Splenic hemangiosarcoma in dogs.

What it is

Hemangiosarcoma (HSA) is a malignant cancer of cells that line blood vessels. When it occurs in the spleen, it forms a mass that can bleed into the abdomen, sometimes catastrophically. The cancer also spreads through the bloodstream, most often to the liver, lungs, and heart.

Splenic HSA is over-represented in Golden Retrievers, German Shepherds, and Labradors, but can occur in any breed and is more common in middle-aged to older dogs.

Signs to watch for

• Sudden weakness or collapse, sometimes followed by recovery within hours (this can correspond to a slow bleed that the body partly resorbs).
• Pale gums and rapid breathing — signs of internal blood loss.
• A distended belly or abdominal discomfort.
• Lethargy and reduced appetite that comes and goes.

A dog that collapses suddenly is an emergency — go to the closest open vet without delay. Slower, intermittent signs deserve a same-day call to your vet.

How veterinarians diagnose it

Initial workup typically includes bloodwork (CBC + chemistry, often showing anemia and signs of blood loss), abdominal ultrasound to identify a splenic mass, and sometimes chest imaging to look for visible metastasis. An echocardiogram may be recommended because HSA can also affect the heart.

A definitive diagnosis usually requires removing the spleen (splenectomy) and submitting it for histopathology, since not every splenic mass is HSA — some are benign. The histology distinction is critical: management and prognosis differ substantially.

What treatment usually looks like

Options depend heavily on whether the dog is stable, whether there's visible metastasis, and the owner's goals.

• Splenectomy — surgical removal of the spleen — addresses the immediate bleeding risk and provides definitive diagnosis. Many dogs recover well from this surgery.
• Adjuvant chemotherapy after splenectomy, typically with doxorubicin-based protocols, can meaningfully extend survival for HSA dogs.
• Palliative care — pain control, transfusion as needed, comfort focus — when surgery isn't pursued.
• Clinical trials — HSA is an active area of veterinary oncology research. Immunotherapies and metronomic protocols are among the approaches under study.

Prognosis

HSA is recognized in the veterinary oncology literature as one of the more challenging canine cancers. Published median survival times after splenectomy alone are typically short (often a few months); adding adjuvant chemotherapy extends median survival meaningfully but remains in the months range for most dogs.

Individual outcomes vary widely. Some dogs live considerably longer than the median; some shorter. Quality-of-life-focused conversations with your veterinary oncologist matter especially in this disease.

Questions to ask your vet

• Did imaging suggest the cancer has already spread to other organs?
• Is my dog a candidate for splenectomy now, or do we stabilize first?
• What's the realistic best-case and median-case picture with each path?
• What does quality-of-life monitoring look like after surgery, and when would we revisit decisions?
• Are clinical trials open that match my dog's specific situation?
• What signs would mean another bleed and what should I do if I see them?
• What does the palliative path look like if we choose that route?

Where to learn more

The American College of Veterinary Internal Medicine (ACVIM) and the Veterinary Cancer Society publish materials on hemangiosarcoma in dogs. Major US vet-school teaching hospitals maintain owner-facing summaries. The Morris Animal Foundation Golden Retriever Lifetime Study has published findings on canine cancer incidence relevant to HSA risk.

For HSA clinical trials, see the SciRouter Vet trial finder.