CONDITION
Hemangiosarcoma
Haemangiosarcoma is a cancer that arises from the cells lining blood vessels. It tends to form tumours that are fragile and prone to bleeding, and it can develop in many parts of the body — most often the spleen, heart, liver, or skin. Because the tumours are blood-filled and can rupture without warning, the first sign may be sudden collapse, weakness, or pale gums, rather than a gradual decline. Many owners arrive on this page after their dog has experienced an unexplained episode of weakness or collapse, or after a mass has been found on imaging or during an examination for something else. The condition can also present as a visible lump on the skin, particularly in areas exposed to sunlight. The internal form is more common in older dogs, and certain breeds — including Golden Retrievers, German Shepherds, and Labrador Retrievers — are seen more frequently. This page explores what an owner may observe, what is happening inside the body, how haemangiosarcoma is investigated and confirmed, and the range of approaches that exist once a diagnosis is made. It also considers the factors that shape outlook and the conversations that often follow.
Why this matters now
Hemangiosarcoma is an aggressive cancer arising from blood vessel cells, most commonly affecting the spleen, heart, or liver in dogs. German Shepherds and Golden Retrievers are particularly predisposed, though any breed can be affected. The tumour often grows silently within internal organs, causing few signs until it bleeds or ruptures. This means diagnosis frequently occurs during a crisis—sudden collapse from internal bleeding—rather than during routine care.
Hemangiosarcoma typically develops as blood-filled masses that grow within organs. These tumours are fragile and prone to rupture, causing acute internal bleeding. Episodes of bleeding may be small and self-limiting initially, causing vague symptoms, before a larger bleed causes collapse. The cancer spreads readily to other organs, particularly the liver, lungs, and heart. Even with treatment, the prognosis is guarded due to the high metastatic rate.
Signals & patterns
Early signals
Episodes of weakness or collapse that resolve
Small bleeds may cause transient weakness followed by apparent recovery as the body reabsorbs the blood.
Lethargy and reduced stamina
Vague tiredness that may be attributed to age can reflect chronic low-grade blood loss.
Pale gums
Anaemia from blood loss causes visible pallor of the mucous membranes.
Distended abdomen
Free blood or an enlarged spleen may cause abdominal swelling.
Later signals
Sudden collapse
Acute haemorrhage causes rapid deterioration, weakness, and inability to stand.
Rapid, weak pulse
Shock from blood loss causes cardiovascular signs.
Breathing difficulty
Pericardial effusion (blood around the heart) causes respiratory distress. Lung metastases may also affect breathing.
Irregular heart rhythm
Cardiac involvement can cause arrhythmias.
Click to read about the biological mechanisms
How this is usually investigated
Investigation often occurs during an emergency presentation but may also follow incidental findings. Staging assesses the extent of disease.
Abdominal ultrasound
Chest radiographs
Echocardiography
Blood tests
Histopathology
Options & trade-offs
Treatment decisions depend on tumour location, extent of disease, and the dog's overall condition. Options range from emergency stabilisation to planned surgery and chemotherapy.
Emergency stabilisation
Intravenous fluids, blood transfusion, and supportive care for dogs presenting in crisis.
Trade-offs: May be lifesaving in the short term. Provides time for diagnostic evaluation and decision-making. Some dogs stabilise well enough for surgery.
Splenectomy
Surgical removal of the spleen when it contains the primary tumour.
Trade-offs: Stops active bleeding. Dogs can live normally without a spleen. Provides tissue for diagnosis. Survival after surgery alone averages one to two months due to metastatic disease.
Surgery with chemotherapy
Combining splenectomy with systemic chemotherapy to address micrometastases.
Trade-offs: Extends median survival to four to six months, with some dogs living longer. Chemotherapy is generally well tolerated.
Pericardiocentesis
Draining blood from around the heart to relieve tamponade.
Trade-offs: Can be lifesaving in cardiac hemangiosarcoma. May need repeating. Provides temporary relief but does not treat the underlying tumour.
Palliative care
Supportive treatment focused on comfort when curative options are not pursued.
Trade-offs: Appropriate for advanced disease or when other treatments are declined. Focuses on quality of remaining time.
Common misconceptions
"A mass on the spleen means cancer"
Approximately two-thirds of splenic masses in dogs are malignant (hemangiosarcoma or other cancers), but one-third are benign. Definitive diagnosis requires histopathology after removal.
"Removing the spleen cures hemangiosarcoma"
Surgery removes the primary tumour but microscopic spread has usually already occurred. Chemotherapy helps extend survival but cure is rare.
"There were no warning signs, so nothing could have been done earlier"
Hemangiosarcoma often causes no obvious signs until rupture. This reflects the nature of the disease rather than missed opportunities. Some dogs do have subtle signs that, in retrospect, may have been related.
Related conditions
Splenic Mass
A splenic mass discovered on imaging or during examination may be a haemangiosarcoma, though other tumour types and benign conditions can also present as masses in the spleen. Because haemangiosarcoma of the spleen is prone to rupture and bleeding, the two pages often describe overlapping presentations—particularly sudden weakness or collapse—and similar diagnostic pathways.
Cardiac Tamponade
Haemangiosarcoma arising in or near the heart can rupture and bleed into the pericardial sac, leading to cardiac tamponade as fluid accumulates and compresses the heart. This is one of the ways haemangiosarcoma may first become apparent, presenting as sudden collapse or weakness rather than a visible mass.
Mammary Tumours in Dogs
Both haemangiosarcoma and mammary tumours are cancers that tend to arise in older dogs and can spread to distant sites, including the lungs, liver, or spleen. In some cases, haemangiosarcoma may be discovered during staging investigations for a known mammary tumour, or the two may be found independently in the same individual.
Osteosarcoma in Dogs
Osteosarcoma and haemangiosarcoma are both aggressive cancers seen more often in certain larger breeds, and both tend to spread to the lungs. Owners whose dogs have been diagnosed with one may later encounter the other, either as a separate tumour or during investigations for signs such as lameness, weight loss, or unexplained decline.
Heartworm Disease in Dogs
Heartworm disease can, in rare instances, mimic some of the signs associated with cardiac haemangiosarcoma—such as exercise intolerance, breathing changes, or fluid around the heart—and both may be considered during investigations of cardiac masses or pericardial effusion. The two conditions have different causes and management pathways, but imaging findings can occasionally overlap.
Understanding that splenic masses require investigation regardless of whether they cause symptoms helps with incidental findings. Being aware that at-risk breeds may benefit from periodic abdominal ultrasound screening is worth discussing. Recognising signs of internal bleeding—weakness, pale gums, distended abdomen—enables prompt action. Knowing that prognosis depends on tumour type helps frame expectations whilst awaiting histopathology.
Last reviewed: 24 April 2026 · Dr Alastair Greenway MRCVS