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Featuring Demian Dressler, DVM and Sue Ettinger, DVM, Dip. ACVIM (Oncology), authors of The Dog Cancer Survival Guide

How Do You Know If Your Dog Has Cancer for Sure? Read Chapter 9: How We Diagnose and Stage Cancer

If you’ve found a lump on your dog, you’re right to be concerned. But no one can tell for sure if a lump is cancer just by looking at it or feeling it. How do you know if your dog has cancer? There’s only one way: test for it.

Some dog cancer tests are simple, painless, and inexpensive. Others are more complex and costly. What tests you need, and how many you need, totally depends upon your dog’s specific case.

In chapter 9 of The Dog Cancer Survival Guide, Dr. Demian Dressler and Dr. Susan Ettinger, his veterinary oncologist co-author, explain the ins-and-outs of each test your dog might get on the way to a diagnosis. We’ve excerpted that chapter below, to help you prepare for your next veterinary visit.

You can get the full book, The Dog Cancer Survival Guide, anywhere books are sold or in our own online shop.

Chapter 9: How Do You Know If Your Dog Has Cancer? How We Diagnose and Stage Cancer

Excerpted from The Dog Cancer Survival Guide: Full Spectrum Treatments to Optimize Your Dog’s Life Quality and Longevity, by Dr. Demian Dressler, DVM, with Susan Ettinger, DVM, Dip. ACVIM (Oncology)

Even the most experienced and skilled oncologist cannot tell you for sure whether a lump is cancerous, just by looking at it or feeling it. To get a diagnosis (determine if cancer is present) we must run at least one test.

Depending upon the diagnosis, we may also need to look for evidence that it has spread, because sometimes it has by the time of diagnosis. What we find will determine the stage of the cancer.

To get a diagnosis of cancer, we need at least one test. To get the stage of cancer, we may need more tests to see if it has spread.

Depending upon the cancer type, several different tests may be used to diagnose and stage your dog’s cancer. Please see Dr. Ettinger’s section of this book for details on the typical diagnosis and staging tests for the most common dog cancers.

Diagnosing Cancer

In order to determine whether a mass is cancerous, we have to run at least one test. A fine needle aspirate may confirm a tumor, and a biopsy is usually necessary to get an accurate diagnosis and enough information to plan treatments.

Fine Needle Aspirate

The first test for malignancy is usually the fine needle aspirate. The vet inserts a very thin needle into the tumor, to draw up a sample of the cells and fluid inside. If the aspirate is taken from an internal organ, the surface area might need to be shaved and disinfected first. The procedure is often done without sedation or anesthesia because it is usually only slightly uncomfortable for the dog. The entire procedure takes about two minutes, and the needle is only in the dog for a fraction of that time.

Fine needle aspirates are done without anesthesia, and take just two minutes to complete.

The vet cannot tell, just by looking at the sample with the naked eye, what kind of cells are inside, so the fluid sample is put on a slide and examined under a microscope by the vet or a cytologist (someone who studies cells). The sample can often confirm that, yes, cancer cells are present (and sometimes what kind) or, no, cancer is not present, at least not in that sample. The sample might also reveal signs of infection or inflammation.

Lymphoma, mast cell tumors, histiocytomas, and cysts are usually easy to diagnose with a simple fine needle aspirate. Other growths may be harder to diagnose this way because some tumor cells clump together so tightly that the vet can’t create enough vacuum inside the needle to pull them apart and up.

In general, fine needle aspirates are most accurate for “fluid-filled” tumors, less accurate for hard tumors.

Other tumors, especially some connective tissue tumors, don’t shed cells easily, so the sample contains only fluid, and no cancer cells. If there are no tumor cells in the sample that does not mean that the tumor is not cancerous. In these cases, the cytology report will label the test inconclusive, and another method will have to be used to diagnose the cancer.

A fine needle aspirate cannot be used to stage cancer (tell how far it has spread) or grade cancer (predict how aggressive it is) with precision. It only shows whether cancer is present in the collected specimen. This is why we so often need to do a biopsy.

Aspirates can only tell us if cancer is present in that little specimen. We might also need a biopsy to get more information about the tumor.


A biopsy is a surgery, typically a small one, which removes tumor tissue for examination by a pathologist. It is more definitive than a fine needle aspirate and can show the type of cancer and its grade (how aggressive it is). All biopsies are done under anesthesia or heavy sedation with pain control, so that the dog cannot feel the surgery.

All biopsies are mini-surgeries, done under anesthesia or heavy sedation plus pain control.

Most biopsies are incisional, which means they don’t remove the entire tumor and some tumor cells are knowingly left in the body. There are several tools that can be used, depending upon the nature and the location of the tumor. These include a scalpel, which takes a wedge-shaped sample, or a core punch, which takes a disk-shaped sample.

Occasionally, an excisional biopsy is done, which is when the entire visible tumor is removed (with a scalpel).

If a tumor is deep in the body (for example, in the liver), a wide needle might be used to do the biopsy. If an even deeper biopsy is needed – for example, a bone sample – a very long core biopsy needle is used. Imaging tools, like ultrasound, are often used to help guide the needle during fine needle aspirates or biopsies of sites deep in the body.

