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Pet cancer: Fact vs. fiction

Evidence-based responses to 10 common veterinary cancer myths

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Cancer awakens myriad feelings and fears in owners that may lead them to seek out information from online resources rife with misinformation.

Believing Dr. Google can lead to a delayed cancer diagnosis and affect treatment decisions.

Because veterinarians play a crucial role in addressing and dispelling false information related to cancer in their patients, we must be ready to set owners straight about top misconceptions about cancer in pets.

1. Only old pets get cancer.

In general, cancer is more common in older people and pets because more time has passed for genetic mistakes to accumulate. Nonetheless, some cancers can commonly occur in younger or middle-aged pets. Histiocytomas are well-known cutaneous round cell tumors of the skin that occur mostly in young dogs, exhibit benign behavior and usually resolve with time.

Other cancers that develop overwhelmingly in dogs 2 years of age or younger include rhabdomyosarcoma, neuroblastoma and nephroblastoma. These cancers arise from primitive tissues and tend to display highly malignant and life-limiting behavior in most cases.

Lymphoma and osteosarcoma also are commonly diagnosed in dogs younger than 2 years old, particularly in predisposed breeds. Occasionally, hemangiosarcoma and mast cell tumors also can occur in young dogs.

2. Environmental toxins are a leading cause of cancer.

Strong evidence suggests that environmental factors play a relatively minor role in cancer development for most cancer types.

A study across all major human cancers showed that, with the exception of a few well-known familial cancers or cancers with known environmental causes (i.e. tobacco-induced lung cancer), cancer incidence correlates directly with the number of stem cell divisions that occur in different tissues. This suggests that, for most cancers, the “bad-luck theory” of cancer incidence prevails.

Similarly, the largest epidemiological meta-analyses conducted in women with breast cancer have found minimal or no link between any environmental factors and breast cancer development.

There are very few epidemiological studies of cancer in veterinary medicine, and none show unequivocal causation. Further research is needed, but the bad-luck theory likely is also the reality for most cancers in our veterinary patients.

3. Surgery, biopsy or aspiration will cause cancer to spread.

No proof exists that surgery, biopsy or aspiration cause cancer to spread to distant organs.

While these procedures may sometimes cause cancer cells to be released into the bloodstream, most are incapable of establishing growth in distant organs unless they already have developed this metastatic ability. In fact, human studies suggest that the benefits of tumor sampling outweigh the risks.

For example, in a study of 2,000 people with pancreatic cancer, those who underwent sampling to obtain a diagnosis had improved outcomes compared to people who did not. The authors theorized that the information available due to the procedure helped speed appropriate cancer treatment. Local tumor spread by seeding after a biopsy procedure does occur rarely, but careful planning and good surgical technique can reduce the impact.

One example in pets is transitional cell carcinoma of the bladder, where seeding to the body wall can occur after surgery or, rarely, after needle aspiration.

Careful oncologic surgery techniques, such as changing instruments and gloves before closing the surgical wound, can reduce the seeding risk.

4. A carbohydrate-free diet can stop cancer growth.

Evidence does exist that cancer cells utilize more glucose and metabolize it through glycolysis more often than normal tissues, but these laboratory findings have not been supported by clinical evidence that low-carbohydrate diets slow cancer growth in people.

There is also concern that a “one-size-fits-all” approach to diet during cancer treatment is inappropriate, as some cancers are more likely to cause cachexia or other nutritional deficiencies.

Despite the fact that a commercially available low-carbohydrate diet is available and marketed for canine cancer patients, research in the field of nutrition and cancer is lacking in veterinary medicine.

So far, this diet has not been shown to improve outcome in pets with cancer. Very low-carbohydrate diets also can cause gastrointestinal upset due to the high fat content and are not safe for all patients.

5. Malignant masses feel different than benign ones.

While softer masses under the skin tend to be benign and firm ones malignant, there are many exceptions to this rule that make it unreliable. In particular, mast-cell tumors are often called “the great pretenders” because they can look and palpate just like a lipoma, skin tag or other benign skin mass. Some soft tissue sarcomas are also encapsulated in fat, causing palpation to be misleading.

For this reason, it is ideal to perform a fine-needle aspiration on all masses to assure a correct diagnosis. If aspiration is nondiagnostic due to poor cellularity or hemodilution, an incisional biopsy may be necessary.

6. If a lymph node feels normal, it’s not cancerous.

In the early stages, metastasis to a lymph node will not change its size, shape or texture, and some abnormal-feeling lymph nodes may just be reactive to the cancer. One study comparing the accuracy of palpation versus fine-needle aspiration found that lymph node palpation was only 60 percent sensitive and 72 percent specific compared to fine needle aspirates, which were 100 percent sensitive and 98 percent specific for cancer metastasis.

7. Side effects from chemotherapy are worse than the cancer itself.

In human medicine, the side effects of chemotherapy can be severe and, sometimes, long lasting. In veterinary medicine, our goals are different because pets are not able to make the choice to undergo cancer treatment. For that reason, veterinarians use much lower chemotherapy doses than what people receive for similar cancers in an effort to avoid side effects and maximize quality of life during treatment.

We aim for 80 percent or more patients having no side effects and, when they occur, most are mild and can be managed at home. If chemotherapy is administered cautiously and tailored to each patient, the benefits far outweigh the harms in the vast majority of our patients.

8. Some pets are too old for cancer treatment.

There certainly are patients that cannot tolerate aggressive cancer treatment, but this is uncommon and not necessarily age related.

For example, a patient may have significant neurologic disease that makes recovery from amputation or other surgery difficult, have hepatic or renal disease that would affect the ability to metabolize chemotherapy or have cardiac disease that makes repeated anesthesia for radiation therapy unwise.

These factors may exist regardless of age, however, and an older dog or cat without these concerns still can be a good candidate for cancer treatment. When planning cancer treatment, it is ideal to look at each patient’s history and medical status individually, with focus beyond their stage in life.

9. Cancer treatment hasn’t changed since I graduated.

As in the human oncology field, our understanding of cancer in pets and how to treat it has progressed significantly over the past 10 years. The past decade has brought new chemotherapy drugs, safer formulations of old chemotherapy drugs and approval of targeted therapies such as Palladia that kill cancer in a different way.

Also, several immunotherapies meant to harness the patient’s immune system to help fight cancer are now approved. Electrochemotherapy has become more widely available to treat nonresectable cancers.

While there still is much research to be done to define the best way to use these new therapies, the future of veterinary cancer therapy is bright.

10. Cancer is a death sentence.

There are many cancers with a low metastatic rate that can be cured if they can be removed surgically or treated with surgical debulking followed by radiation therapy. These include low-grade soft tissue sarcomas and mast-cell tumors, among others.

Even for those with a tendency to spread, not all cancers move quickly, and growth rate is highly dependent on cancer type. Anal sac adenocarcinoma and thyroid carcinoma, for example, are notoriously slow progressing despite high metastatic rates. This is why the consideration of the unique behavior of each individual tumor type is important when communicating prognosis to owners.

Even when an aggressive cancer is diagnosed, focused palliative care often can improve the quality and, to some degree, quanity of life an owner may have with their pet.


Dr. Katherine Skorupski is an associate professor of clinical medical oncology at the University of California, Davis, School of Veterinary Medicine.

Originally published in the April 2017 issue of Veterinary Practice News. Did you enjoy this article? Then subscribe today! 

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