Center of Clinical Comparative Oncology (C3O), Department of Clinical Sciences, Faculty of Veterinary Medicine and Animal Science, Swedish University of Agricultural Sciences (SLU), Uppsala, Sweden
The majority of the case load in small animal practice consists of dogs. However, in some specialized clinics, cats predominate. In all, cats are the fastest growing segment of companion animal services. Cats get most of the tumors that occur in dogs. It is, however, important to know the differences regarding prognosis, treatment, and management/rehabilitation. Cats should not be treated the same way as small dogs. This lecture will focus on some aspects of feline oncology and discuss some tumor entities especially important in cats.
It is hard to differentiate subtle symptoms of cancer in dogs from other diseases, but it can be even more of a challenge in cats. Cancer is an old animal's disease in most cases. Hence, coexisting diseases are very common and must be evaluated simultaneously with the cancer to construct an optimal treatment plan and make a correct prognosis. Typical clinical signs that may occur in cats with cancer are lumps and bumps, abnormal odors, vomiting/diarrhea, non-healing wounds, weight loss, reduced appetite, coughing or difficulty breathing, tiredness or depression, increased thirst/frequent urination, and evidence of pain.
As coexisting diseases often occur, a minimal database consisting of complete blood count (CBC), biochemistry panel, and urinalysis is mandatory since coexisting disorders will inevitably affect the possible treatment options and prognosis. Depending on suspicion, diagnostic imaging and fine needle aspirates and/or biopsies will be performed before a definite treatment plan is decided.
In most cancers, surgery is the treatment of choice. The goal is to minimize tumor burden. In many cancers, a complete cure can be achieved if the tumor is diagnosed in an early phase and complete resection with clean surgical margins is obtained. In many cases, the tumor has spread by the time of diagnosis so that a complete resection is not possible. Then, minimizing the tumor burden (cytoreductive surgery) with adjuvant treatment (often in the form of chemotherapy and/or radiation treatment) can be satisfactory. Especially in cats, it is important not to exceed the surgical "dose." Cats do not respond favorably when their food intake is impaired because, e.g., fatty liver will rapidly develop. It is also important for cats to be able to groom and if this is reduced their quality of life will decrease dramatically.
Chemotherapy is widely used in feline oncology. To succeed in treatment, a basic understanding of differences between cats and dogs is mandatory. Certain drugs, such as doxorubicin and lomustine, must be used at lower dose to avoid intolerable side effects. Alopecia can occur with doxorubicin therapy, especially in dog breeds with continuously growing hair (e.g., Poodle, Schnauzer, etc.), and cats will occasionally lose their whiskers. The alopecia usually resolves after therapy. Cats do not seem to develop cardiomyopathy with doxorubicin use. However, acute renal failure can occur in cats after a single dose of doxorubicin, and liposome-encapsulated doxorubicin (Doxil®) has been shown to cause chronic renal failure months after therapy. Cisplatin causes fatal pulmonary edema in cats and should therefore never be administered. The sister substance, carboplatin, is safe to use in cats and is useful for a wide variety of cancers in both dogs and cats. The drug 5-fluorouracil (5-FU) causes rapid-onset, fatal neurotoxicity in cats. In general, cyclophosphamide, Oncovin®, and prednisone (COP)-based protocols are well tolerated in cats. However, cyclophosphamide can cause sterile hemorrhagic cystitis and where it does, this treatment needs to be stopped.
Although not available everywhere, radiation therapy (RT) can be an excellent tool in cancer treatment for cats. Especially in cases where the surgical dose cannot be high enough to completely remove the tumor, RT can be used in complement to ensure important body functions for the cat. Facial/oral surgery is often very complicated in cats, as they do not tolerate even short periods of inappetence/anorexia without liver damage. Tube feeding can be a short-term solution, but the cat needs to be able to feed itself to maintain quality of life. In these circumstances, RT can treat local disease in this area and despite side effects with RT, quality of life is better in the long term with this treatment modality.
Specific Cancers in Cats
This cancer type is very common in both cats and dogs. In cats, the tumor is even more aggressive, with more than 85% of the mammary tumors (MTs) being malignant. More than one-fourth will metastasize to the lungs and/or other sites and are often fatal. Since the tumors are so aggressive, radical surgery at an early stage is very important. If the tumor is < 3 cm at surgery, the median overall survival is over a year. Sometimes adjuvant chemotherapy is recommended, as the metastatic potential is so high in cats.
