Dogs: Warning Signs & Types of Cancer

Warning Signs in Dogs

Early Detection is Key

Pets have become members of our families, and as we take better care of them, they are living happier, longer lives. Despite this, or perhaps because of this, cancer is one of the leading causes of death in dogs and cats. Early detection is key to a better outcome, and this is why we have come up with the 10 warning signs of cancer that every pet owner should know.

Oral Odor

Oral tumors do occur in pets and can cause a pet to change its food preference (i.e. from hard to soft foods) or cause a pet to change the manner in which he/she chews their food. Many times a foul odor can be detected in pets with oral tumors. A thorough oral examination with radio­graphs or CT scan, requiring sedation, is often necessary to determine the underlying cause.

Straining to Urinate

Straining to urinate and blood in the urine usually indicate a urinary tract infection, however, if the straining and bleeding do not resolve rapidly with antibiotics or are recurrent, cancer of the bladder may be the underlying cause. Cystoscopy or other techniques that allow a vet­erinarian to take a biopsy of the bladder are useful and sometimes necessary to establish a definitive diagnosis in these cases.


Unexplained lameness (especially in large or giant breed dogs) is a very common sign of bone cancer. Radiographs of the affected area are useful for detecting cancer of the bone.


A dry, non-productive cough in an older pet should prompt chest radiographs to be taken. This type of cough is the most common sign of lung cancer. Please remember there are many causes of cough in dogs and cats.

Unexplained Bleeding

Bleeding from the mouth, nose, gums or blood in the urine or stool, that is not due to trauma should be examined. Although bleeding disorders do occur in pets, they usually are discovered at a younger age. If unexplained bleeding starts when a pet is old, a thorough search should be undertaken.

Chronic Vomiting or Diarrhea

Unexplained vom­iting or diarrhea should prompt further investiga­tion. Tumors of the gastrointestinal tract can often cause chronic vomiting and/or diarrhea. Radio­graphs, ultrasound examinations and endoscopy are useful diagnostic tools when this occurs.

Chronic Weight Loss

When a pet is losing weight and you have not put your pet on a diet, you should have your pet checked. This sign is not diag­nostic for cancer, but can indicate that something is wrong. Many cancer patients have weight loss.

Abdominal Distension

When the “stomach” or belly becomes enlarged rapidly, this may suggest a mass or tumor in the abdomen or indicate bleed­ing that is occurring in this area. A radiograph or an ultrasound of the abdomen can be very useful in this situation.

An Enlarging or Changing Lump

Any lump on a pet that is rapidly growing or changing in texture or shape should have a biopsy. Lumps belong in biopsy jars, not on pets.

Swollen Lymph Nodes

These “glands” are located all throughout the body but are most easily detect­ed under the jaw or behind the knee. When lymph nodes are enlarged they can suggest a common form of cancer called lymphoma. A biopsy or cytol­ogy of these enlarged lymph nodes can aid in the obtaining a diagnosis.

Common Cancers in Dogs

Thyroid Carcinomas in the Dog

Thyroid tumors account for 1.2 % to 3.8 % of all tumors in the dog and typically develop in older dogs with a median age of 9-11 years. There is no gender predisposition but Golden retrievers, Boxers, and Beagles are over represented. Most thyroid carcinomas are non-functional (meaning they don’t produce thyroid hormones); 60% of patients have normal thyroid function; 30% are hypothyroid (under active thyroid); 10% are hyperthyroid (over active thyroid). Approximately 30-40% of thyroid carcinomas will have already metastasized (spread) at the time of diagnosis and ~80% will ultimately develop metastasis.

Thymoma in Dogs

Thymomas are rare tumors that arise from the epithelium (lining) of the thymus gland in the dog and cat. They are typically diagnosed in older animals, with the median age in dogs being 9 years and in cats it is 10 years. There is no breed predilection but medium and large dogs are overrepresented. Thymomas are considered benign or malignant based on their clinical features rather than on histologic (under the microscope) features. Benign thymomas do not invade into adjacent structures within the chest cavity while malignant thymomas do invade adjacent structures

Soft Tissue Sarcomas in Dogs

Soft tissue sarcomas (STS) make up a large category of tumors that arise from connective tissue. This category includes tumors of fibrous tissue, fat, smooth muscle, nerves, and lymphatic vessels. The diagnosis of soft tissue sarcoma includes fibrosarcomas, malignant peripheral nerve sheath tumors, histiocytomas, myxosarcomas, liposarcomas, lymphangiosarcomas, and undifferentiated sarcomas. STS comprise ~15% of all skin/subcutaneous (under the skin) tumors. These tumors are typically very invasive to the surrounding tissue but generally have a low risk of spreading (metastasis).

Primary Lung Tumors in Dogs

Primary lung tumors (cancer originating in the lung) are uncommon in dogs and account for less than 1% of all tumors. It is most common in older dogs, but no specific breed or sex is predisposed. They are almost always malignant (invasive with the potential to spread).

