Hemangiosarcoma is an aggressive cancer of the blood vessel lining that primarily affects dogs, and it carries one of the worst prognoses of any canine cancer. The tumors grow from the cells that form blood vessels, which means they can develop almost anywhere in the body but most commonly appear in the spleen, heart, and liver. Because these tumors are filled with blood and fragile, they often rupture without warning, causing life-threatening internal bleeding. An estimated one in five Golden Retrievers will develop hemangiosarcoma in their lifetime, though any breed can be affected.
Where Hemangiosarcoma Develops
The spleen is the most common site. Splenic hemangiosarcoma often grows silently for weeks or months, producing no obvious symptoms until the tumor ruptures. The heart is the second most common location, specifically the right atrium, where a tumor can cause fluid to accumulate around the heart and compress it. The liver is another frequent site, and tumors there also carry a high risk of rupture and internal bleeding.
A less dangerous form, called cutaneous (skin) hemangiosarcoma, can appear on or just beneath the skin. Dogs with thin coats and light-colored skin are more susceptible, and UV exposure plays a role. Skin tumors confined to the surface carry a significantly better prognosis than the visceral (internal organ) forms, though tumors that extend into deeper tissue layers behave more aggressively.
Breeds at Highest Risk
Golden Retrievers face the greatest risk, with roughly 20% developing hemangiosarcoma during their lives. German Shepherds, Labrador Retrievers, Portuguese Water Dogs, and other large-breed dogs also have elevated rates. The cancer typically strikes middle-aged to older dogs, usually between 8 and 13 years old, though it can occur earlier. Researchers at Cornell have identified molecular differences between cancerous and healthy spleen tissue, including an overproduction of certain structural molecules in tumor tissue that could eventually serve as early-detection biomarkers. They’ve also begun identifying distinct subtypes of hemangiosarcoma with different genetic profiles, which may one day allow for more targeted treatment.
Signs and Symptoms
The cruelest feature of this cancer is how invisible it stays until a crisis hits. Many dogs show no symptoms at all until a tumor ruptures. When that happens, the signs come on suddenly and can include:
- Collapse or severe weakness
- Rapid or labored breathing
- Pale or white gums
- A distended abdomen (from blood pooling in the belly)
- Sudden death
Before a rupture, some dogs show vague, intermittent symptoms. They may seem tired, eat less, or have episodes of unexplained weakness that resolve on their own. These “waxing and waning” episodes can happen when a small tumor bleeds, temporarily stabilizes, and then bleeds again. Owners often describe a dog who seemed fine one day and collapsed the next. That pattern is unfortunately typical.
How It’s Diagnosed and Staged
Many cases are first detected during an emergency visit after a tumor ruptures. Ultrasound can reveal masses on the spleen or other organs, and free fluid in the abdomen often signals active bleeding. A definitive diagnosis requires a tissue sample, usually obtained during surgery to remove the affected organ or mass.
Staging determines how far the cancer has spread. In Stage I, the tumor is confined to its original site and smaller than 5 cm. Stage II involves a ruptured tumor, one larger than 5 cm, or cancer that has reached nearby lymph nodes. Stage III means distant spread, most commonly to the lungs or liver. Unfortunately, by the time most dogs are diagnosed, metastasis has already occurred at a microscopic level, even when imaging looks clean.
A newer screening option, liquid biopsy blood tests, is becoming available. One validated test detected hemangiosarcoma in about 74% of confirmed cases while correctly identifying cancer-free dogs 98.7% of the time. This technology measures fragments of tumor DNA circulating in the bloodstream. It’s not perfect, missing roughly one in four cases, but it represents a step toward catching the disease before a crisis.
Treatment Options and What to Expect
For splenic hemangiosarcoma, emergency surgery to remove the spleen is typically the first step. Dogs can live normal lives without a spleen, and surgery addresses the immediate danger of bleeding. However, surgery alone provides a median survival time of only about 1.6 months, because microscopic cancer has almost always spread to other organs by the time the primary tumor is found.
Adding chemotherapy after surgery extends survival. Dogs treated with both surgery and chemotherapy had a median survival of roughly 3.4 months. Those who completed three or more chemotherapy cycles did meaningfully better, with a median survival around 200 days (about 6.5 months) compared to 55 days for dogs who received fewer cycles. Some individual dogs live much longer. In one study, survival ranged from 6 days to over 4 years, though long-term survivors are the exception.
For tumors on the heart, surgery is more complex and carries greater risk. Treatment decisions depend on the tumor’s size, location, and whether the dog is stable enough for anesthesia.
Quality of Life During Treatment
Most dogs tolerate chemotherapy far better than humans do. Side effects like nausea, decreased appetite, and low energy tend to be mild and short-lived. The goal in veterinary oncology is always maintaining quality of life, not just extending it. Your veterinarian or veterinary oncologist will monitor your dog closely and adjust treatment if side effects become significant.
Supplemental Approaches
One supplement that has drawn attention is a compound derived from the turkey tail mushroom, called polysaccharopeptide (PSP). A study at the University of Pennsylvania tested it in 15 dogs with splenic hemangiosarcoma who received no other cancer treatment after diagnosis. Dogs in the highest dose group had a median survival of 199 days, which exceeded the previously reported median of 86 days for dogs receiving no treatment beyond surgery. Some dogs in the trial lived beyond a year on the supplement alone. The study was small and preliminary, but the results were notable enough to generate further investigation.
The Realistic Outlook
Hemangiosarcoma remains one of the most difficult cancers to beat in veterinary medicine. The combination of late detection and aggressive spread means that most dogs with the visceral form live weeks to months after diagnosis, even with treatment. Long-term survival beyond a year is possible but uncommon for splenic or cardiac cases.
For owners of high-risk breeds, regular veterinary checkups that include abdominal palpation and periodic ultrasound screening after age 6 or 7 can sometimes catch tumors before they rupture. Catching the disease earlier doesn’t guarantee a cure, but it can mean a calmer, more controlled path to treatment rather than an emergency room crisis. Emerging blood-based screening tests may improve early detection further, particularly for breeds like Golden Retrievers where the lifetime risk is so high.