Veterinary oncology

ALSO KNOWN AS: Animal cancer management, companion animal cancer management, domestic animal cancer management

DEFINITION: Veterinary oncology is officially defined as studying tumors in animals, excluding humans. The most common usage of the term currently refers to the study and management of cancer in domestic and, more typically, companion animals. Most veterinary oncologists are trained primarily in treating cancer in dogs and cats. Many become involved in diagnosing and treating or directing the management of cancer in other companion species, such as birds, rodents, rabbits, horses, pet ruminants, aquatic animals, and exotic animals, in cooperation with the attending small-, large-, or exotic-animal veterinarian.

Veterinary oncology is informative for the study and treatment of cancer in people because some animals can serve as models of human neoplasia. These cancers can occur naturally or be experimentally induced. One example of naturally occurring cancer in animals possibly informing the treatment of humans involves the use of dogs as models of osteosarcoma in children since the tumors show very similar behavior in dogs and children, growing most often on the metaphysis and acting aggressively. Additionally, as in children, osteosarcomas in dogs are associated with rapid bone growth (in large-breed dogs), trauma such as bone fractures, and gender (male canines and humans are more susceptible), among other correspondences. Another example is the development of the canine melanoma vaccine at the University of Wisconsin School of Veterinary Medicine. Animals also serve as models of experimentally induced neoplasia or metastasis, such as nude mice that receive implants of cancerous human cells to test pharmaceutical interventions and study tumor biology.

Subspecialties: Veterinary oncology comprises veterinary medical and veterinary radiation oncology. Medical oncologists are diplomates of the American College of Veterinary Internal Medicine, and radiation oncologists are diplomates of the American College of Veterinary Radiology. An unofficial specialty is surgical oncology. This field has no diplomate status, but these veterinarians are trained in special surgical procedures and techniques to remove cancerous tissue from animals (mostly dogs and cats).

Cancers treated: The increase in life expectancy in companion animals due to vaccines, antiparasitic medications, and general improvements in health care has resulted in increased cancer incidence associated with advanced age. Some cancers are more prevalent in certain species and more responsive to treatment than others.

One of the most common blood cancers in domestic animals is lymphoma (lymphosarcoma), which is comparable to non-Hodgkin lymphoma in people. In dogs, the incidence rate varies depending on the breed, with Airedales, basset hounds, boxers, bulldogs, bullmastiffs, golden retrievers, Saint Bernards, and Scottish terriers believed to be among the most susceptible, and dachshunds and Pomeranians among the least. Incidence is estimated to be about 0.1 percent in susceptible breeds, making it the most common blood cancer in this species. This disease can generally be managed for a few years if caught early but not cured.

Lymphoma is even more prevalent in cats than in dogs or people, accounting for about 90 percent of feline blood cancers and about 30 percent of all feline tumors, and is associated with the feline leukemia virus (FeLV) and the feline immunodeficiency virus (FIV). Since the late 1990s, the FeLV vaccine has lowered the incidence of certain forms of lymphoma in cats. However, feline lymphoma is generally less responsive to treatment than the canine variety.

Mast cell tumors are among the most common canine cancers. They are most often found on the skin and are the most common skin malignancy in dogs. Certain breeds, such as boxers, bulldogs, and Boston terriers, have shown an increased incidence of this disease, which is amenable to treatment if caught early. An internal form of the disease is less responsive to treatment.

Other veterinary cancers treated include hemangiosarcoma, a very aggressive tumor of endothelial origin that affects the blood vessels and is seen mainly in dogs, most commonly in the heart, skin, or spleen. Greater prevalence has been noted in German shepherds, golden retrievers, and poodles. This condition generally has a very poor prognosis despite treatment.

Another cancer with a very poor prognosis is vaccine-associated feline sarcoma. This unusually aggressive but relatively rare sarcoma is believed to be promoted by inflammation accompanying the administration of certain vaccines and possibly other injected medications in cats. It has proven to be unresponsive to most treatment modalities, including surgery, and the location of the growth is a key prognostic factor. Vaccination-site guidelines issued by the American Association of Feline Practitioners (AAFP) have decreased the likelihood that an inoperable growth will develop on the trunk. If an aggressive sarcoma develops on the leg, the limb can be amputated, increasing the chance of a cure.

Another aggressive cancer, osteosarcoma, occurs primarily in large-breed dogs. It is also most successfully treated with surgery, generally amputation of the affected limb, but it is difficult to catch before it metastasizes, usually to the lungs.

Mammary cell tumors are the most common neoplasia in female dogs and are third after skin tumors and lymphoma in cats. About 40 to 50 percent of canine mammary tumors are benign, while about 80 to 90 percent of feline mammary tumors are malignant. The best treatment for dogs is prevention since spaying before the first heat virtually eliminates the risk of developing this tumor. Treating cats with progestin drugs increases their risk of developing mammary cancer. Complete surgical resection offers the best chance of a cure in dogs, while in cats, the disease has usually metastasized to the lungs by the time of diagnosis.

