Understanding and Managing Hemangiosarcoma in Dogs

Hemangiosarcoma in dogs is one of the most aggressive and challenging cancers to detect early. Often called a “silent killer,” it develops in the cells that form blood vessels and can grow rapidly in vital organs: most commonly the spleen, heart, skin, and liver. Tragically, many dog owners only discover it after a sudden health emergency, which makes the diagnosis feel even more disorienting and frightening.

If that’s where you are right now, know that it’s okay to feel overwhelmed. We’ll will walk you through what hemangiosarcoma is, the signs to watch for, and what your options look like, so you have somewhere to start.

What is hemangiosarcoma?

Hemangiosarcoma (HSA) is a type of cancer that starts in the blood vessel cells. It grows quickly and can spread to other parts of the body, especially the lungs, liver, or the heart. Because it’s related to blood vessels, tumors can be filled with blood, making them prone to rupture. This rupture is often what leads to a sudden crisis, like internal bleeding or collapse. Sadly, hemangiosarcoma is both common and deadly, with an estimated 50,000 cases diagnosed annually.

Hemangiosarcoma occurs primarily in the spleen, followed by the heart, skin, and liver. While hemangiosarcoma in internal organs usually goes unnoticed until a severe symptom appears, skin hemangiosarcoma may show up visually as a red or purple mass. By the time the tumor is diagnosed, cancer cells have likely spread to other parts of the body, which is one of the things that makes this disease so hard to treat.

Symptoms to watch for

In its early stages, hemangiosarcoma may show little to no signs. But if you know what to look for, you might catch subtle changes in your dog that warrant a vet visit. Here are some symptoms that may appear:

• Lethargy or tiredness
• Brief episodes of weakness or collapse, followed by seeming “normal” again
• Pale or white gums (a sign of internal bleeding)
• Rapid or labored breathing
• Swollen or distended abdomen
• Sudden collapse, which is often linked to a tumor rupture

If you notice sudden collapse, pale gums, or difficulty breathing, this is a medical emergency. These may signal that a tumor has ruptured, causing internal bleeding, which requires immediate vet care.

What causes hemangiosarcoma?

The cause of hemangiosarcoma isn’t fully understood yet. What researchers do know is that there’s likely a genetic component. About 30% of diagnosed dogs carry a gene mutation called PIK3CA, which is thought to play a role in how the cancer develops. Certain breeds also appear to be at significantly higher risk, including Golden Retrievers, Portuguese Water Dogs, Boxers, German Shepherd Dogs, Labrador Retrievers, and Flat-Coated Retrievers, though the research on specific breed risk varies by study. Environmental factors, like UV exposure, may contribute to the skin form of hemangiosarcoma.

Because so much of this comes down to genetics, there’s often nothing an owner could have done differently. This isn’t a diagnosis that comes from something you missed.

How is it diagnosed?

Diagnosing hemangiosarcoma often requires multiple tests since its symptoms can overlap with other conditions. Your vet might start with blood work to check for anemia, but to confirm the presence of tumors, they may recommend these tools:

• An abdominal ultrasound to detect tumors in organs like the spleen or liver
• Chest X-rays to see if the cancer has spread to the lungs
• A CT scan (advanced imaging) for detailed views
• An echocardiogram (heart ultrasound), especially for suspected cardiac cases
• A biopsy, which involves taking a sample of the tumor to confirm the diagnosis

In some cases, a biopsy may not be possible until surgery is performed, especially for tumors in internal organs.

Treatment options

We know this is a lot to take in. Treatment decisions are hard, and there’s no one-size-fits-all answer. Your vet is your best guide here, but understanding the general options can help you go into that conversation feeling more prepared.

The ideal treatment of hemangiosarcoma involves surgery to remove the tumor before metastasis occurs, followed by chemotherapy to delay the progression of metastatic disease.

• Surgery: Most common for tumors in the spleen or skin. For splenic hemangiosarcoma, this means removing the spleen (splenectomy). For dogs with hemangiosarcoma of the spleen treated with surgery alone, the median survival time is one to three months.
• Chemotherapy: Adding chemotherapy, using protocols that include the drug doxorubicin given repeatedly at two to three-week intervals, increases the median survival time to four to six months, making the combination of surgery and chemotherapy the preferred and most effective treatment available.
• Palliative care: Pain management and supportive treatments may be chosen to prioritize your dog’s comfort and quality of life if surgery isn’t an option. This is a valid and loving choice.

The prognosis for tumors localized to the skin is better than for tumors of the spleen or other internal organs. When caught early, surgery may fully remove the tumor in skin cases.

Prognosis and life expectancy

This is one of the hardest parts of hemangiosarcoma, and we won’t soften the truth. For internal forms of hemangiosarcoma, the prognosis is serious. For internal forms, even with treatment, most dogs have a median survival time of 4-6 months. Less than 10% are alive a year post-diagnosis. Without treatment, survival is often just weeks. On the other hand, skin hemangiosarcoma has better outcomes if caught and treated early.

Is it covered by pet insurance?

Yes, hemangiosarcoma is covered under Lemonade Pet’s base accident and illness policy, as long as it isn’t pre-existing. That includes eligible costs like diagnostic testing, surgery, and chemotherapy that are typically part of treatment.

It’s worth understanding how pre-existing conditions work here: if signs or symptoms were present before your policy was active or during your waiting period, the condition won’t be eligible for coverage. Because hemangiosarcoma can develop without obvious warning signs, having a policy in place while your dog is healthy is the clearest path to making sure you’re covered if something develops.

Before we go

We know how devastating a hemangiosarcoma diagnosis can be. It’s one of the most aggressive cancers dogs face, but what matters most is the time and comfort you can give to your pet. Stay in close communication with your vet, and explore all your options: whether it’s surgery, palliative care, or anything in between. And remember, support systems like pet insurance can help relieve some of the financial stress so you can focus on what’s most important: loving your dog.

A few quick words, because we <3 our lawyers: This post is general in nature, and any statement in it doesn’t alter the terms, conditions, exclusions, or limitations of the policies issued, which differ according to your state of residence. You’re encouraged to discuss your specific circumstances with your own professional advisors. The purpose of this post is merely to provide you with info and insights you can use to make such discussions more productive! Naturally, all comments by, or references to, third parties represent their own views, and Lemonade assumes no responsibility for them. Coverage may not be available in all states. Please note that statements about coverages, policy management, claims processes, Giveback, and customer support apply to policies underwritten by Lemonade Insurance Company or Metromile Insurance Company, a Lemonade company, sold by Lemonade Insurance Agency, LLC.  The statements do not apply to policies underwritten by other carriers.