Treating Aggressive Cancer in Pets

Joanne Lynn Intile, DVM, MS, DACVIM
Published: December 12, 2013
Treating Aggressive Cancer in Pets

Last week, I explained the terminology surrounding histiocytic diseases, with a focus on the neoplastic variants (e.g. localized and disseminated histiocytic sarcoma). This week I will focus on clinical signs, staging tests, and treatment options for animals.

Pets with histiocytic sarcoma typically have non-specific clinical signs, including weight loss, decreased appetite, vomiting, diarrhea, coughing, weakness, or lameness. Signs relate to where the disease is located. For example, limping would be common in an animal with a tumor surrounding a joint, whereas coughing would be seen in an animal with a lung mass.

Unfortunately, these signs do not allow us to know what is going on in that animal as a “whole.” Therefore, staging tests are recommended to provide a baseline prior to starting any further treatment plans, and to also serve as a means to assess future response to therapy.

When we stage a cancer this means we are searching for evidence of spread of disease. Many people are familiar with the term “stage” for cancer, but often have an incorrect perception of what it really means.

The stage of a particular tumor can only be determined after all recommended tests are completed. Though physical exams, blood work, and biopsy reports can be helpful in establishing reference points for patients, further testing is necessary to truly establish a specific stage of disease.

For histiocytic tumors, typical staging tests include the physical exam, a complete blood count, a serum chemistry panel, a urinalysis, three view thoracic radiographs (chest X-rays), abdominal ultrasound, lymph node aspirates, and a bone marrow aspirate.

Staging tests are important because they will dictate the kinds of treatments we will recommend. When all staging tests are performed, we typically have one of two options for pets with histiocytic sarcoma:

For cases where we perform staging tests, and we do not see evidence of disease in other regions of the body, surgery to remove the primary tumor will be recommended. Often we will recommend follow up with chemotherapy.


For cases where disease is detected in multiple regions of the body at one time, surgery is typically not recommended, and systemic therapy with chemotherapy would be the treatment of choice. Chemotherapy would be considered a palliative form of treatment in such cases, as a cure would be unlikely. However, it would be expected to extend a good quality of life for a patient, and would be a very reasonable treatment option.

Sometimes, even when tumors are confined to one area of the body, owners will not elect to pursue surgery, or do not complete all the recommended staging tests. These are the most challenging cases to manage, as my ability to predict prognosis is hampered by a general lack of information about the patient’s status, as well as a lack of ability to foresee how the tumor(s) will impact that animal in a long term sense.

The chemotherapy drug I recommend for treating pets with histiocytic sarcoma is a drug called CCNU. This is an oral form of chemotherapy administered once every 3-4 weeks. CCNU is generally very well tolerated in pets. Upset stomach signs (vomiting/diarrhea/poor appetite) are very rarely seen.

The most common side effect we can see with CCNU is a low white blood-cell count.  This can occur anywhere from 5-10 days following treatment in a dog, but can be much less predictable, and occasionally quite prolonged, in cats. I generally recommend dogs have a blood count check one week after receiving this drug, whereas I recommend cats have weekly blood work performed to be able to better assess their numbers.

CCNU can also cause liver damage in dogs, so liver values are monitored carefully during the course of treatment. We often recommend owners administer a commercially available liver protecting supplement at home, as one study suggested this could reduce the frequency of liver value elevation following treatment with CCNU. Cats seem fairly “immune” to this complication, though we still monitor them carefully.

For dogs with localized histiocytic tumors removed surgically, treatment with CCNU can control reoccurrence/spread of disease for an average of 8 months and average survival times can exceed 18 months with surgery and repeated chemotherapy treatments. Quality of life on this treatment plan is generally excellent.

For dogs with measurable histiocytic tumors (either a solitary tumor not removed surgically or with multiple tumors present at the time of diagnosis), CCNU has a 30-50 percent chance of reducing the overall disease burden by at least 50 percent. Treatments are repeated as long as a response is detected, and for the average case, this would be about 3-4 months (approximately 3-5 treatments). The average survival in such cases could likely be about 6 months.

Other chemotherapy drugs can be used when CCNU is ineffective, or if patients do not tolerate this drug treatment. This can range from intravenous forms of treatment (e.g., doxorubicin) to anti-angiogenesis treatments such as metronomic oral chemotherapy or tyrosine kinase inhibitors.

An interesting new treatment option for histiocytic sarcoma is bisphosphonate therapy. Bisphosphonates are drugs given primarily to prevent osteoporosis in women because they inhibit resorption and breakdown of bone. Though not “traditional” chemotherapy drugs, bisphosphonates also can destroy different tumor cells, and preliminary data in petri dishes suggests they can be effective in killing cancerous histiocytes, making them an interesting option for treating histiocytic sarcoma in pets.

As is typical for most cancers, the use of chemotherapy to treat histiocytic diseases in cats is less studied and most reports are of the anecdotal/case-report variety.

The take home message is although histiocytic sarcoma is an aggressive form of cancer in pets, treatments do exist, and in many cases referral to a veterinary oncologist is the next step. They are the best-equipped individuals to explain your options and will work closely with your primary care veterinarian to provide your pet the best possible outcome.

Dr. Joanne Intile

Image: nalinratphi / Shutterstock

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