Other cancers commonly treated by veterinary oncologists are bladder cancers (transitional cell carcinoma most commonly), prostate cancer (dogs are the only nonhuman species with a significant occurrence of this neoplasia), melanomas, oral and cutaneous squamous cell carcinoma (common in cats), transmissible venereal tumors, and various other sarcomas, carcinomas, and round-cell tumors in cats, dogs, and other species.

Training and certification: For medical oncology, candidates for certification and diplomate status must apply for membership in the American College of Veterinary Internal Medicine (ACVIM). The college requires these candidates to be graduates of an accredited or approved veterinary school, licensed to practice veterinary medicine in a state, province, or other country, in satisfactory moral and ethical standing, and to pass both the general medicine and oncology certification examinations.

The general exam for medical oncology consists of two parts—comprehensive, which tests knowledge of general medicine common to all species, and a more specialized section oriented toward the individual’s field of interest, that is, small animals (mostly dogs and cats) or large animals (mostly horses and ruminant species). To take the general exam, qualified veterinarians must have completed an approved one-year internship (or equivalent) in either small- or large-animal medicine and surgery and a two-year residency in veterinary oncology under the direct supervision of a diplomate of the ACVIM. To take the certification exam in veterinary medical oncology, veterinarians must also submit a letter from their oncology training program supervisor, other letters of recommendation, a case log, and publications.

For radiation oncology, a candidate for certification must be a graduate of an accredited or approved veterinary school, maintain proper ethical and moral standing, be licensed to practice veterinary medicine and surgery in one or more states, and have completed a one-year internship in small- or large-animal medicine and surgery. The candidate must have completed a veterinary radiation oncology program approved by the American College of Veterinary Radiology (ACVR) executive council. Additionally, the candidate must have completed a two-year residency training program in veterinary oncology, submitted references, and passed the written and oral general and radiation oncology exams. The written exam tests the candidate’s knowledge of general veterinary oncology, radiation physics and dose calculations, radiation biology, and clinical aspects of radiation oncology.

Surgical oncologists are the American College of Veterinary Surgeons (ACVS) diplomates. The unofficial designation of “surgical oncologist” follows the successful completion of a fellowship in surgical excision of tumors and cancerous tissue from animals.

Services and procedures performed: Veterinary oncologists diagnose, stage, and plan treatment strategies for their animal patients. Most of the time, they are not expecting or planning a cure for the malignant or metastatic conditions. Rather, they want to effect a remission or control the tumor’s growth to provide the animal with a good quality of life. Generally, extremely toxic treatment modalities are avoided in animals because of their shorter life spans, high costs, inconvenience to owners (traveling to distant treatment centers, caring for a sick pet, and so on), and the fact that animals primarily live in the moment and cannot rationalize discomfort.

For diagnosis and staging, veterinary oncologists examine blood work (complete blood count, blood chemistries); employ various imaging modalities, depending on need, availability, and cost (X-rays, magnetic resonance imaging, computed tomography); and analyze the results of biopsies, cytology, and bone-marrow aspirates. A histopathologist graded the cancer cells for conditions like mast cell tumors. Not all tests are conducted on all animals. Staging is critical to planning the treatment process and forming a prognosis. After staging, veterinary oncologists often work with surgeons if the tumor is resectable.

If the cancer has metastasized or spread to distant lymph nodes, most medical oncologists will plan adjunctive chemotherapy. Chemotherapy is also prescribed for diffuse cancers, such as lymphoma, that are not amenable to surgical intervention. Growths that cannot be resected or can be only partially resected are often treated with local radiation therapy directed by a radiation oncologist. Some veterinary referral or academic centers offer orthovoltage or megavoltage units (linear accelerator, cobalt, or cesium). However, others outsource these therapies to human cancer treatment clinics that work directly with the veterinary oncologist. Other forms of radiotherapy, such as brachytherapy or systemic radiotherapy (for cats' thyroid tumors), are also offered to veterinary patients.

Animal oncology employs almost every chemotherapeutic agent used for human chemotherapy, and specially trained veterinary nurses often administer the drugs under the direct supervision of the oncologist. The veterinary oncologist can also offer therapy for pain and the side effects of treatment, such as nausea and vomiting following chemotherapy or oral mucositis following radiation therapy. The oncologist follows the animal’s progress throughout treatment and modifies the protocol in case of complications, such as severe bone marrow suppression. The veterinary oncologist should provide a good idea of the animal’s prognosis and help the client make informed decisions about treatment options while considering costs and quality of life for the animal and owner.

Related specialties and subspecialties: Veterinary oncologists and clients work closely with practitioners in the fields of veterinary pathology, veterinary clinical pathology, veterinary dermatology, veterinary emergency and critical care, veterinary small-animal internal medicine, veterinary large-animal internal medicine, and veterinary surgery to treat and diagnose neoplastic conditions.

Bibliography

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