November is Pet Cancer Awareness Month, a subject we return to each year at Boundary Bay Veterinary Specialty Hospital (BBVSH). Our board-certified veterinary oncologists regularly diagnose and treat patients with cancer, making this an important opportunity to help pet owners better understand a disease that can be both complex and frightening. This year, we’re taking a closer look at the biology behind cancer: what happens when normal cells become cancerous, how does cancer spread, and why does understanding a particular cancer matter for treatment?
As Dr. Sarah Charney, DVM, DACVIM (Oncology), DACVR (Radiation Oncology), board-certified veterinary medical and radiation oncologist and co-owner of BBVSH, explains, cancer isn’t a single disease. Understanding the cancer process can help pet owners make sense of why veterinarians recommend certain tests, why cancers can behave very differently and how that information helps guide the choices that come next. For families facing a cancer diagnosis, that knowledge can be an important part of replacing some of the fear with understanding — and hope.
What Happens When Normal Cells Become Cancerous?
Your pet’s body is made up of trillions of cells. Normally, those cells follow carefully regulated processes controlling when they grow, divide and die. Cancer develops when genetic changes disrupt those controls, allowing abnormal cells to survive and reproduce when they normally would not.
This process is known as malignant transformation. As cancer develops, cells can acquire characteristics that allow them to continue dividing, resist apoptosis — the programmed cell death that normally removes damaged or unnecessary cells — and, in some cancers, invade surrounding tissues.
Over time, abnormal cells may form a tumor. Benign tumors don’t metastasize, or spread to distant parts of the body, although they can still cause significant problems because of their size, location or effects on surrounding structures. Malignant tumors — cancers — can invade nearby tissues and may spread elsewhere in the body.
Cancer isn’t one disease. There are many different cancers arising from different types of cells and behaving in very different ways. That’s why knowing that a pet “has cancer” is only the beginning of understanding the disease.
Why Do Pets Develop Cancer?
There usually isn’t one simple answer. Cancer develops through genetic changes within cells, and both inherited and acquired factors can contribute to those changes.
Genetics can influence cancer risk, which is one reason certain cancers occur more frequently in some breeds. Age is also important: as pets get older, their cells have had more time to accumulate genetic changes. Environmental exposures, some viruses and other biological factors may contribute to particular cancers as well.
For most individual pets, however, it isn’t possible to point to one event and say, “that’s what caused the cancer.” Once cancer is suspected or diagnosed, the more useful questions become: What type of cancer does my pet have? How is it behaving? Has it spread? And what can we do about it?
How Can Cancer Grow and Spread?
For a tumor to continue growing, its cells need oxygen and nutrients. Some tumors stimulate the development of new blood vessels through a process called angiogenesis, creating a blood supply that helps support continued growth.
Some cancers remain confined to the place where they began. Others acquire characteristics that allow them to invade nearby tissues and metastasize — spread from the original tumor to other areas of the body. Cancer cells may enter the bloodstream or lymphatic system, travel to another location and establish new tumors.
Different cancers have different patterns of spread, and understanding that biological behavior helps veterinarians determine which areas of the body should be evaluated. This is one reason staging is an important part of assessing many cancers.
Depending on the cancer, staging may involve bloodwork, sampling lymph nodes or other tissues, X-rays, ultrasound, CT or other diagnostic imaging. Staging assesses how extensive the cancer is within the patient, providing information that can help the oncologist evaluate prognosis and determine which treatment options may be appropriate.
How Do Veterinarians Identify the Type of Cancer?
Finding a lump or abnormality doesn’t tell us everything about the cells inside it. A family veterinarian or veterinary specialist may begin the investigation with a physical examination, bloodwork, imaging or a fine needle aspirate, which collects cells for cytology — examination of individual cells under a microscope.
In other cases, a biopsy is needed to obtain a larger tissue sample for histopathology. This allows a veterinary pathologist to examine the cells as well as how they are arranged within the tissue. For some cancers, histopathology can also provide a grade, which describes characteristics of the tumor and may offer information about how aggressively it is expected to behave.
It’s useful to distinguish grade from stage. Grade describes characteristics of the tumor itself; stage describes the extent of cancer within the patient. Depending on the type of cancer, both can provide important information when considering prognosis and treatment.
