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The Daily Vet by petMD

The Daily Vet is a blog featuring veterinarians from all walks of life. Every week they will tackle entertaining, interesting, and sometimes difficult topics in the world of animal medicine – all in the hopes that their unique insights and personal experiences will help you to understand your pets.

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Last week I introduced you to Duffy, an older Golden retriever, whose seemingly simple limp turned out to be a harbinger for the devastating diagnosis of osteosarcoma. This week I want to go over some of the available staging tests designed to look for spread of this type of cancer, as well as provide my clinical insight into their value and utility.

The recommended treatment of choice for dogs with osteosarcoma of a weight-bearing bone is amputation of the affected limb. In only very specific cases, we may consider localized excision of the affected portion of the bone without pursuing an amputation (i.e., limb-sparing surgery). More information on this procedure will follow in a subsequent article.

Osteosarcoma is a highly metastatic tumor. The most common locations where the cancer will spread to are the lungs and to other bones. At the time of diagnosis, greater than 90 percent of dogs will test negative for spread of disease. Yet even with immediate removal of the tumor, most dogs will develop metastatic tumors within a few short months after surgery. This indicates that the cancer already spread before the primary tumor was removed, but existed at a level below our ability to detect it. The average lifespan is only expected to be about 4-5 months with amputation alone.

Given the propensity for this cancer to spread to the lungs and other bones, historically we used radiographs (X-rays) of the lungs along with our physical exam findings as the main ways to assess for spread. There are some limitations to these diagnostic tests though; in order for a metastatic tumor to be visible on a radiograph, it must be about 1cm3 in size, which is estimated to be about 1 billion cancer cells. It doesn't take a medical degree to know that’s a huge amount of cancer cells. We also know animals do not show signs of pain the same way people do, and physical exams can be notoriously insensitive for picking up the discomfort associated with a metastatic tumor within another bone.

Advanced diagnostic tests with increased sensitivity for detecting spread of osteosarcoma tumors are now more readily available. We now recommend a thoracic CT scan as this imaging modality is superior to radiographs for picking up smaller tumors within the lungs and are also better at localizing tumors to specific portions of this tissue. We also can perform nuclear scintigraphy, which is a diagnostic test useful for picking up tumors in other skeletal bones.

CT scans and nuclear scintigraphy are wonderful testing options, but tend to be limited in their availability, are expensive, and have the downside of requiring heavy sedation and/or general anesthesia. They also have their own particular false positive and false negative rates and are qualitative tests, meaning they rely on human interpretation and operator error, which sometimes contributes to confusing results.

Some veterinarians recommend performing abdominal ultrasounds as a screening test on dogs with bone tumors. The odds of a bone tumor spreading to an internal organ would be exceedingly low, but the odds of an abdominal ultrasound picking up one or more abnormalities of indeterminate significance would be moderate. Typically this leads to further tests, which themselves may or may not be conclusive. All the while we have a painful patient and confused and emotional owners who are simply looking for the right thing to do for their dog.

Advanced testing options are great, but when I discuss their utility with owners, I really try to put the focus on determining what their goal is for their dog. We have to ask ourselves what we will do with the results of the test before performing it, and will these results alter the recommended treatment plan?

Dogs with osteosarcoma are painful, and although there are several available palliative treatment options, each falls considerably short in their ability to control pain when compared to amputation. If a CT scan shows hundreds of tiny tumors throughout all lung lobes, I agree the prognosis for long-term survival is poor. But do we not consider amputation of that pet’s limb to control pain while they are still asymptomatic for spread? What if the scan shows two tumors, or just a possible tumor? How do we decide the right answer?

In my opinion, whether metastases are detected or not at the time of diagnosis, surgical amputation of the affected limb in an otherwise asymptomatic dog is something I will recommend in nearly all cases. I didn’t always feel this way, and this stance is something I’ve adopted through my years of working as an oncologist trying to medically manage the discomfort of dogs with bone tumors.

Of course, not every owner elects for amputation, and not every dog is a candidate for this surgery (e.g., they may have severely debilitating orthopedic or degenerative neurological diseases that hamper their ability to ambulate even with four limbs). In those cases, we have several options for palliation of pain, each with it’s own varying success rates, which will be the subject of next week’s article.

I discussed the option of pursuing advanced testing with Duffy’s owners and they elected to pursue the thoracic CT scan, bone scintigraphy, and the abdominal ultrasound, which fortunately were all negative for any spread or intercurrent disease, with the exception of a pesky suspicious 4mm nodule in one of his left lung lobes.

And thus began the discussion of amputation versus palliative care for Duffy.

To be continued…

Dr. Joanne Intile

Image: Utekhina Anna / Shutterstock

Comments  2

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  • Duffy
    07/31/2013 06:13pm

    Would Duffy be a candidate for amputation AND excision of the lung tumor? Is it at all possible that the 4mm "thing" in his lung isn't cancer?

    "The average lifespan is only expected to be about 4-5 months with amputation alone."

    Might there be post-surgical treatment that would give the dog a good quality of life? Chemo? Radiation?

    It makes sense that osteosarcoma would spread to other bones, but is there a medical reason that it seems to head toward the lungs and not usually internal organs?

  • 08/01/2013 08:11pm

    Very informative. I would appreciate it very much if there were an article on Mast Cell Tumors, specifically occurring on the head. My dog has been diagnosed with this type of cancer. Because of his age we are doing palliative treatment only with Benedryl and Tagamet with the addition of digestive enzymes to his raw diet. Georgie is a Puggle, age of 10 or 11. My main question is whether a tumor located on the top of the skull, slightly behind one ear, could metastasize to the brain or the ear or is most likely to travel to the digestive tract, liver and hind legs. I also want to know what the signs are that indicate it is kinder to euthanize.

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