Staging for Canine and Feline Cancer Patients – Using Diagnostic Imaging to Look Inside Your Pet

PetMD Editorial
May 09, 2016
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In this multipart series, I’m covering the topic of staging cancer patients and why doing so is important in the process of determining if our companion canines and felines have detectable cancer or if they are in remission.

The staging process is one in which I have to partake on an ongoing basis with my dog, Cardiff, so I’m well aware of the sometimes frustrating process of having increased concern for cancer recurrence based on abnormalities that are discovered in his diagnostics. But if Dr. Avenelle Turner (Cardiff’s oncologist at Veterinary Cancer Group) and I didn’t stay on top of all facets of his internal workings, we could overlook minor abnormalities that can collectively create a bigger picture of concern for his whole body health.

Unfortunately, staging doesn’t just involve one simple diagnostic test. Instead, many types of tests are used to create a complete picture of a pet’s health. Part 1 covered the basic concepts of staging, Part 2 dealt with blood diagnostics, Part 3 gave the scoop on poop and pee, and now in Part 4, I will shed light on diagnostic imaging.

Radiographs: Using Still Life to Look Inside

Also known as x-rays, radiographs are a routine and relatively simple means of looking inside of our pets’ bodies to determine the states of normal tissues or the presence of abnormalities. 

Radiographs create a static (still) image that permits veterinarians to get a basic picture based on organ systems and structures appearing white, black, or in varying shades of gray.

Until the advent of digital radiography, film was exclusively used. Fortunately, digital radiography has become highly utilized by veterinarians as there are numerous advantages over film, including improved imaging quality and less patient and employee exposure to x-rays.

Very dense structures such as bones and metal look white on x-rays, as all of the x-ray beams are blocked by the high density and do not enter the imaging plate or piece of film. Air can be seen inside organs like the lungs, trachea (windpipe), stomach, intestines, and other organs, which appear black as air has no density to block the x-ray beams. Muscle, fat, skin, and solid organs like the spleen, liver, and other structures appear in various shades of gray.

At least two radiograph views are needed to create a 3-D image in the mind of the doctor who is reviewing the images so that what is really going on in the body can be best understood. A pet’s body or limb will be viewed from the right or left side in a lateral (“Lat”) projection and a bottom to top view in a ventrodorsal (“VD”) projection (or vice versa in the dorsoventral [“DV”] projection).

No sedation or anesthesia is generally needed for radiographs to be taken, but dogs and cats that aren’t amenable to being positioned due to behavioral or health reasons may need to be sedated or anesthetized to attain suitable radiographs.

Cardiff now has radiographs of his chest and abdomen every 3 to 4 months to look for evidence of other disease processes or for the presence of lymphoma in other tissues, including the lymph nodes that are contained within his chest cavity that course along his esophagus (“food tube”) and blood vessels.

Radiographs are great to get a baseline of normal and abnormal, but they don't always provide more specific information about a particular organ system. For example, both times that Cardiff had a small intestinal tumor that was causing the intestinal diameter to reduce and prevent food and fluid from properly moving through, radiographs of his abdomen did not reveal the presence of the masses. They were discovered via ultrasound, which has been the more crucial diagnostic test in determining whether Cardiff is still in remission or is having a recurrence of intestinal T-cell lymphoma.

Ultrasound: Viewing the Internal Body in Motion

Whereas radiographs create a static image, ultrasound produces a real-time, moving picture of your pet’s internal organs.

The abdominal organs and tissues like the heart and blood vessels are better imaged via ultrasound than are structures like bones, joints, lungs, and others. Ultrasound waves do not penetrate air or very dense structures (bones, metal, etc.), so looking inside the chest cavity for abnormalities is relatively non-diagnostic unless the heart and blood vessels are the organs being evaluated.

An ultrasound of the heart is called an echocardiogram and is a crucial component of thoroughly evaluating heart appearance and function.

Adriamycin (Doxorubicin), one of the many chemotherapy drugs Cardiff has received, has a toxic effect on the heart, so I've repeatedly pursued echocardiograms as part of Cardiff’s ongoing staging process in an effort to reduce the use of the drug. Radiographs can give basic information about the heart, such as its overall size and whether particular structures within and around it are enlarged or shrunken, but the echocardiogram sheds light on how well the heart valves are functioning to prevent blood from flowing in an abnormal direction (against the flow).

Generally, patients don’t need to be sedated or anesthetized for an ultrasound to be performed, but behaviorally challenging pets may need to be mildly sedated to be still enough to be appropriately positioned and for the few to many minutes needed to complete the ultrasound. Additionally, the site being evaluated via ultrasound is commonly clipped free of hair, and alcohol or ultrasound gel is applied to the skin to facilitate the entry of ultrasound waves into body tissues, all of which may be upsetting to the animal.

Magnetic Resonance Imaging and Computed Tomography: Imaging for Higher Sensitivity Areas

When radiographs and ultrasound don’t quite create a complete image of a pet’s internal structures, other imaging techniques like magnetic resonance imaging (MRI) and computed tomography (CT) are needed.

MRI is the preferred imaging technique to look at structures like the brain, spinal cord, nerves, and intervertebral discs. CT scans are primarily used to look for masses occupying the space within soft tissue structures like the lungs or nasal cavity, or in body cavities like the chest or abdomen.

According to Southern California Veterinary Imaging (SCVI), a “recent study in the Journal of College of Veterinary Radiology found that CT scans are five to six times more sensitive than radiography at detecting soft tissue nodules (metastasis) within the lungs.”

Both MRI and CT take multiple images in sequence while coursing over the targeted body part.  The slice-like images can then be viewed to see the progression of normal and abnormal findings. MRI and CT are the best way to determine the extent to which a disease process is affecting an organ or body system.

Unlike with radiographs and ultrasound, MRI and CT require the patient to be fully anesthetized so that the body part needing to be studied is completely still.


Nuclear Imaging: A Closer Look at the Bones

Sometimes, more advanced tests need to be performed to detect the presence of cancer when radiographs, ultrasound, MRI, or CT scans just can’t quite seem to find the abnormal cells. 

Nuclear imaging involves the injection of radioactive isotopes into the body which move to areas of tissue where there is increased cellular activity. One of the most practical applications of nuclear imaging being used in the cancer staging process is during bone scans.

When there is a disease process like osteosarcoma (OSA, a malignant bone cancer), the cancer is rapidly growing and damaging bone cells. SCVI reports that “30-50% of bone loss must be present in order for the changes to be visible on x-rays,” so the bone scan can help veterinarians to identify areas of concern that may merit biopsy or amputation and to confirm the OSA diagnosis before evidence of bone loss can even be seen using radiographs. Earlier identification of the cancer means that the disease can be treated quicker and can spare the patient pain and potential metastasis to other sites.

Well, you now have a sense of what goes into the elaborate process involved in staging your pet for cancer. As the process isn’t a simple one, it’s important to have a positive relationship with your veterinarian or veterinary oncologist to help guide you through the series of choices one can pursue in providing the most appropriate cancer management plan for your pet. 

dog utrasound, pet cancer

Maria and Dr. Rachel Schochet of SCVI performing an abdominal ultrasound.