Internal organs can also be biopsied using an endoscope: a small tube is threaded into the organ, and then a tiny machine with a fiber-optic camera is sent through the tube. The machine also has little arms that can grab and cut tissue samples, which are then pulled back through the tube.

Long needles and/or endoscopes are used to biopsy tumors found deep in the body.

Once the biopsy specimen is removed, it is sent to a pathology lab in a preservative, where it is prepared and embedded in a clear plastic. The specimen is sliced very thin, stained, and then examined under a high-powered microscope. The pathologist looks for several things, including the cancer type and grade. Knowing this can help us decide whether and where to look for cancer spread. It can also help us to plan how wide and deep an incision
to make during a curative surgery.

Your veterinarian cannot examine the biopsy. Specially-trained pathologists do that in a separate lab.

Because of all of the steps involved, it usually takes five to ten days to receive your biopsy report after the actual surgery is performed. As you may have already experienced, it feels like ten years.

Despite the expense, the anesthesia, the recovery time, and the stress associated with a biopsy, it is a good idea to get one. Without it, you and your team are in the dark about the true nature of your dog’s specific cancer case, or whether your dog even has cancer.

Without a biopsy, it is hard to tailor a treatment plan.

Get a Biopsy after a Curative Surgery

In addition to their usefulness as an initial diagnostic tool, biopsies are also done after curative surgeries. In these cases, the removed tumor will be sent to the pathology lab and examined along with the margins, or edges, to see if there are cancer cells present. This is called a comprehensive margin evaluation. This type of examination can help a vet to know if another surgery or another treatment is in order, because, although some tumors seem very distinct from the surrounding tissue, they may have microscopic cancer cells spreading out in a halo. If the pathologist doesn’t find any microscopic tumor cells, he pronounces the margins “clean” or “complete.”

A comprehensive margin evaluation is done to see if the surgery was “complete.”

Clean margins in a sample may mean that the cancer is cured or unlikely to recur. If the pathologist finds cancer cells on the border of the removed tissue, he pronounces them “dirty” or “incomplete,” because it is very likely that cancer cells are left in the dog, too. In these cases, your vet or oncologist may suggest another surgery or some other treatment to target the remaining cancer cells.

Staging Cancer

Once a diagnosis is made, your vet or oncologist will probably want to stage the cancer, or check for spread. Not all cancers spread, but most do.

Cancer cells can spread in two ways, and the first type of spread is local invasion, sometimes called local involvement or local spread. To visualize this, picture a crab (cancer is actually the Latin word for crab). The body of the crab is like the tumor.

Tumors may spread using local invasion. They send out legs into the tissues around the tumor.

The long crab legs are tumor cells reaching into normal tissues that surround it. These legs burrow through cell walls, create new blood vessels to steal oxygen from the bloodstream, and secrete acid to lower the pH in the surrounding tissues. Tumors hijack normal tissues and replace them with more tumor cells. This is how a tumor which begins in the nasal cavity can spread next door into the bones of the skull.

The second way that cancer spreads is by metastasis, also called distant involvement or distant spread. In this kind of spread, the tumor releases cancer cells into the circulation (bloodstream or lymphatics), so they can sail away and find distant places to live. This is how a cancer that begins in the spleen can also be found as close as the local lymph nodes, or as far away as the liver, lungs, or bone marrow.

Tumors may spread using metastasis, also called distant involvement or distant spread. They send cancer cells into the circulation.

Finding out what a cancer’s stage is, or how much it has spread, will tell you and your vet how advanced your dog’s case is. It will tell you what to expect over the course of the disease, and the prognosis. All of this information can really help you decide which of the available treatments to use. Depending upon what the biopsy report shows us, we may need to look for spread with an ultrasound-guided aspirate or biopsy, X-rays, ultrasound, CT scans, urinalysis, various blood tests, or a bone marrow biopsy. To find out if the cancer has entered the lymphatic system, the lymph nodes may also be checked with a fine needle aspirate, or even a biopsy.

Cancer is staged by testing for spread or metastasis. Once you know that, you can find out how far along the cancer is.

The tests will vary depending upon the type of cancer, its stage, and your own treatment and budget preferences. As Dr. Ettinger points out, a lot of money can be wasted on tests that you may not need, so be clear with your vet or oncologist about your budget. Reading the rest of this book, including the tests Dr. Ettinger uses for the most common canine cancers, will give some guidance about how far is far enough for you and your dog.


No matter how many tests we run, we don’t yet have tools that can detect micrometastasis. In micrometastasis, a very small number of cancerous cells spread throughout the body, using the circulation system; they do not form tumors right away, but have the potential to bloom into full cancers later on. These cells may not be found in testing, even with our most sophisticated imaging tools.

Their invisible spread is one of the reasons I recommend so many “outside the box” therapies in Full Spectrum cancer care. By using as many non-invasive therapies as possible, we may increase the odds of reducing micrometastasis.

Even if you get “clean margins” or think your dog’s cancer is in remission, it’s wise to assume that micrometastasis is still possible. That’s why keeping up the non-invasive therapies in steps two through five of Full Spectrum Cancer Care for at least two years is usually a good idea.

Found that FREE chapter helpful? Get the full 500-page book The Dog Cancer Survival Guide for only $9.99. There are 40 more Super Helpful Chapters, 5 Incredible Appendices, and 1 Fantastic Index in The Dog Cancer Survival Guide.

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