10 Signs of Cancer in Dogs

Cancer is a topic that no pet parent wants to think about. But the fact is that one in three dogs will eventually develop some form of cancer during their lifetime.

If caught early, roughly half of all canine cancers are treatable. That’s why it’s so important to learn the most common signs of cancer in dogs.

Just remember that many conditions, not just cancer, can cause similar clinical signs, and that the signs may vary depending on the type of cancer.

Identifying these symptoms is the first step, and the next step is to visit your veterinarian to make the correct diagnosis.

Signs Your Dog Has Cancer


Unexplained Lumps and Bumps

Several forms of cancer can cause lumps or bumps on your dog’s body, including a mammary gland tumor.

Mammary Gland Tumors in Dogs

This type of cancer occurs primarily in female dogs that aren’t spayed as well as those spayed after 2 years of age, although male and female dogs of any age and breed may develop mammary tumors.

Certain breeds, including Poodles and various spaniel dog breeds, have an increased risk.

This cancer usually begins as one or more nodules in the nipple area, which may become inflamed and swollen. Malignant mammary tumors tend to spread to nearby lymph nodes and mammary glands if left untreated.


Not all noticeable tumors are serious, however. Lipomas are common but benign fatty tumors that grow in the layer just beneath the skin.

These tumors most frequently occur in the trunk, armpit and groin areas, but they can also grow internally. It is not uncommon for elderly dogs to develop multiple lipomas, which feel like soft, moveable lumps under the skin.  

While lipomas are not life-threatening, they can interfere with movement if they become large enough, and internal lipomas may compress internal organs.  


Osteosarcoma is the most common bone cancer in dogs. While tumors usually occur in the long bones of the limbs, osteosarcoma can affect any bone.

This bone cancer grows very quickly and frequently spreads to other areas of the body, especially lymph nodes, lungs and other bones. Because of its aggressive nature, osteosarcoma is usually detected after the cancer has already spread. 

Dogs with osteosarcoma may appear to be in pain and walk with a limp, and the affected limb may be swollen.

Large and giant dog breeds have the highest risk of developing osteosarcoma.

Pigmented Sores

Darkly colored sores are a sign of melanoma, a cancer of the pigment-producing cells.

Melanomas in dogs tend to affect the mouth and lips, and they can also be found on their nail beds, footpads and eyes.

Specific signs will depend on where the tumor is located and may include a swollen paw, eye discharge or sores in the mouth.

Breeds with darkly pigmented oral tissues, such as the Chow Chow’s tongue, have an increased risk of developing melanoma.

Surgical removal can be difficult, as malignant melanoma tends to be locally invasive and spreads to deeper tissues and bone.

Swollen Lymph Nodes

Several types of cancer can cause lymph nodes to feel more prominent.

Lymphoma is a common malignant cancer that accounts for up to 20% of all canine tumor cases.

This cancer affects the lymphocyte, a type of white blood cell that plays an important role in immune function.

While most lymphoma cases begin in the lymph nodes, lymphoid tissues in the visceral organs, skin and bone marrow can also be affected.

Lymphoma can affect dogs of all ages and breeds, but Golden Retrievers and Boxers are among the most overrepresented pure breeds.

Swollen lymph nodes in the neck, knee and armpit regions are typically the first to be noticed.

Several types of canine cancers can also spread locally to nearby lymph nodes, causing them to enlarge. These include melanoma, osteosarcoma and mammary gland tumors.

Wounds That Won’t Heal

A particularly aggressive form of cancer known as the mast cell tumor, may present as a skin lesion that just won’t resolve.

This cancer affects mast cells, which are immune cells involved in allergic and inflammatory reactions. These cells are located throughout the body, but tumors tend to concentrate in the vessels and nerves near the skin, mouth and nose.

Less commonly, the gastrointestinal, respiratory and urinary systems may be targeted.

Abnormal growth of these cells causes an uncontrolled release of histamine, irritating the area surrounding the tumor.

Mast cell tumors are most common in older, purebred dogs, including the Boxer, Boston Terrier, Bulldog and Schnauzer.

Gastrointestinal Problems

The massive release of histamine associated with mast cell tumors can also cause significant problems with the gastrointestinal system, leading to stomach ulcers, vomiting and diarrhea.

Less dramatic signs of GI upset, such as decreased appetite, may occur from several other forms of cancer, including osteosarcoma and lymphoma.

Sudden Weakness or Collapse

While weakness can arise from a number of factors, sudden collapse is an alarming but common symptom of hemangiosarcoma, a cancer of the cells lining the blood vessels.

Hemangiosarcoma is a rapidly spreading form of malignant cancer that most frequently targets the heart, skin, spleen and liver.

This form of cancer is most common in the German Shepherd, Golden Retriever and other large breeds.

Since dogs usually show only mild warning signs, cases of hemangiosarcoma may not be detected until the cancer has reached an advanced stage.

Unfortunately, one of the most common initial signs of hemangiosarcoma involves sudden collapse due to massive internal bleeding, usually from a ruptured spleen.

Labored Breathing

Several of the canine cancers mentioned above are capable of spreading to the lungs, where they may cause respiratory distress.

Melanoma, hemangiosarcoma, mast cell tumor and osteosarcoma can all cause labored breathing and coughing with metastasis.

Unexplained Weight Loss

Weight loss that has no apparent cause may be a side effect of cancer, particularly with hemangiosarcoma, lymphoma and osteosarcoma.

Weight loss typically occurs because of the metabolic demands of the tumor, or because your dog is in pain and discomfort, resulting in anorexia and decreased activity.

Dogs with oral melanoma may find eating and swallowing to be difficult, resulting in weight loss.


While a variety of issues can cause lethargy, cancer can cause increased sleep and a reluctance to exercise and play.

Specific cancers that are frequently associated with lethargy include lymphoma and osteosarcoma.

By: Dr. Natalie Stilwell

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