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Needle Aspiration vs. Biopsy for Diagnosing Canine Tumors

Updated: Nov 28, 2023



When it comes to tumors in dogs, one of the most important things to do is figure out exactly what you're dealing with. While an experienced vet can make educated guesses, the truth is that it's essentially impossible to fully diagnose a tumor through means like examination, palpation, and simple imaging.


The only way to know what a tumor is is to perform a test on the cells themselves. That means extracting cells from the tumor and examining them using histopathology techniques.


There are two ways to harvest those cells: a traditional biopsy and a fine needle aspiration. Which is better, which should you use, and what do you need to know?


Correcting Terminology


First and foremost, it's worth taking a moment to clear up a common misconception. While we often refer to these two tests as a needle aspiration and a biopsy, that's not actually accurate. The reality is that both procedures are kinds of biopsies and actually work in very similar ways: using a needle to take a sample of the tissue of a tumor. There are other kinds of biopsies as well, such as those used when taking samples of skin cancers.


The two kinds of biopsies using needles are, appropriately enough, called needle biopsies. The two we're primarily concerned with are FNA and CNB.


FNA stands for Fine Needle Aspiration. This kind of biopsy uses a single small needle guided into a tumor and a syringe to withdraw fluids and small samples of cells from the tumor. This sample is then examined.


CNB stands for Core Needle Biopsy. These work in essentially exactly the same way: using a needle inserted into a tumor to withdraw a sample. The difference is that a core needle biopsy uses a larger needle and excises a larger, contiguous sample of cells.



There are a variety of other kinds of biopsies and a handful of other kinds of cell samples that can be used for cytology and histopathology. These include:

  • Endoscopic biopsies, where an endoscope is inserted into the body and used to collect larger samples of a tumor. These are often used for taking samples of lesions in the intestines, esophagus, sinuses, and similar bodily cavities.

  • Laparoscopic biopsies, which use small incisions to look inside the belly area (and similar biopsies for the chest and lungs, known as thoracoscopic and mediastinoscopic.)

  • Open biopsies, which are surgical removals of part or all of a tumor to examine it; these are used on small tumors that would be difficult to take a sample of without nabbing the whole thing or where the tumor is generally removable without other issues.

  • Skin shaves, which take surface layers of skin to diagnose skin lesions.

  • Punch biopsies, which take deeper layers of skin to diagnose skin lesions.

  • Liquid biopsies, which take samples of fluids like blood to check for relevant disease or the signs of disease elsewhere.

For the most part, we're just focusing on the two kinds of needle biopsies in today's post.


The Benefits and Drawbacks of Fine Needle Aspiration


Fine needle aspiration biopsies have a few benefits and a few drawbacks. They aren't perfect for every situation; after all, if they were, we wouldn't use anything else.


First and foremost, the fact that the needle used for the sampling is very small means that, often, little or no anesthesia is necessary to take a sample. If you've ever gotten a vaccine shot done well enough that you don't even feel it, that's the kind of needle being used for fine needle aspiration.


Another benefit of the small needle is that there is virtually no risk of complications. It's considered minimally invasive, with basically no risk of anything other than minor irritation in almost all cases. While any penetration of the skin barrier – especially for deeper tumors – can carry some risks, the smaller the needle, the fewer the risks.



The sample taken with a fine needle aspiration can be immediately analyzed if your practice is equipped with the appropriate kinds of tools on hand. Even a simple microscope examination can give some idea of what the tumor is.


As far as drawbacks are concerned, there are generally two when it comes to a fine needle aspiration.


First, the smaller needle can sometimes have difficulty gathering enough material and providing enough cells for diagnostic imaging to be conclusive. Certain kinds of tumors are filled with fluids or blood, which can fill the syringe and leave little or no room for the cells you want to sample to examine. This means that, for certain lesions and certain cancers, fine needle aspiration can be nondiagnostic. In particular, sarcomas of various types are difficult to confirm with fine needle aspiration because the cells in a sarcoma don't separate from the tumor readily.


"Fine needle aspiration is one of the most effective diagnostic and biopsy methods available today. It is also one of the least invasive diagnostic procedures and causes virtually no pain for pets. In fact, the needle used for aspiration is typically thinner than needles used for vaccination. However, fine needle aspiration is not 100% effective. The effectiveness of fine needle aspiration biopsy is 70% to 80%." - Wag.

If a cytological sample proves inconclusive, repeating the cytological sampling might be an option. Occasionally, the quality of the sample preparation can vary, and a repeat procedure with careful handling might yield a more definitive result. Should histopathology be necessary, a core needle biopsy is not the sole method available.


Although core needle biopsy, also known as tru-cut, is one option, it is infrequently my preference. Alternatives like punch biopsy or surgical wedge resection are often more suitable and are methods I tend to favor.


The Benefits and Drawbacks of Core Needle Biopsies


Core needle biopsies use a larger needle to take larger samples. Since the needle is larger, though, it has all of the drawbacks of a more invasive procedure.

  • It generally requires anesthesia, depending on the location of the tumor.

  • The larger hole left by the needle has a higher (though still minimal) risk of infection or other complications.

  • The larger sample can be more painful and take longer to heal than a fine needle aspiration.

