Why is my veterinarian NOT recommending chemotherapy?

The results of a study titled Survey of UK-based veterinary surgeons’ opinions on the use of surgery and chemotherapy in the treatment of canine high-grade mast cell tumour, splenic haemangiosarcoma and appendicular osteosarcoma were recently published. The study examined what percent of general veterinarians recommended chemotherapy for the three specific tumor types listed in the title, along with what chemotherapy protocols they recommended, and the reasons why post-operative chemotherapy would not be recommended for cases.

The facts tell us:

Mast cell tumors are the most common skin tumors diagnosed in dogs. The biological behavior of mast cell tumors is variable and best predicted by the grade of the tumor, which is assigned by a pathologist examining the biopsy. Chemotherapy is recommended to lower the risk of regrowth and/or spread of high grade tumors.

Splenic hemangiosarcoma is an aggressive tumor of the cells lining blood vessels. The prognosis with surgery alone (splenectomy) is 2-3 months. Chemotherapy can extend the expected lifespan to approximately 6 months after surgery. Some dogs can live a year or more following completion of such treatment.

Appendicular (limb) osteosarcoma is the most common primary bone cancer in dogs. Tumors are painful, and amputation of the affected limb is recommended to provide immediate relief. Amputation alone doesn’t alter a dog’s expected survival time (4-5 months) because the vast majority of dogs will go on to develop metastases within their lungs or to other bones in that time frame. Chemotherapy is recommended after amputation to increase survival time, typically to about one year, with 10-15% of dogs living 2 years.

Looking back at the study, I found several surprising conclusions.

  • General practitioners were more likely to recommend surgery for mast cell tumors and splenic hemangiosarcoma than for osteosarcoma.

As outlined above, the ideal treatment for all three tumor types is surgery. Yet the study pointed to evidence of veterinarians being less willing to recommend amputation than splenectomy (removal of the spleen) or surgical excision of a skin mast cell tumor. The authors speculate veterinarians might view amputation as excessively disabling. Yet, they point out several studies have examined owners’ opinions of outcome for their pets following amputation surgery and shown favorable responses, whereas similar studies are lacking for splenectomy or mast cell tumor removal.

Most owners are reluctant to pursue amputation for their pets, despite obvious evidence of pain. They frequently counter my opinion their pet is in discomfort, even when the dog is unable to bear weight on the affected leg. They perceive amputation as drastic and incapacitating.

While I understand an owner’s perception being skewed, it’s difficult for me to determine why a veterinarian would feel the same. Especially when knowing surgery would be a means of improving quality of life, rather than debilitating it. Veterinarians must be able to discuss amputation as a feasible and standard option for dogs with osteosarcoma out of responsibility to provide a treatment option that will eliminate pain from their patients.

  • General practitioners were more likely recommend chemotherapy for high grade mast cell tumors than for splenic hemangiosarcoma or osteosarcoma. The most common reason why general practitioners did not recommend chemotherapy for hemangiosarcoma or osteosarcoma was because they questioned the efficacy of treatment for those diseases, yet 51% and 36% of veterinarians did not know a current protocol for each disease, respectively.

Scientific evidence tells us, for all three tumor types, survival time can be extended when chemotherapy is added following surgery. Specific protocols are recommended for each disease based on data from research studies proving the efficacy of such treatment.

I understand the difficulty of keeping current on oncology treatment in pets and I wouldn’t expect a general practitioner to be more successful than I am in doing so. But the data supporting the efficacy of chemotherapy for splenic hemangiosarcoma or appendicular osteosarcoma is actually more established and straightforward to interpret than what’s available for mast cell tumors.

A lack of knowledge isn’t a valid excuse for not offering treatment. Veterinarians are responsible for seeking out options for their patients, and this includes recognizing when it’s time to involve the expertise of a specialist. Owners are appreciative of their veterinarians who are willing to learn from their pets and would likely be happy to know their vet communicated with an oncologist regarding their care.

  • General practitioners most frequently prescribed masitinib (Kinavet®) for dogs with high grade mast cell tumors (40%), with all other potential treatment options (e.g. intravenous vinblastine or oral CCNU) being offered only by 11% or less of respondents.

There are several treatment options for high-grade mast cell tumors in dogs. In terms of efficacy, it’s difficult to say which would be “the best” treatment as there’s no study directly comparing response rates and survival times among the choices. Therefore, I offer owners several options for treatment, and our decisions are made based on objective parameters such as number of required trips to the hospital, concern for side effect, and cost.

Masitinib is an oral form of chemotherapy registered for treating mast cell tumors in Europe, similar to toceranib (Palladia®) which is licensed in the US. Masitinib and toceranib belong to a family of drugs called tyrosine kinase inhibitors, which are small molecular inhibitors. The mechanism of action of small molecule inhibitors is different from ‘traditional’ chemotherapy drugs, which are more directly cytotoxic.

Though not specifically accounted for in the study, I suspect the increased frequency of prescription of Masitinib for mast cell tumors by general practitioners reflects the perceived relative “ease” of administration as compared with injectable vinblastine or even oral CCNU.

There is widespread perception by pet owners and veterinarians that tyrosine kinase drugs are less toxic, less intensive, and less risky than injectable or oral chemotherapy options. While I can’t argue there’s appeal for owners to give their pet’s chemotherapy at home rather than via a trip to the veterinarian’s office, small molecular inhibitors are no less hazardous or costly, and require more intensive monitoring than other forms of chemotherapy in order to be safely administered.

Of course, there are always two sides to a story. In the study, the general practitioner’s answers were based off a questionnaire, with no opportunity to explain their responses, leaving much to question about the conclusions made. The study was done in the United Kingdom, where there are likely differences in access to specialty medicine compared to where I work in the US. I’m also aware of the difficulties faced by general practitioners regarding struggling to explain the benefits of referral to a veterinary oncologist or cancer care in pets. But I can’t avoid considering the results as they are presented.

People assume my job is depressing because I’m the one having conversations with owners about a diagnosis of cancer, however it’s the general practitioners who are the frontline in communication. The results of this study indicate how important their role is in learning the correct information to disseminate, as well as the manner in which it is discussed. And the underlying importance of the veterinary oncologist in ensuring the facts are preserved and the patient’s best interests are kept at the forefront.

For more information on finding a board certified veterinary oncologist for your pet see www.ACVIM.org.

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