Is Your Dog’s Tumor Spreading? How To Know If Cancer Has Metastasized

What is Metastasis?

Metastasis refers to the spread of cancer cells from the original tumor site to other parts of the body (National Cancer Institute, 2022). This occurs when cancer cells break away from the primary tumor and enter the bloodstream or lymph system, allowing them to travel to other organs and tissues where they can form new tumors. Metastasis is a hallmark of malignant cancers and is often more dangerous than the primary tumor itself (, 2022).

When cancer metastasizes, it is still named after the part of the body where it originated. For example, breast cancer that spreads to the lungs is still called metastatic breast cancer, not lung cancer. Metastatic cancers can be very challenging to treat and have a poorer prognosis compared to non-metastatic cancers. Stopping metastasis is a major goal of cancer treatment.

Common Signs of Metastasis in Dogs

Some of the most common signs of metastasis in dogs include:

Lethargy/Increased Sleep – Dogs that are normally active and energetic may start sleeping more and acting lethargic as the cancer spreads. This is due to the cancer sapping energy reserves and the body working harder to fight the disease [1].

a dog being examined by a veterinarian

Loss of Appetite – Metastatic cancer often causes a decreased appetite in dogs. The cancer itself and side effects of treatments like chemotherapy can lead to nausea, mouth sores, and other problems that make eating uncomfortable [1].

Unexplained Weight Loss – Despite eating the same amount, dogs with metastatic cancer often experience weight loss and muscle wasting. This occurs as the cancer consumes energy and protein stores [2].

Persistent Lameness or Swelling – The spread of cancer to areas like bones, joints, or lymph nodes can cause limping, stiffness, and swelling in dogs’ legs or other body parts [1].

Difficulty Breathing – Metastasis to the lungs or chest can lead to labored breathing, coughing, or shortness of breath as the cancer impairs respiratory function [2].

Diagnostic Tests

There are several diagnostic tests that can be used to determine if a tumor has metastasized in a dog:

Fine Needle Aspirate

A fine needle aspirate involves inserting a thin needle into the tumor or enlarged lymph node to collect cells. These cells are then examined under a microscope by a veterinary pathologist to determine if they are cancerous. This is a minimally invasive procedure that does not require anesthesia.


A biopsy involves surgically removing a small piece of the tumor and examining it under a microscope. This provides information about the type of cancer cells present and how aggressive they appear. Biopsies are considered the gold standard for cancer diagnosis but do require sedation or anesthesia [1].

Blood Tests

Certain blood tests like complete blood counts, biochemistry profiles and clotting tests can sometimes detect abnormalities that may indicate cancer. However, blood tests alone cannot definitively diagnose cancer.

Imaging Tests

X-rays, ultrasound, CT scans, MRI and nuclear scintigraphy can all be used to look for tumors or signs of metastasis in other areas of the body. These imaging modalities allow veterinarians to see inside the body and evaluate the size, shape and location of tumors [2].

Common Sites of Metastasis

When cancer spreads in dogs, it often metastasizes to these key areas:

The lungs are a frequent site for metastasis in dogs. Cancer cells can spread through the bloodstream from primary tumors like osteosarcoma, hemangiosarcoma, and mammary tumors. Signs of lung metastasis include breathing difficulties, coughing, and exercise intolerance.[1]

The liver contains a rich blood supply, making it susceptible to metastatic cancer. Hemangiosarcoma, lymphoma, and mammary tumors commonly spread to the canine liver. Symptoms include vomiting, diarrhea, jaundice, weight loss, and abdominal swelling.[2]
a ct scan image showing cancer spread in a dog's body

Lymph nodes

Lymph nodes are part of the lymphatic system, which filters fluid from tissues. Enlarged lymph nodes are a common indicator that cancer has spread from sites like the mouth, skin, mammary glands, and extremities.[3]

The brain is a relatively rare site for metastasis, most often originating from mammary tumors, melanoma, and hemangiosarcoma. Neurological symptoms like seizures, pacing, and behavior changes may signal brain metastasis.[1]

Metastatic bone cancer causes pain, swelling, and fractures. Osteosarcoma, mammary tumors, lung tumors, and melanoma commonly spread to bones.Early detection gives the best chance for successful treatment.[2]

Treatment Options

There are several main treatment options for dogs with metastatic cancer, with the goal of controlling the spread of cancer and managing symptoms.

Surgery may be an option to remove the primary tumor and any isolated metastases, if the cancer has not spread too far. This can provide temporary control of the cancer in some cases. However, surgery alone is rarely curative for metastatic cancer in dogs.

Chemotherapy involves using drugs that target and kill cancer cells. It can help slow the spread of metastases and extend a dog’s life. Common chemotherapy drugs used in dogs include vincristine, cyclophosphamide, doxorubicin, and carboplatin. Chemotherapy can have significant side effects, so dogs require close monitoring.

Radiation therapy uses high energy beams to kill cancer cells and shrink tumors. It is usually used along with surgery or chemotherapy. Radiation can help control metastasis to specific areas. Side effects may include skin irritation and fatigue.

Pain management is a key part of treatment, using medications like NSAIDs, opiates, and steroids to keep the dog comfortable. Additional medications may help with nausea, diarrhea, and other side effects of treatment.

Treatment plans are tailored to each patient based on the type of cancer, location of metastases, and overall health. Often a combination approach is used. Quality of life is emphasized when formulating a treatment strategy.

