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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.

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


Comments  12

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  • White Count
    12/12/2013 11:19pm

    "The most common side effect we can see with CCNU is a low white blood-cell count."

    I've had several kitties that ended up anemic from various things (mostly kidney failure) and had excellent results using injectable Procrit. Yeah, it's expensive, but (1) the response was worth the cost and (2) critters get such tiny amounts that a 6-pack of small vials lasts quite awhile.

    Is there any reason an animal on CCNU with a low white count couldn't benefit from Procrit?

  • 12/14/2013 06:54pm


    Thanks for the question. You are correct that Procrit (or some other veterinary version of this drug) can be used in pets to stimulate red blood cell production. This is most often employed for pets with chronic kidney disease.

    We have similar drugs that can be useful in stimulating white blood cell production (Procrit would be specific for red cells not white cells).

    Some people use them if animals have low white blood cell counts from chemo. I stay away from them unless they have a problem rebounding from a low count for a few reasons:

    1) most animals are completely asymptomatic for low white blood cell counts and do not encounter any complications from it
    2) most animals will rebound from a low count within 2-4 days
    3) most of the stimulatory drugs are made for people and with repeated use, animals will build up antibodies, which will neutralize the effect of the drugs
    4) since they are human drugs they are very expensive, maybe even to the point of being cost prohibitive (a lot more than Procrit)
    5) in patients with cancers of white blood cells, using drugs that stimulate further production of white blood cells is controversial because we ask ourselves 'are we also stimulating the cancer cells to multiple' - this is very controversial and theoretical

    I will use them if a patient has prolonged depressed counts and we are facing marked risk for infection.


  • Pets cancer
    12/14/2013 05:52am

    What are the symptoms of cancer in pets..?? What is their behavior when they have cancer.


  • New Research?
    08/12/2014 06:58pm

    Hello Dr. Intile,

    Our 4 legged companion Biggs (9 year old Bombay) has been diagnosed with Histiocytic sarcoma. We understand the condition carries a poor prognosis, however we are attempting to do our best to for him.

    He is currently in decent health w/ some light lethergic symptoms.

    Are you aware of any new research on the topic of HS in Felines?

    I have attempted to extensively research such things as the Biphosphonate options as well as the biphosphonate treatments with dox or vincritine to synergisticly improve the effects.

    Unfortunately for us this research seems to be almost exclusively discussed for canines.

    I would greatly appreciate any insight.


  • 08/13/2014 11:48am


    I'm sorry to hear about your diagnosis for your kitty Biggs. You are right that Histiocytic sarcoma is typically an aggressive cancer with very little information available for treatment in cats. I hope you've been able to find a veterinary oncologist nearby to help you through the treatment process, or at least to be able to go over different treatment options and then you can decide if anything sounds like something you would like to try. Many oncologists treat cats with CCNU, but you are right that there's some evidence for doxorubicin and pamidronate for potential therapy. I think any of those are reasonable options. Most veterinary oncology research occurs at veterinary schools, so depending on where you live, you may have access to that. In my experience, how cats do with this disease/treatment often depends on how much disease they have in their bodies and how sick they are at the time of diagnosis. You are doing all you can for your companion. Hang in there and good luck.

  • 11/08/2014 07:44pm

    my cat was also diagnosed with this awful cancer in August. Have you tried chemotherapy?

