A diagnosis of cancer is a pet owner’s greatest fear. Part of this anxiety is based in fear of the unknown. Canine and feline cancer is oftentimes misunderstood, and pet owners may not be aware of the great advances in the specialty of veterinary oncology, which has provided improvements not only in the quality of a pet’s life but also potentially adding quantity to that pet’s life by slowing cancer spread or by even offering a cure.
The concept of cancer can be overwhelming, so this November, The Meowing Vet has provided pet owners information on the basics of cancer in dogs and cats for National Pet Cancer Awareness Month. This article teaches owners common oncology terminology and informs them of the common cancers and clinical signs of cancer in pets as well as how cancer is diagnosed and treated in veterinary medicine (including ways in which this differs from human oncology).
Scroll down to learn more about the basics of cancer in dogs and cats…
Terminology
Your veterinarian can feel a mass in Fido’s abdomen. Your head’s spinning at the news. “Is this the same things as a tumor? Does this mean that Fido has cancer?!” Before getting too worked up, we need to differentiate the following terms and provide definitions to some key words that you may hear in a discussion with your vet.
- Mass: a nondescript term which may be applied to various swellings, enlarged organs or other bodily structures, as well as tumors (i.e. a tumor is a mass, but a mass isn’t necessarily a tumor)
- Tumor: an abnormal growth (or mass) in or on the body caused by uncontrolled growth or irregular division of cells of various origins; may be benign or malignant
Neoplasm: synonym for tumor- Neoplasia: the process by which unregulated, abnormal cell growth can cause a tumor (or neoplasm) to develop; not necessarily synonymous with cancer, as neoplasia may be either benign or malignant (only malignant neoplasia is used interchangeably with the term cancer)
- Benign: characterization of a tumor that tends to be slow-growing and does not invade into surrounding structures or spread (i.e. metastasize) throughout the body; i.e. a relatively “good” tumor in comparison to a malignant tumor
- Malignant: characterization of a tumor that grows quickly, invades into surrounding tissues and structures, and has the ability to spread (metastasize) throughout the body; i.e. a “bad” tumor
- Cancer: the presence of a malignant neoplasia (i.e. a neoplasm may be benign or malignant, and only a malignant neoplasm denotes the presence of cancer); cancer is dangerous because its spread can impair the normal functions of various organs throughout the body, causing them to fail
Metastasis: the spread of malignant cancer cells into the lymphatic system or bloodstream, where they can then spread throughout the body; the lungs are a common site for metastasis; related terms: metastatic (adjective), metastasize (verb)- Grade: characteristics of tumor cells to determine how rapidly they may grow and spread
- Stage: the classification of cancer based on the size and local infiltration of a tumor as well as whether or not is has metastasized to other organs; characterized as stages 0-V, with each stage carrying a subsequently worse prognosis
- Stage 0: no cancer, just the presence of some abnormal pre-cancerous cell(s) which may have the propensity to develop into cancer
- Stage I: early stage cancer in which only a single small tumor involving a single tissue or organ is present (excluding bone marrow)
- Stages II & III: a larger cancer that has spread to nearby organs or lymph nodes
- Stage IV: advanced or metastatic cancer which has spread throughout the body
- (Stage V for lymphoma/leukemia: when lymphoma has spread into the blood or bone marrow, manifesting as leukemia)
- Paraneoplastic syndrome: a systemic disorder caused by substances produced by cancer or secondary to the presence of a specific tumor type; examples of paraneoplastic syndromes include anemia (low blood cell count) and hypercalcemia (high blood calcium)
Stable disease: cancer that has not entered into partial remission yet has not progressed (i.e. grown or spread) after 4 weeks of treatment- Remission: a state in which a tumor targeted by treatment begins to be reduced in size; may be either partial remission or complete remission; not to be mistaken with a cancer that has been cured (i.e. just because a patient has entered remission does not mean that he or she is cancer-free)
- Surgical margins: the edges of a surgically removed tumor, which are analyzed by a pathologist to see if any cancer cells persist at the edges of the excised tumor (i.e. “didn’t get it all”) or if the margins are free/clear of cancer cells (the latter of which may mean that the tumor was fully removed during surgery if there has been no further spread or metastasis)
- Oncology: the study of cancer
- Oncologist: a veterinary oncologist is a veterinarian who has undergone special residency training to become a board-certified specialist in the discipline of oncology; if a diagnosis of cancer is made by your primary veterinarian (i.e. general practitioner), he or she may refer your pet to an oncologist for specialized diagnostics and treatment
- Pathologist: a veterinary pathologist is a veterinarian who has undergone advanced training to specialize in the discipline of pathology, the study of the causes and effects of diseases; clinical pathologists analyze laboratory samples for diagnostic purposes (i.e. your primary vet may send diagnostic samples to a veterinary pathologist for expert analysis if cancer is suspected in your pet)
What causes cancer? What is its prognosis?
