the benefit of a “tissue context”. Finally, the amount of sample is limiting, and so additional tests require additional samples. In general, a fine needle aspirate is a suitable first-line approach to narrow the list of differential diagnoses, although in many cases, it can provide all the information needed to formulate a treatment or management plan.
Incisional biopsies are, by nature, more invasive than fine needle aspirates. However, contemporary tools allow biopsies to be obtained safely and at reasonable cost. Often, a “tru-cut” biopsy tool, which is similar to a needle with a larger gauge or channel, can be used in a manner that is only slightly more invasive than a fine needle aspirate. This larger needle is inserted into the tissue, and as it penetrates, it also “cuts”, allowing a small core to remain embedded within the needle channel. This core of tissue is then removed from the needle and can be used to make imprints for cytopathology, and it can be placed in a preservative for additional tests. Most often, the preservative is formalin, which is used to “fix” the tissue to prevent changes in its shape or form. The formalin-fixed tissues are then embedded in paraffin wax for processing and storage. The most common test done by a pathologist is examination of formalin-fixed and paraffin embedded tissue that has been subjected to several additional processing and staining steps under the microscope. The advantages of an incisional biopsy are that there is more tissue available for analysis, and the architecture, or the context of how cells relate to each other, are preserved. In addition, the preservation of the tissue in a paraffin block allows the pathologist to perform additional tests without the need to obtain additional samples. This provides a more complete picture to accurately classify and stage the disease. Because tissue imprints can be obtained from these samples, a provisional diagnosis can also be obtained. The disadvantages are that it is more invasive (it usually requires at least a small “nick” in the skin to allow the needle to be placed into the tissue), it requires heavy sedation or general anesthesia, it takes longer to process and read, and consequently, it is more costly. In some cases, it is necessary to remove a large piece of tissue (“wedge biopsy”) in order to provide the pathologist sufficient material to reach a diagnosis.
Excisional biopsies are done when there is an opportunity to remove the whole tumor without significantly added risk to the patient.
To read about how biopsy samples are used in cancer studies, click here
This document was prepared in collaboration with the Briard Medical Trust
University of Minnesota College of Veterinary Medicine | University of Minnesota Cancer Center
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