Palliative Care

Palliative care is an important consideration for dogs with metastatic cancer to help improve their quality of life. The goals of palliative care are to relieve pain and provide comfort when a cure is not possible.

Pain management is a major component of palliative care. Metastatic cancer can cause significant pain due to the spread of tumors. Veterinarians may prescribe pain medications like NSAIDs, opioids, and steroids to control pain. Nutritional support is also important to maintain strength and weight. Appetite stimulants may be used if the dog is not eating well.

a dog receiving pain medication from its owner

Other aspects of palliative care include treating nausea, managing other symptoms, providing favorite foods, acupuncture, and addressing mobility issues. The overall focus is on maximizing comfort and quality of life for the remaining time the dog has left.

Palliative care allows owners to pamper their dog and create positive memories together through the end of life journey. It aims to give dogs a peaceful passing when further treatment is no longer recommended.


The prognosis for dogs with metastatic cancer depends on the type of cancer and extent of spread. Some cancers have better survival times than others once they metastasize. According to the Riney Canine Health Center at Cornell University, “Some cancers are so aggressive that even with a full treatment, most dogs will not live beyond another 3-4 months. For example, if your dog’s lungs are full of tumors (metastatic disease), intensive therapy may provide weeks to a few months of good quality life, but is not likely to provide a cure.”

According to Today’s Veterinary Practice, the traditional prognosis for dogs with advanced metastatic disease was 1-2 months survival. However, with newer treatments available, survival times are improving for some types of metastatic cancers in dogs. Still, the prognosis remains very serious once cancer has metastasized.

It’s important to discuss your individual dog’s prognosis with your veterinarian, as every case is different. Factors like your dog’s overall health, type of cancer, extent of metastases, and treatment plan will impact the prognosis.


There are some steps dog owners can take to help prevent cancer or detect it early:

Get routine veterinary exams. Wellness visits allow vets to get a baseline on your dog’s health and notice any changes early on. Physical exams and bloodwork can sometimes detect cancer before symptoms arise.

Know your dog’s risk factors. Some breeds are prone to specific cancers due to genetics. Discuss any breed-related cancer risks with your vet.

Limit sun exposure. Long hours in the sun raise the risk of certain cancers in dogs, like lymphoma and skin cancer. Use dog-safe sunscreen and limit midday hours in the sun.

Maintain a healthy weight. Obesity is a risk factor for some dog cancers. Stick to your vet’s recommended diet and exercise plan.

Avoid tobacco smoke. Secondhand smoke can cause lung cancer and other cancers in dogs. Don’t let anyone smoke in your home or around your dog.

Use caution with lawn chemicals. Herbicides and pesticides may be linked to increased cancer risk, especially in certain breeds. Consider organic options.

Get recommended vaccines. Some vaccines may help prevent cancer-causing infections in dogs, like the leptospirosis vaccine.

Spay/neuter after maturity. Altering too early may increase certain cancer risks. Discuss timing with your vet.

Coping as an Owner

Receiving a cancer diagnosis for your dog can be emotionally devastating. As an owner, you play a critical role in providing emotional support and focusing on your dog’s quality of life throughout treatment or end-of-life care.

Be sure to give your dog plenty of love and affection. Gentle petting, brushing, massaging, or just sitting together can help relieve stress for both you and your dog. Consider taking pictures or videos to cherish your memories.

Monitor your dog for signs of pain, discomfort, or changes in behavior. Discuss any concerns immediately with your veterinarian so adjustments can be made to improve your dog’s comfort and quality of life. Be open about your feelings with your vet as well.

As your dog’s health declines, you may need to consider euthanasia. In-home euthanasia services allow your dog to pass away peacefully at home, surrounded by familiar sights, sounds, and smells. Your vet can provide guidance on when it’s time and help facilitate in-home euthanasia services.

Don’t hesitate to lean on your support system of family, friends, and other pet owners during this difficult time. Joining pet grief support groups can also help you process your emotions.


a person hugging their dog comfortingly

Metastasis occurs when cancer cells spread from the primary tumor to other areas of the body. In dogs, some of the most common primary tumor sites are the mammary glands, skin, lungs, and lymph nodes. While metastatic cancer can be difficult to treat, early detection and prompt treatment provide the best chance for remission and quality of life.

Some key points to remember include:

  • Non-specific signs like lethargy, decreased appetite, and weight loss can indicate metastasis. Look for lumps, abnormal discharge, enlarged lymph nodes, lameness, or breathing issues.
  • Diagnostic tests like blood work, imaging, biopsies, and lymph node aspirates help determine if and where cancer has spread.
  • Common sites of metastasis are the lungs, liver, brain, and bones. Treatment should address all affected areas.
  • Treatment options include surgery, chemotherapy, radiation therapy, immunotherapy, and supportive palliative care.
  • Early detection and treatment when cancer is still localized gives the best prognosis.
  • Providing excellent nutrition, avoiding toxins, spaying/neutering, and controlling parasites may help reduce cancer risk.

While a metastatic cancer diagnosis is very difficult for dog owners, focusing on providing the best quality of life possible with treatment can make a significant difference. Staying attuned to any changes and alerting the vet quickly at the first sign of symptoms is key to maximizing treatment efficacy. With an informed, caring owner, even dogs with advanced cancer can have meaningful time and comfort.

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