Diagnostic Approach to Pets with Cancer
Dogs and cats with cancer are typically presented by their owners either because of the physical presence of the mass if it is visible or palpable, or because of the effect of the cancer on the body and its functions. Neoplasia is defined as new growth. Cancer is the state of having a neoplasia in the body. Malignancy is generally reserved for those cancers that have some ability to metastasize, whereas benign cancers are strictly local lesions. A "tumor" is generally considered a solid neoplasia, as opposed to a hematogenous neoplasia such as leukemia. It is important to establish the owner's knowledge base and understand where they have gained that knowledge in order to best communicate with them about the approach to their pet's diagnostic workup.
If physical examination fails to reveal a mass, minimum database should include complete bloodwork, and radiographs and ultrasound should be offered. If a tumor is found, it should be needle aspirated if it is accessible. If the mass is in a body cavity, ultrasound guidance may be necessary. Needle aspirates of urinary tract masses should be avoided, if at all possible, as seeding of carcinomas to the abdominal wall has been seen. An incisional biopsy should be performed if the information will change the treatment or change the owner's willingness to treat. Biopsies should be performed such that the biopsy tract can be removed with the tumor at the time of definitive treatment. Drains should not be used after tumor removal as this complicates the possibilities of additional surgery or radiation therapy.
A general practitioner may also be asked to "screen" a pet for cancer. The emergence of biomarkers such as TK1, cCRP, and haptoglobin, as well as the assessment of vitamin D status which may influence both risk and response to cancer therapy, offers new insight into the evaluation of animals with cancer. These must be interpreted in the context of clinical findings.
Including an oncologist in the evaluation and treatment planning of a pet with cancer can improve patient outcome, as well as satisfaction for both the client and the general practitioner. A referral may be necessary if the equipment required for treatment is unique and not routinely available. The primary treatment modalities for treating cancer are surgery, radiation, and chemotherapy. While radiation facilities are limited due to the equipment required, both surgery and chemotherapy can be performed in private practice. Surgery and radiation are used for local control of cancer, whereas chemotherapy can be used to improve both local and systemic control.
Chemotherapy is not difficult to administer, but a commitment to understanding the drugs, necessary administration precautions, and each drug's side effects is critical. Some drugs are vesicants and can cause severe tissue sloughing if not properly administered. Even if the general practitioner chooses not to include injectable chemotherapy in their practice, there are oral chemotherapy agents that can provide treatment options to a client without the need for travel to a specialist. For some general practitioners, a referral for comprehensive cancer treatment is the best option, whereas in other practices a great deal of cancer care can be done in-house with periodic communication with a local specialist or teaching hospital.
Chemotherapy is the least technically demanding cancer treatment that can be performed by virtually any practitioner. It is not the actual physical administration of the drugs that is the crucial step, but the required knowledge of the proper use and dosing of these drugs. Anticancer drugs tend to have a very narrow therapeutic index and small increases in dose can result in huge increases in toxicity.
A few helpful hints about successful chemotherapy are as follows:
Never "round up" for chemotherapy drugs, always round down if needed.
Never break tablets. You will expose the owner unnecessarily to mutagenic agents and most chemotherapy tablets are not meant to be split such that the drug is not necessarily evenly distributed within.
To achieve the desired dose intensity (amount of drug per amount of time), dosages of oral chemotherapy can be spread over different periods of time, special sizes can be compounded, or the dose can be decreased slightly if appropriate to accommodate.
Be sure to tell the owner that some side effects may occur, but even if we expect side effects, we can still treat them. Educate your owners preemptively about when to worry and what they can do at home. This may include monitoring rectal temperatures (fevers in chemotherapy patients should always be treated as an emergency, and antiemetics can be provided for the owner to have if needed).
It is critical that a veterinarian be familiar with the general and unique toxicities of chemotherapy agents. Some unique toxicities are listed below. It is not difficult to get a biopsy, make a diagnosis, read about or consult about a chemotherapy of choice, and inject the drug. However, both veterinarian and pet owner will be frustrated and will not continue treatment if the toxicities are not addressed.