Some cancers may also benefit from molecular or genetic testing that further characterizes the disease. BBVSH patient Lucy the Wonderdog, for example, developed more than one type of cancer over several years. Molecular diagnostics provided more precise information about her cancers and helped inform treatment decisions as her care progressed.
Read Lucy the Wonderdog’s story and learn how molecular diagnostics are changing cancer care for pets.
At BBVSH Bellingham, our board-certified oncologists also have access to a range of cancer diagnostics, and our Oncology team can work with other BBVSH specialty services when a patient’s diagnosis or treatment requires a multidisciplinary approach.
Why Isn’t Every Cancer Treated the Same Way?
Because cancers don’t all behave the same way. The type of cells involved, whether the cancer is localized or has spread, its biological behavior and the individual patient can all influence treatment decisions.
For a localized tumor, surgery may offer an opportunity to remove the cancer. Chemotherapy uses medications to kill cancer cells or interfere with their ability to grow and divide and can be particularly useful for cancers that are systemic — affecting the body more broadly — or have a significant risk of metastasis. Radiation therapy uses targeted ionizing radiation to damage cancer cells and may be used alone or in combination with surgery or chemotherapy.
Some cancers may also be treated with targeted therapies, which act on particular molecular features or pathways involved in cancer growth, or immunotherapy, which uses the immune system to recognize or respond to cancer.
Treatment doesn’t always mean pursuing remission or cure. Palliative care focuses on controlling pain and other symptoms and maintaining quality of life and may be an appropriate choice for some pets and families.
There is no one-size-fits-all approach. The treatment plan depends on factors such as tumor type, grade and stage, expected biological behavior, your pet’s overall health, the potential benefits and adverse effects of treatment, and your family’s goals.
Is Chemotherapy Different for Pets?
This is one of the questions veterinary oncologists hear frequently. Many people understandably associate chemotherapy with the difficult side effects experienced by friends or family members undergoing human cancer treatment. In veterinary oncology, quality of life is a central consideration when treatment decisions are made.
Side effects can occur, including nausea and decreased appetite, and patients are monitored so treatment can be adjusted when needed. In general, pets tend to tolerate chemotherapy side effects better than people. The goal isn’t simply more time — it is more good days.
There are also many misconceptions about what cancer treatment will mean for a pet and their family, from concerns about chemotherapy side effects to assumptions about prognosis.
What Cancer Warning Signs Should Pet Owners Know?
Cancer can affect almost any part of the body, so there isn’t one set of symptoms. A new or changing lump, unexplained weight loss, decreased appetite, persistent sores or swelling, unexplained bleeding, difficulty breathing or swallowing, recurring vomiting or diarrhea, persistent coughing, pain or a noticeable decrease in normal activity are all changes worth discussing with your family veterinarian.
These signs don’t necessarily mean your pet has cancer. Many can occur with other medical conditions, but a persistent or unexplained change is worth investigating.
What Comes Next After a Cancer Diagnosis?
A cancer diagnosis can feel like the moment everything changes. But the word cancer alone doesn’t tell you what happens next. The next step is understanding your pet’s particular cancer: where it started, what type it is, its grade and stage when those measures apply, how it is expected to behave, and what treatment options are available.
Your family veterinarian remains an important part of that process and may recommend referral to a board-certified veterinary oncologist for further diagnostics and treatment planning. At BBVSH Bellingham, our board-certified Oncology team sees patients for cancer consultation, diagnosis and treatment, developing individualized plans based on the cancer and the needs of the patient and family.
For some pets, that means surgery, chemotherapy, radiation or a combination of treatments. For others, it may mean managing symptoms and prioritizing comfort. Families also often have questions about caring for a pet during cancer treatment, including what role nutrition may play.
[COMING SOON: Nutrition and Cancer — What Should You Feed a Pet With Cancer?]
A cancer diagnosis can be frightening, but it doesn’t tell you everything about what lies ahead. Learning more about your pet’s particular cancer — its type, biology and behavior — can replace some of that fear with a clearer understanding of the options available.
There can be hope in that knowledge: hope for treatment, hope for more good days, or simply the confidence to make informed decisions centered on your pet’s comfort and quality of life.
Understanding Cancer in Dogs and Cats: Frequently Asked Questions
What happens in a dog’s or cat’s body when cancer develops?