The development of cancer may arise from a multitude of factors, including heredity as well as hormonal or environmental variables. Therefore, determining the exact inciting cause of cancer for most patients is difficult. Because so many cancer types exist, the prognosis for each type differs, and each individual animal afflicted with a particular type may also face a different outcome based upon the time in the disease process in which the cancer was diagnosed and treatment was initiated, the stage of the cancer and how quickly it is growing, the development of worrisome side effects during treatment, and the presence of any co-existing diseases that may complicate the cancer and its treatment options.
Signs That Your Pet May Have Cancer
Pet owners should take note of the following top 12 warning signs, which are common in the emergence of cancer in a dog or cat.
- Abnormal or rapidly enlarging growth or swelling, including a distended abdomen or swollen lymph nodes (your pet’s external lymph nodes may be enlarged in the neck, chest, under the front limbs, in the groin, and behind the knees)
- Weight loss
- Loss of appetite or vomiting
- Difficulty eating or swallowing
- Lethargy and intolerance to exercise
- Coughing or difficulty breathing
- Lameness
- Changes in urination or defecation, including straining to urinate, more frequent urination, or developing diarrhea or constipation
- Abnormal bleeding or discharge
- Wounds that do not heal
- Malodor
- Pale gums
Diagnostics
Your description of your pet’s behavior at home, paired with a thorough physical examination performed by your veterinarian, are the most important tools to begin a cancer investigation. Based upon abnormalities detected upon your pet’s physical exam, your vet may wish to perform bloodwork, a urinalysis, and imaging such as radiographs (X-rays) or ultrasound. If a discernible mass is detected, direct sampling from the inside of that structure is typically advised, allowing cells to be sampled and analyzed from within it. Tests include a fine needle aspirate or biopsy. 
A fine needle aspirate (FNA) is performed by placing a sterile needle directly into the tissue or organ of interest to extract cells. Cytology is then performed when the isolated cells are analyzed under a microscope to examine them for any abnormalities. In contrast, a whole tissue chunk (and not just individual cells) is removed during a biopsy procedure via a sterile needle, punch technique, or surgical removal (excisional biopsy), thereby allowing for more details to be ascertained from the collected tissue via a process called histopathology. Surgical removal and histopathologic testing of an enlarged lymph node may also be recommended in some scenarios. Furthermore, some blood cancers may arise from the bone marrow or else other cancer types may spread to it, so sampling bone marrow via an aspirate or core biopsy may also be performed in certain cases. Other specialized tests (such as flow cytometry, the CADET® BRAF urine test for canine bladder and prostate cancers, etc.) may be performed to gather further information on particular cancer types.
Once a cancer diagnosis is reached, additional diagnostic tests may be recommended before treatment can be initiated. For instance, prior to the administration of the chemotherapy drug doxorubicin (Adriamycin®) which can cause side effects to the heart, a full cardiac workup (including an echocardiogram and electrocardiogram/ECG) should be performed to monitor heart health. During treatment, frequent re-checks are often required to ensure that the treatment choice is safe and effective, the most common of which is bloodwork (CBC) to monitor for leukopenia (low white blood cell count), a common side effect of many chemotherapy drugs.
General Treatment Options for Cancer
When a pet owner is met with news that a beloved dog or cat has cancer, he or she may immediately balk at the thought of putting a pet through treatment due to the notoriously unpleasant side effects that cancer treatments (particularly, chemotherapy) cause in human patients. However, even if we use similar treatment options in veterinary medicine, cancer is managed a bit differently in our pets. The goal of cancer treatment in human medicine is to result in a complete cure in most cases. Therefore, human cancer patients are often met with an artillery of heavy-duty treatments in hopes of achieving that cure; therefore, more side effects may arise.
