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Tips for managing cancer patients in general practice, and when to refer (Proceedings)
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.
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 work up.
When presented with a sick animal, 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.
A general practitioner may also be asked to "screen" a pet for cancer. The diagnostics listed above constitute a logical approach to such a pet, however it must be emphasized to the owner that there is no definitive test to say whether or not a pet has cancer. Screening tests such as cancer antigen levels commonly available in human medicine are not routinely available in veterinary medicine. There are however tests that may help screen a geriatric patient for specific cancer as in the case of the VBTA (veterinary bladder tumor antigen) test for bladder cancer or in serum tests such as calcium levels (can be elevated in lymphoma and anal sac adenocarcinoma) or alkaline phosphatase (can be elevated with osteosarcoma). It is important to understand the limitations of these tests in order to apply them to a given animal patient.
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 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.
Unique toxicities of selected chemotherapeutic agents
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 it doesn't hurt to have metoclopramide, metronidazole, or pepto on hand for GI issues).
• 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, but both veterinarian and pet owner will be frustrated and will not continue treatment if the toxicities are not addressed.
• 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 3000/µl prior to chemotherapy administration.
• Chemotherapy drugs not typically considered significantly myelosuppressive are vincristine, prednisone, and l-asparaginase.
• Myelosuppression is generally at its worst 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.
• 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 pre-existing cardiac abnormalities.
• The complete pathogenesis of the cardiotoxicity of doxorubicin is not fully understood but the currently accepted primary process is the generation of iron-based free radicals that damage cardiac myocytes. The heart lacks catalase and depends on glutathione for free radical scavenging, so is uniquely sensitive to this damage.
• Tachyarrythmias 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.
Support for the chemotherapy patient includes medication to alleviate nausea and diarrhea. The general practitioner must also be familiar with side effects unique to certain chemotherapy agents. Pets receiving radiation should be closely monitored for healing and adequate pain management, and must be restricted from self-mutilation. Surgery patients require pain and incision management, and may require nutritional support. Many palliative treatments are available for clients who do not choose definitive care.
It is important to be familiar with the resources available to the general practitioner in the area of the practice. Questions to investigate when choosing to include cancer treatment in private practice include:
• Where is the closest radiation facility, and what types of radiation are available?
o For a complete listing: https://cvmsecure.missouri.edu/vcs/equipment.aspx
• Who is the closest oncologist and what is the availability of that person?
o Try www.vetcancersociety.org
• What clinical trials are available within the area?
o Try www.vetcancersociety.org
• From whom can I order chemotherapy drugs and how long does it take to receive them?
• Do the chemotherapy drugs require special handling, storage or have a limited shelf life?
• Is my staff willing to participate in managing a cancer patient?
• What steps should be taken to properly handle a chemotherapeutic agent?
• What are the general costs of treatment options and what should I charge?
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.
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Henry CJ, Tyler JW, McEntee MC, et al. Evaluation of a bladder tumor antigen test as a screening test for transitional cell carcinoma of the lower urinary tract in dogs. Am J Vet Res 2003;64(8):1017-1020.
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Gillette EL, LaRue SM, Gillette SM. Normal tissue tolerance and management of radiation injury. Sem Vet Med Surg (Small Anim) 1995;10(3):209-213.