Unique toxicities of selected chemotherapeutic agents
Doxorubicin in dogs
Limit total cumulative dose, pretreat with dexrazoxane, or give as slow infusion
Doxorubicin in cats
Cisplatin in dogs
Ensure adequate renal function prior, fluid diuresis
Vincristine in dogs (peripheral neuropathy)
No prevention, monitor closely and discontinue drug
Cisplatin or 5-fluorouracil in cats
Never give to cats!
Pretreat with steroids, antihistamines, monitor for 30 min
Sterile hemorrhagic cystitis
Mesna or switch to chlorambucil
It is important to perform a complete blood count prior to each chemotherapy administration to ensure adequate bone marrow function. Most oncologists prefer a neutrophil count greater than 2500/µl prior to chemotherapy administration.
Myelosuppression is generally at its worst/lowest point (called the nadir) about 7–10 days after therapy and when neutrophil counts are low, patients may be susceptible to secondary infections including life-threatening sepsis and/or pneumonia.
If the dog is asymptomatic and neutrophil counts are above 1500/µl, typically no treatment is required.
If the dog is symptomatic (inappetant, lethargic, but not febrile) or if counts are less than 1500/µl, then prophylactic antibiotics should be instituted such as cephalexin or TMS.
If the dog is symptomatic and febrile, it should be hospitalized for IV fluids and IV antibiotics to cover gram-negative and anaerobic bacteria, as well as gram-positive skin flora. Typical combinations include ampicillin (or cefazolin) plus enrofloxacin, plus or minus metronidazole.
If the dog experiences febrile neutropenia or a neutrophil count less than 1000/µl, then subsequent doses of the most recent chemotherapy drug should potentially be reduced by 10–25%.
For potentially nephrotoxic drugs, it is important to obtain renal indices before each treatment. This should consist of a minimum of BUN, creatinine, and urine specific gravity.
For cardiotoxic drugs, specifically doxorubicin, there is no method of monitoring that will predict whether a patient will have clinical signs associated with cardiomyopathy.
Cardiotoxicity typically manifests after the course of chemotherapy has been completed. It is prudent to offer an echocardiogram prior to therapy to be sure there are no preexisting cardiac abnormalities.
Tachyarrhythmias are among the earliest clinical signs of doxorubicin cardiotoxicity.
Cisplatin causes a fatal pulmonary edema in cats.
5-fluorouracil causes a fatal neurotoxicity in cats.
When to Refer
Clients should be referred to a specialist if they request a referral, if the proposed treatments are beyond the scope of the practice, or if there are unique treatments or clinical trials offered only by the specialist. It is important to discuss the decision to include cancer treatment in your practice with your staff. If communication surrounding treatment decisions is consistent and supportive, the client will be satisfied with the outcome regardless of success or failure. A treatment may be beyond the scope of a general practice if it involves supportive measures not available at the practice (such as ventilation for a thoracotomy), if the tumor is very near vital structures creating a challenging surgical approach, if potential complications are daunting (such as extravasation of a chemotherapy agent), or if the treatment is not available (such as radiation). Support for the client entails thorough communication of treatment options, including pros and cons and cost versus benefit. Once the client's treatment goals are identified, decisions regarding treatment can usually be readily made. Once decisions are made and are humane for the pet, the client should be supported in their decisions.
Cancer treatment can be rewarding for the practitioner, the client, and the patient. Good quality and increased quantity of life can be granted to animals with cancer if a proper approach is used. The inclusion of cancer treatment can be a practice builder for the generalist. Referral to a specialist should be offered and a specialist can be used to fill in any steps the practitioner needs, whether that is creating a treatment plan for the practitioner, helping fine tune a protocol the practitioner is already familiar with, or managing the case from beginning to end.
1. Takada S. Principles of chemotherapy safety procedures. Clin Tech Small Anim Pract. 2003;18(2):73–74.
2. Ehrhart N. Principles of tumor biopsy. Clin Tech Small Anim Pract. 1998;13(1):10–16.
3. Gillette EL, LaRue SM, Gillette SM. Normal tissue tolerance and management of radiation injury. Semin Vet Med Surg (Small Anim). 1995;10(3):209–213.