However, because our canine and feline patients have much shorter lifespans than us humans, the goal with veterinary oncology may not to always achieve a cure; instead, the goal is often to improve quality of life and lengthen lifespan as much as possible with hopes that the cancer at least enters into remission. If remission (but not complete eradication of the neoplasm) is achieved, the cancer may grow so slowly that it does not interfere greatly with the animal patient’s expected natural lifespan. Since achieving a cure may not always be the goal for our pets who are afflicted by cancer, our treatment protocols are typically less intense than for human counterparts, thus causing less nasty side effects. In fact, the majority of patients receiving chemotherapy will not experience any major side effects (up to 80% of dogs receiving chemo do not develop any adverse effects), greatly improving our pets’ quality of life while they battle it out with cancer. That said, certain treatment options and various chemo drugs can result in specific side effects. Most treatment methods work by killing cancer cells, but healthy cells can sometimes become damaged inadvertently, resulting in adverse reactions.
The categories for cancer treatment are:
Surgery: surgical removal (if possible) of the cancerous organ/tissue or amputation of a limb affected by neoplasia; may be curative in some cases if the cancer has not spread locally or metastasized and if surgical margins are clear- Radiation therapy (a.k.a. radiotherapy): a tumor is exposed to high doses of radiation in hopes of killing cancerous cells; may be used with an intent to cure, palliate (reduce pain), or shrink tumors prior to/after surgery
- Chemotherapy (“chemo”): the use of cytotoxic (cell-killing) medications to destroy certain cancerous cells
- Targeted biotherapy: newer treatment options that are emerging with the goal of targeting only cancerous cells while preserving nearby healthy cells which may otherwise be damaged if radiation or chemo were used
Other options:
- Adjunctive therapies: medical intervention that is paired with the traditional cancer treatments to minimize side effects experienced by the dog or cat undergoing treatment for cancer; includes hydration management, nutritional support, anti-nausea medication, and pain management
- Palliative care: therapies (similar to those also used in adjunctive therapies) to keep a patient comfortable and improve quality of life when cancer treatment is no longer an option and until the proper time for euthanasia is decided upon
What about prednisone?
An old adage in the veterinary community states, “No animal should die without first the attempt of prednisone.” While this phrase may not necessarily apply to all cases, prednisone (and its relative, prednisolone – both corticosteroid drugs), is a common mainstay in many cancer treatment plans. Prednisone primarily works by reducing inflammation and swelling, therefore reducing pain and discomfort while improving a pet’s appetite. Prednisone may even act as an affordable chemotherapeutic agent for certain cancers, such as feline intestinal lymphoma. In other cases, it may be an important adjunctive therapy or palliative option to keep pets comfortable and improve quality of life when cancer is incurable. or other treatment options are not possible.
Types of Tumor Classes
As discussed, tumors arise when a population of cells starts to go haywire, dividing and growing in an unregulated manner. Different cell lineages may be culpable, causing one of the following tumor classes to develop:
- Carcinoma: arise from epithelial cells, which comprise the skin and lining of internal organs
- Sarcoma: arise from mesenchymal cells, which makes up connective tissue of bone and soft tissues (muscle and fat)
- Round cell tumor (a.k.a. discrete cell tumor): may arise from a variety of discrete homogenous cell populations; make up approximately 90% of neoplasms in dogs and cats, including lymphoma, histiocytoma, mast cell tumor, transmissible venereal tumor, plasma cell tumor, melanoma, multiple myeloma, etc.
- Neuroendocrine tumor (a.k.a. naked nuclei neoplasm): arise from cells that release hormones into the bloodstream in response to nerve impulses received from the nervous system (including thyroid, adrenal, pancreatic, chemodectoma, and carcinoid cancer types)
The Most Common Cancers in Dogs and Cats
Cancer is a relatively common disease in pets, with most cancer types increasing in prevalence as a dog or cat ages. Certain cancer types are more common in dogs vs. cats, as explained below. Furthermore, certain breeds may be predisposed to the development of specific cancer types. Though the cancers addressed here are the most common cancer types affecting dogs and cats, many other cancers exist, and many different cancer cell lineages can develop from a multitude of organs and organ systems.
