Hemangiosarcoma in Dogs: A Silent Cancer of Blood Vessels

By Tailwisp Editorial TeamUpdated October 17, 20263 min read

Hemangiosarcoma is often called a silent killer because dogs can seem well until a tumor bleeds.

What it is

Hemangiosarcoma is a malignant tumor of cells that line blood vessels. Because these tumors are full of blood, they can rupture and cause internal bleeding. The spleen is the most common site, followed by the right atrium of the heart, the liver and skin.

Who is affected

Middle-aged to older dogs, especially German shepherds, golden retrievers, Labradors, boxers and Portuguese water dogs. Skin forms in light-skinned, short-coated dogs are linked with sun exposure and are often less aggressive. See dog sunburn and skin cancer.

Signs

Early on, signs may be vague or absent:

• Intermittent lethargy
• Pale gums
• Reduced appetite
• Swollen belly

When a tumor bleeds, signs can appear suddenly:

• Collapse or weakness
• Rapid breathing and racing heart
• Very pale or white gums
• Distended abdomen

Seek emergency care immediately. See dog emergency warning signs and anemia in dogs.

Diagnosis

Vets use bloodwork, abdominal ultrasound, chest X-rays and echocardiography to find tumors and look for spread. Fluid from the abdomen may be tested. A definitive diagnosis requires analysis of tumor tissue, usually after surgery.

Treatment

• Emergency stabilization: fluids, oxygen and sometimes blood transfusion
• Splenectomy to remove a bleeding splenic tumor
• Chemotherapy, such as doxorubicin, to slow spread and extend survival
• Alternative or palliative approaches, including supportive care
• Surgery for skin forms with wide margins

Many splenic masses are benign, such as hematomas, and the only way to know is to examine the tissue.

Outlook

Visceral hemangiosarcoma is aggressive. Survival with surgery alone is often measured in weeks to a few months, and chemotherapy may extend it somewhat. Skin forms have a better outlook. Your vet or oncologist can explain what to expect.

Questions to ask your vet

Ask whether the mass can be sampled safely, what the risks of surgery are, how likely malignancy is, what a staging work-up involves and what palliative options exist if you choose not to pursue surgery. Ask about quality of life and how to plan for emergencies.

Planning for emergencies

Dogs with a known splenic mass can bleed again at any moment. Save the number of your nearest emergency clinic, know how to transport your dog calmly and keep an eye on gum color, breathing and energy. Ask your vet which signs mean you should go immediately.

Common mistakes

• Assuming sudden weakness will pass
• Waiting overnight
• Skipping chest imaging
• Ignoring a swollen belly

Troubleshooting

• My dog collapsed but recovered: call your vet right away, as a bleed may have resolved temporarily.
• Gums seem pale: go to the vet.
• My dog is slower than usual: book a prompt exam.

A simple plan

1. Know the emergency signs.
2. Locate your emergency clinic.
3. Get imaging if a mass is suspected.
4. Discuss options with a specialist.

Real-life examples

A shepherd collapses in the yard and has an emergency splenectomy, then begins chemotherapy. A retriever's splenic mass turns out to be benign after removal. A boxer receives palliative care when surgery is not possible. Outcomes vary.

Making decisions

You may have little time. Ask your vet to explain choices clearly and honestly.

Splenic masses that are not cancer

Not every splenic mass is hemangiosarcoma. Hematomas and other benign lesions are also common in older dogs, which is why tissue analysis matters. Ask your vet about the odds in your dog's case, and whether imaging suggests spread, but understand that the only certain answer comes from examining the removed tissue.

The takeaway

Hemangiosarcoma can bleed without warning. Treat sudden collapse as an emergency and discuss options with your vet.

This article is general educational information, not veterinary advice. Every dog is different. If your dog is unwell, or you are unsure about a symptom, diet change or medication, contact your veterinarian.