Multiple Myeloma in Dogs

Multiple Myeloma (MM) is a cancer of plasma cells, which are specific types of B-lymphocytes (white blood cells) which produce antibodies as part of the body’s immune system. Multiple Myeloma is a relatively uncommon cancer and there is no gender or breed predilection in dogs. The average age at the time of diagnosis is 8-9 years


Melanoma is the most common tumor found in the mouth of dogs, and the second most common tumor found is on the digits (toes). There is a predisposition for male dogs and certain breeds seem to be overepresented, including Scottish terriers, Cocker Spaniels, Gordon Setters, Chow Chows, and Golden retrievers.  They are locally invasive tumors, often infiltrate deep into the bone (of the jaw or toe), and have a high rate of metastasis (spreading). Cats can also get melanomas but it is much less common than in the dog.

Histiocytic Sarcoma in Dogs

The most common tumor of the spleen in dogs is hemangiosarcoma (HSA). Up to 50% of dogs with splenic HSA are in DIC (disseminated intravascular coagulation, or inability to clot blood) at the time of presentation. Unfortunately, with surgery alone, the average survival times are only around 3 months. Liver biopsy is essential to differentiate between liver metastasis (spread) and benign hyperplasia (increased tissue growth).  HSA does not always start in the liver or spleen; it can also start in the skin, subcutaneous tissue, or the heart. Stage I cutaneous HSA may be cured with aggressive surgical resection. Radiographs (x-rays) of the lungs are required to rule out pulmonary metastasis (tumors in the lungs). Cardiac HSA is a common cause of pericardial effusion (fluid surrounding the heart) in dogs. HSA in cats is rare but occurs most commonly within the abdomen (spleen, liver or kidneys) or subcutaneous tissue (under the skin)

Chronic Lymphocytic Leukemia (CLL) in Dogs

Chronic Lymphocytic leukemia (CLL) is a condition in which mature (i.e., “normal”-looking) lymphocytes (a type of white blood cell) accumulate in the body (including in the bone marrow and spleen). This results in elevated circulating lymphocyte counts on CBC blood tests. CLL typically occurs in middle age to older dogs and is often found “accidentally” when blood work is being performed for other reasons. CLL usually progresses slowly and patients can typically be medically managed and live well for years.

Canine Urinary Bladder Neoplasia

Urinary bladder cancers are more common in female dogs, with Scottish Terriers, West Highland White Terriers, Shetland Sheepdogs, and Beagles being overrepresented. Transitional cell carcinoma (TCC) is by far the most common type of bladder neoplasia (cancer) encountered in dogs.

Canine Mast Cell Tumors

Mast cell tumors (MCT) are the most common malignant skin tumors in dogs and among the most common tumors overall. MCT most commonly occur as solitary lumps or masses in the skin and occasionally dogs can have multiple MCTs. These tumors can have a very variable appearance–they can resemble fatty (lipomatous) masses or may be reddened (erythematous) and/or ulcerated. MCT may change quickly, often waxing and waning in size  (become larger, then smaller). They do this because mast cells contain granules of histamine, heparin and other chemicals that when released into the body can cause swelling, redness, and increased stomach acid production.

Canine Mammary Gland Tumors

Mammary tumors (breast cancers) are the most commonly diagnosed tumor in intact female dogs older than 7 years of age. Male dogs can also develop mammary tumors, but rarely. Several breeds are prone to developing mammary cancer including Poodles, English Spaniels, English Setters, and Terriers. About 50% of mammary tumors are malignant (invasive to surrounding tissue with a high risk of spreading) and 50% are benign—this is very similar to the statistics for breast cancer in women.

Canine Lymphoma

Lymphoma and lymphosarcoma (LSA) are interchangeable terms. Lymphoma in dogs is very similar to non-Hodgkin’s lymphoma in people. Lymphoma represents 7% of all cancers in dogs. Most affected dogs are between 5-9 years of age, but the disease can occur in dogs of any age. Generalized lymphadenopathy (lymph node enlargement) in an otherwise healthy dog is the most common presentation. Hypercalcemia (high blood calcium) occurs in 20% of dogs with lymphoma. Administration of glucocorticoids (steroids) prior to confirming a diagnosis can make obtaining the diagnosis much more difficult and lead to the cancer becoming chemotherapy resistant.

Appendicular Osteosarcoma

Appendicular osteosarcoma (bone cancer of the leg) is most common in giant and large breed dogs, with an average age of onset of 7 years. The most common site for the development of osteosarcoma (OSA) is the distal radius (near the wrist), followed by the proximal humerus (near the shoulder) and less commonly proximal and distal femur and tibia (hips to ankles). While acute lameness due to pathologic fracture (disease-related bone break) occurs in some dogs, most present with a history of progressive lameness over several weeks.

Acute Leukemias-Lymphoid and Myelogenous

Acute leukemia is a systemic cancer characterized by the infiltration of immature lymphocytes or myelocytes (two types of white blood cells) called in the bone marrow (and commonly in the liver and spleen as well). Patients with acute leukemia are typically quite ill due to their disease. Affected animals are typically young (less than 5 years of age). There is a male gender predilection in some studies.