Dogs
Between 25-33% of dogs will be affected with cancer at some stage. The most common canine cancers include:
Lymphoma (also called lymphosarcoma): a cancer involving lymphocytes, a type of white blood cell; once spread to the bloodstream, it is termed leukemia- Hemangiosarcoma: a cancer derived from the lining of blood vessels; most commonly affects the spleen, skin, heart, and liver; (not to be confused with benign hemangioma)
- Mast cell tumors: arises from mast cells, a white blood cell that participates in allergic reactions (may affect the skin or internal organs)
- Osteosarcoma: primary bone cancer
- Melanoma: arises from pigmented melanin cells and most commonly affects the oral cavity
- Mammary gland adenocarcinoma: “breast cancer” is more likely to arise in un-spayed (i.e. intact) female dogs (spaying a dog before her first heat cycle greatly reduces her risk of developing mammary gland cancer later in life); can also affect un-spayed female cats
- Primary lung cancer: these pulmonary tumors begin in the lungs themselves rather than develop secondary to metastasis of another tumor in the body
- Anal gland adenocarcinoma: cancerous tumors of the anal sacs
What about lipomas?
Lipomas are benign fatty tumors and are quite common in dogs as they age, arising primarily under the skin or between muscle tissue. Occasionally, cats can be affected, too. Most lipomas require no treatment, but some can grow large enough that they impede movement, cause discomfort, or crowd internal organs, thus requiring surgical removal. Rarely, a benign lipoma can turn malignant, becoming a liposarcoma (a true cancer) that also requires treatment. Differentiating a lipoma from a liposarcoma involves a biopsy.
Dog Breed Predispositions include, but are not limited to:
- Boxer dog: mast cell tumors, lymphoma
- Golden retriever, Labrador retriever, German shepherd, Bichon frise: hemangiosarcoma
- Rottweiler, Great Dane, and other large or giant breed dogs: osteosarcoma
- Chow chow and Shar Pei: oral melanoma
- Scottish terrier: transitional cell carcinoma of the bladder
Cats
Though cancer is more common in dogs than in cats, felines (particularly older cats) are most susceptible to developing the following cancer types:
Lymphoma: intestinal lymphoma is one of the most common cancers, mainly occurring in older cats- Soft tissue sarcomas: a malignant tumor arising from connective tissue, such as muscle; vaccine-associated sarcomas (a.k.a. feline injection-site sarcomas, or FISS) are overall uncommon, but some sarcomas may arise secondary to post-inflammatory effects following certain injections administered to cats (primarily, vaccines against Rabies virus and feline leukemia virus/FeLV, yet only 1-2 out of every 10,000 cats receiving these vaccinations go on to develop an associated tumor)
- Squamous cell carcinoma: a skin cancer triggered by exposure to the sun’s harmful UV rays; cats with white ears are at increased risk (Interesting fact: Although squamous cell carcinomas are associated with UV light exposure in humans and cats, squamous cell carcinomas that arise in dogs are not associated with UV rays.)
What about feline leukemia virus (FeLV)? Does it really cause leukemia in cats?
Though infection with the feline leukemia virus (FeLV) can cause cats to become very ill, primarily due to impairment of the immune system, most infected felines do not develop cancer consequentially. However, around 10% of FeLV-positive cats (mainly younger cats) will develop some form of cancer if FeLV transforms into the feline sarcoma virus, primarily resulting in lymphoma (and leukemia if it spreads to the bloodstream) or sarcoma.
While not all cancers may result in a complete cure for your dog or cat, not all cancer diagnoses automatically mean a death sentence for your pet either if met with prompt detection and appropriate treatment. The realm of veterinary oncology continues to develop, offering better treatment options to improve the quality of life and lifespans of our pets.
For additional information on the latest research involving cancer treatment in dogs and cats, please visit the following resources:
- Animal Cancer Foundation
- Veterinary Cancer Society
- The National Canine Cancer Foundation
- Vet Specialists
– Maranda Elswick, DVM
If your pet is displaying the clinical signs discussed above, The Meowing Vet urges you to schedule an appointment with your local veterinarian.