  • Maine Coon
    11/08/2014 07:41pm

    My 15-year old cat Tommy was diagnosed with histiocytic sarcoma in August of this year. It was a devastating diagnosis! The growth originated on his left hind tarsus and metastasized to the left popliteal lymph node, in addition to possibly having metastasized to the sublumbar lymph nodes in the groin. I was referred to a large veterinary research hospital where we determined that chemotherapy with CCNU 17.5 mg was the best option for Tommy. After his first chemotherapy dose, Tommy's limping decreased and after the second dose he is no longer limping and his overall demeanor has improved. He is taking Denamarin and has had no severe side effects so far, besides some soft stool. He has gained weight back and after the third dose of CCNU, the tumor has softened and shrank significantly. Tommy is no longer in pain and I am so grateful for the chemotherapy working so well with Tommy. Even though I understand that his prognosis is guarded, I hope for continued success with the chemo and that his cancer will be able to be fully controlled. I wanted to share Tommy's story since not much is known about this type of cancer in cats. The Oncology department gave the CCNU a 40% chance of success with Tommy's tumor. So far it appears as if this treatment is working. I will post an update on Tommy periodically.

  • 11/11/2014 01:41am

    Thank you for sharing Tommy's story. Please post updates on how things are going!

  • 12/06/2014 07:01pm

    Hi there, it's been a month since I have posted about Tommy, so I wanted to leave an update on him. He received an abdominal ultrasound and chest x-ray the other day in order to re-stage the cancer. There is no evidence of metastasis to other organs and the lymph nodes in the groin are no longer enlarged. He has gained weight (from 12.8 pounds up to 14 pounds) and is very playful, active and happy. The vets are very pleased and the cancer seems to be under control at this point. The ultrasound did show mild changes to the liver, which the vets think could be due to age (he will turn 16 in May) or possible side effects from the CCNU. In addition to the CCNU, I am giving him some holistic/supplemental treatments, which my vets are aware of. I give him the following supplements:
    1. Coq10 (every other day)
    2. Pet Alive C-caps for cellular health (every day)
    3.Purina Vet diet probiotics (every day)
    4. Dancing Paws Feline Multivitamin (every day)
    5. Organic Flaxseed oil (3x week)
    Additionally, Tommy is on a low-carb, high protein diet by Merrick's. Hope this post gives other people hope for their pet!

  • Survivor!
    06/09/2015 01:08am

    Hi ~
    I'm a little late to this article, but thought I'd share our experience.
    Our dog Murphy is a retriever mix. At 7 yrs old he was diagnosed with a bone tumor in his right front leg, probably osteosarcoma, but the biopsy was inconclusive. We went ahead with amputation and eventually found out that it was actually histiocytic sarcoma. It was cd18 +, so he had 6 doses of CCNU along with taking Denamarin to protect his liver. The CCNU was tolerated well. We were told that if we were lucky we might get 12-18 months with Murphy. After he completed his chemo, another tumor was found, this time on his left shoulder blade. We decided not to do any further chemo at that time.
    Well, it has now been 2 years and 2 months since Murphy's amputation and he is still going strong! The tumor on his shoulder blade has not increased in size and his lungs continue to be clear. He is on a grain-free diet but doesn't take any supplements. He gets chest x-rays every 3 months to check for metastasis.
    He amazes us every day! [img][URL=http://s76.photobucket.com/user/Donna_Novak_Hansen/media/20150526_174320-1_zpshxnmawkx.jpg.html][IMG]http://i76.photobucket.com/albums/j23/Donna_Novak_Hansen/20150526_174320-1_zpshxnmawkx.jpg[/IMG][/URL][/img]

  • Tommy has passed away
    09/06/2015 09:17pm

    It is with a heavy heart, that I have to report that my Tommy has passed away. He suddenly started breathing fast and heavier than normal and the vet found fluid in his lungs. The fluid was drawn but Tommy only temporarily improved. He was admitted to Auburn University ICU in a last ditch effort to find out what was going on. He could no longer breathe without being in an oxygen chamber and the decision was made to let him go. It was the hardest decision. The vets informed me that it is unclear whether pulmonary toxicity is a side effect of cumulative CCNU treatments in cats or if the cancer had spread, even though the ultrasounds and x-rays were always clear.

  • 09/07/2015 02:14am

    I am very sorry to hear about the loss of Tommy. Thank you for sharing his story and treatment outcome. This will help many other owners I am sure. I am sorry again. Joanne

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