The larger sample, however, provides one notable benefit: a greater likelihood of diagnosis from the sample. Larger needles taking core samples of tumors provide better samples, particularly of the kinds of tumors that resist diagnosis from fine needle aspirations.



Core needle biopsies will require some amount of protection at the site of the biopsy. That will likely mean the patient needs an Elizabethan collar to prevent them from licking or chewing at it until it heals. In the case of deeper biopsies, particularly of the liver, excessive bleeding is a primary risk, though it's not frequent in most cases.


Are There Drawbacks to Biopsies?


No.


At least, not such that it should dissuade you from performing them.


Biopsies, particularly core needle biopsies, do have a small risk of complications. These can range from infection to the rare risk of more serious complications, particularly in lung biopsies. Otherwise, the greatest risk comes from anesthesia, not the procedure itself.


It may seem strange, but I typically don't recommend a bone biopsy for limb bone cancers. You might ask why. It's because these particular tumors often show up clearly on X-rays, making a biopsy often surplus to requirement. There is complexity in bone tumors as they are an amalgam of different cells, including tumor cells, cartilaginous cells, inflammatory cells, and normal cells as well.


This means that a small biopsy sample might not give a true reflection of the whole tumor and could result in inaccurate diagnoses. It's important in these cases to analyze the entire tumor to get the most accurate diagnosis possible. While this applies to limb bone cancers, it isn't always applicable with other types of lesions. In light of this, skipping a biopsy each time could prevent us from detecting key diagnostic information. There's also the possibility that it might slow down the patient getting the potentially necessary treatment they need.


Note: Limb-sparing treatments exist, including new treatments gaining steam in the veterinary community. For a full rundown of our analysis of these options, check out this article.



Without proper information, you can't make informed decisions, so your outcomes are going to be worse. Fortunately, biopsies are generally highly effective. Moreover, both FNA and CNB are complementary; they work together to diagnose different kinds of tumors from different approaches and minimize the chances of error and misdiagnosis. There's a growing push to perform both kinds of biopsy at the same time for this very reason.


Some people may worry about "tumor seeding" or the penetration of a tumor, causing it to "leak" or spread cells that then spread the cancer. While there are very rare confirmed cases of this happening, the incidence rate is extremely low, and much of the information about it happening came from a doctor who has since been discredited and whose license was revoked. It shouldn't be a concern.


Which Method of Biopsy is Better?


The truth is that both kinds of biopsy are good, and they cover the bases that the other lacks. For that reason, it can be highly beneficial to perform both kinds of biopsy at the same time, to have both kinds of samples to perform both cytology and histopathological testing on.


Getting a cytological examination done by a vet is typically fast and easy, yet often requires another review by a clinical pathologist to give a final diagnosis. This extra step usually requires an additional one to two days. We only resort to a biopsy if the cytological examination results remain unclear. The time it takes to get biopsy results may differ, usually falling somewhere between three to ten days.



Looking for a second opinion? At Hope Vet, one of our primary services is to take a look at the histology report you get from a sample and help offer our opinions on it. Our experts are well-versed in all sorts of canine cancers, both common and uncommon, and will get back to you with our report within 72 hours. If you're unsure of the diagnosis or just want confirmation, we're happy to help. Just reach out to get started or ask us any questions you may have.


Once you know what kind of tumor you're dealing with – and in particular, if it's cancerous, and if so, what kind of cancer – you can proceed to treatment options. Treatments, of course, can range from surgery to chemotherapy to novel immune treatments, with more being developed every year.


Does Every Lump Need a Biopsy?


While you can use your judgment, the answer is generally going to be yes. Almost always, a suspicious lump, lesion, or tumor should be biopsied. The opportunity cost for a fine needle aspiration – that is, the cost in terms of risk, healing, anesthesia, and so on – is virtually nonexistent. Even the actual costs come down to whatever your practice charges for an office visit. A fine needle aspiration generally runs between $25 and $200, making it an inexpensive way to diagnose many lesions.


This is especially important for dogs for two reasons. The first is that the range of possible treatments for most canine cancers, while growing over the years, is still relatively small. There are only so many different treatment options before the potential avenues are exhausted. Secondly, a key element to treatment is catching malignant disease as early as possible. Aspirating a lump when it's first discovered and small has a much greater chance of catching something early enough to address it before it becomes a life-threatening problem.



Core needle biopsies have a larger opportunity cost, and that's where the judgment aspect comes into play. You may feel that a lump is less likely to be dangerous or that it's slow-growing enough that you can wait. You may also be examining a patient where the risks of anesthesia are high enough that a more invasive procedure isn't viable. These are all case-by-case details, however, and in most cases, a biopsy is still better than not.


How We Can Help


At Hope Vet, we're a team of dedicated veterinary specialists. Our goal is to provide you with additional resources and options for diagnosis, analysis, and second opinions. Whether you have tricky internal medicine cases, canine cancers, dermatological issues, or other problems in your patients, it's not uncommon to face challenges outside of your specialties. That's where we come in. By reaching out and requesting a consult from our team, you can get our expert opinions in just a few days, helping you provide better, more detailed care for your patients.



All you need to get started is to reach out and request a consultation or drop us a line with any questions. We're here for you so you can be there for your patients.

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