Lymphoma (lymphosarcoma, LSA) is not only the most common neoplasia in cats (~ 25%), but it also is the most common hematopoietic malignancy in cats, with the gastrointestinal form representing nearly 80%. Combination chemotherapy is standard care and can lead to complete remission rates of approximately 30–65% with a median duration of remission of 3–7 months. The alimentary form is not always evident for the clinician and symptoms may be vague, consisting of weight loss, inappetence, vomiting, and sometimes diarrhea. Important differential diagnoses are parasitic infestation, inflammatory bowel disease, and pancreatitis. Sometimes, acute surgery needs to be done to ensure gastric/enteral passage. Medical treatment is always necessary. Cats can have slow-growing (indolent) forms of lymphoma. These do not respond well to chemotherapy; however, overall survival can sometimes be quite long. When discussing treatment for a cat with lymphoma, it is important to always test feline leukemia virus (FeLV)/feline immunodeficiency virus (FIV) status, as positive cats have worse prognosis.
Squamous Cell Carcinoma
Despite being a tumor of the skin, squamous cell carcinoma (SCC) often develops in the oral cavity of the cat. As oral surgery (especially in aboral locations) is very complicated in cats, this is often a fatal disease as local progression will disable the cat and impair eating. The tumor responds poorly to chemotherapy, especially if large. Palliative treatment with pain medication and anti-inflammatory substances (i.e., COX-inhibitors) has sometimes been reported to result in long-term survival. Radiation therapy has some success in achieving local control without too many side effects. The tumor rarely metastasizes to distant locations. Squamous cell carcinoma of the nasal plane and/or pinnae mostly affects cats with light hair and little pigmentation. Chronic exposure to ultraviolet (UV) light is one likely cause of this tumor. If a cat has burns in these areas, the owner should be instructed to apply sun screen. This can, however, be complicated in cats. Amputation of the ears can be successful. Nasal plane surgery is more complicated, but local treatment with, e.g., photodynamic therapy (PDT)3 or RT has been reported to result in long-term survival with good quality of life.
Injection (Vaccination)-Induced Sarcoma4
Vaccine-associated sarcomas are relatively common in the USA, but less common in other countries. These sarcomas are believed to be associated with administration of certain vaccines, but have also been reported with other injections. Vaccine-associated sarcomas are comparably more prevalent than fibrosarcomas at non-vaccine sites.
These sarcomas develop as a result of the chronic local inflammatory response induced by injection/vaccine. As not all cats develop sarcomas after vaccination, it is speculated that some form of genetic predisposition exists. If sarcoma develops, aggressive surgery must be carried out as soon as possible. If the tumor is located in the neck (interscapular area), surgery often includes excision of part of the spine. Different combinations of adjuvant treatment with RT and/or chemotherapy have been studied. It seems, however, that radical surgery alone has the best overall survival results.5 To avoid complicated surgery, many countries now have programs instructing veterinarians to vaccinate/inject in areas where surgery is easier to perform with clean margins, such as hind limbs.
1. von Euler H. Tumours of the mammary glands. In: Dobson J, Lascelles BDX, eds. BSAVA Manual of Canine and Feline Oncology. 3rd ed. Gloucester, UK: BSAVA; 2011: 237–247.
2. Taylor SS, Goodfellow MR, Browne WJ, Walding B, Murphy S, Tzannes S, et al. Feline extranodal lymphoma: response to chemotherapy and survival in 110 cats. J Small Anim Pract. 2009;50(11):584–592.
3. Bexfield NH, Stell AJ, Gear RN, Dobson JM. Photodynamic therapy of superficial nasal planum squamous cell carcinomas in cats: 55 cases. J Vet Intern Med. 2008;22(6):1385–1389.
4. Novosad CA, Bergman PJ, O'Brien MG, McKnight JA, Charney SC, Selting KA, et al. Retrospective evaluation of adjunctive doxorubicin for the treatment of feline mammary gland adenocarcinoma: 67 cases. J Am Anim Hosp Assoc. 2006;42(2):110–120.
5. Martano M, Morello E, Ughetto M, Iussich S, Petterino C, Cascio P, et al. Surgery alone versus surgery and doxorubicin for the treatment of feline injection-site sarcomas: a report on 69 cases. Vet J. 2005;170(1):84–90.