Cancer develops when genetic changes disrupt the normal controls that tell cells when to grow, divide and die. Abnormal cells can continue reproducing when they normally would not, resist programmed cell death and, in some cancers, invade surrounding tissues or spread to other parts of the body.
Why do pets develop cancer?
There usually isn’t one single cause. Cancer develops through genetic changes within cells, and factors such as inherited genetics, age, environmental exposures, some viruses and other biological influences may contribute. For most individual pets, it isn’t possible to identify one specific event that caused their cancer.
Are all tumors in dogs and cats cancerous?
No. Benign tumors don’t metastasize, or spread to distant parts of the body, although they can still cause medical problems because of their size, location or effects on surrounding tissues. Malignant tumors are cancers that can invade nearby tissues and may have the ability to spread elsewhere in the body.
How does cancer spread in pets?
The spread of cancer from its original location is called metastasis. Cancer cells may invade nearby tissues or enter the bloodstream or lymphatic system and travel to other parts of the body. Different cancers have different patterns and risks of metastasis, which is one reason identifying the specific cancer is so important.
What does staging cancer in a dog or cat mean?
Cancer staging evaluates how extensive the disease is within the patient and whether it has spread beyond its original location. Depending on the type of cancer, staging may include bloodwork, lymph node or tissue sampling, X-rays, ultrasound, CT or other diagnostic imaging. This information can help a veterinary oncologist assess prognosis and determine appropriate treatment options.
What’s the difference between cancer grade and cancer stage?
Grade describes characteristics of the tumor itself and, for some cancers, can provide information about how aggressively it may be expected to behave. Stage describes how extensive the cancer is within the patient’s body. Depending on the type of cancer, both can be important when considering prognosis and treatment.
How do veterinarians find out what type of cancer a pet has?
Veterinarians may use a combination of physical examination, bloodwork, diagnostic imaging and cell or tissue sampling. A fine needle aspirate collects cells for cytology, while a biopsy provides tissue for histopathology. In some cases, molecular or genetic testing can provide additional information about the cancer and help guide treatment decisions.
Why aren’t all cancers in pets treated the same way?
Cancer isn’t one disease. Different cancers originate from different types of cells and can have very different biological behaviors. Treatment decisions may depend on the cancer type, grade and stage, whether it has spread, the pet’s overall health, expected benefits and adverse effects of treatment, and the family’s goals.
What cancer treatments are available for dogs and cats?
Depending on the cancer, treatment may include surgery, chemotherapy, radiation therapy, targeted therapies, immunotherapy, palliative care or a combination of approaches. Treatment is individualized because the most appropriate approach depends on the particular cancer and patient.
Do dogs and cats tolerate chemotherapy better than people?
In general, pets tend to tolerate chemotherapy side effects better than people, although side effects such as nausea and decreased appetite can occur. Quality of life is a central consideration in veterinary oncology, and patients are monitored so treatment can be adjusted when needed. The goal is to balance cancer treatment with maintaining comfort and more good days.
What signs of cancer should I watch for in my dog or cat?
Possible warning signs include a new or changing lump, unexplained weight loss, decreased appetite, persistent sores or swelling, unexplained bleeding, difficulty breathing or swallowing, recurring vomiting or diarrhea, persistent coughing, pain or changes in normal activity. These signs can also occur with many non-cancerous conditions, so persistent or unexplained changes should be evaluated by a veterinarian.
When should my pet see a veterinary oncologist?
Your family veterinarian may recommend a veterinary oncologist when cancer is suspected or diagnosed and additional diagnostics, staging or specialized treatment planning would be helpful. A board-certified veterinary oncologist can help identify and characterize the cancer, explain how it may behave, discuss treatment and palliative-care options, and develop an individualized plan for the patient.
Is veterinary oncology available in Bellingham, Washington?
Yes. Boundary Bay Veterinary Specialty Hospital in Bellingham sees oncology patients, with board-certified veterinary oncologists providing cancer consultations, diagnosis and treatment. The Oncology team can also work with other BBVSH specialty services when a pet’s cancer care requires a multidisciplinary approach.
What should I do if my pet has just been diagnosed with cancer?
Start by gathering information about your pet’s particular cancer rather than assuming that the diagnosis alone determines what comes next. Understanding where the cancer originated, what type it is, its grade and stage when applicable, how it is expected to behave and which treatment options are available can help you and your veterinary team make informed decisions centered on your pet’s quality of life.
