The Canine Ledger

A Mass on the Spleen: Why "Two-Thirds Are Cancer" Is Out of Date

A splenic mass found by chance is more often benign than cancer. One that has bled is more often cancer. What recent studies show, and what to ask.

Written by thecanineledger

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If your dog suddenly collapses, is weak, has pale gums, or has a swollen belly, go to a vet or emergency clinic now. A mass on the spleen can bleed into the belly, and that is an emergency. → Emergency Signs in Dogs: When to Go Now, Not Tomorrow

Owners told their dog has a mass on the spleen often hear a grim rule of thumb: two-thirds of splenic masses are cancer, and two-thirds of those are haemangiosarcoma, an aggressive cancer of blood vessels. Recent studies suggest that rule is too simple, and for some dogs, too gloomy.

While a splenic mass is serious and needs a plan, the odds depend a great deal on how the mass was found. This page explains the difference.

1. Found by chance, not bleeding: more often benign

Many splenic masses are found by accident, on a scan done for something else. A study of 105 dogs who had the spleen removed for a mass like this, with no bleeding, found that “74 of 105 (70.5%) patients had benign splenic lesions and 31 (29.5%) had malignant neoplasia” (Cleveland & Casale, J Am Vet Med Assoc 2016).

The authors concluded that “Incidentally found, nonruptured splenic masses or nodules without associated hemoperitoneum were most commonly benign.” (Haemoperitoneum means blood in the belly.)

2. A mass that has bled: more often cancer

The picture changes when a mass has bled. In a study of 197 dogs with bleeding into the belly from the spleen, not caused by injury, “Histopathologic findings indicated malignancy in 144 of the 197 (73.1%) dogs”, and haemangiosarcoma was the diagnosis in 64% of all the dogs (Millar et al., J Am Vet Med Assoc 2021).

The same study found that a benign result isn’t always the end of the story. Nine of 53 dogs with benign results died early, and the authors suspected a cancer that hadn’t been found.

3. All splenic masses together

A 2025 study of 436 dogs who had the spleen removed, whatever the reason, found that “Out of all splenic masses regardless of presentation (eg, incidental, nontraumatic hemoabdomen), 60.7% were benign lesions” (Swanton, Coleman & Baldeon, J Am Vet Med Assoc 2025).

It also found that where on the spleen a mass sits made no difference, but more than one mass did: “The odds of a malignant diagnosis were 1.6 times higher (95% CI, 1.1 to 2.5) when there were masses in multiple locations compared to a single location in the spleen.”

The limits. All three studies looked back at hospital records, and only included dogs who had the spleen removed. Dogs whose owners chose euthanasia or no surgery aren’t counted, which can shift the figures.

Our note: This is our summary of three studies. Other studies may disagree.

4. Why the scan alone often can’t tell

A benign mass, such as a haematoma (a blood-filled swelling) or nodular hyperplasia, can look like a cancer on ultrasound. The definite answer usually comes from examining the spleen after it is removed.

Before surgery, vets often check the chest and the rest of the belly for signs that a cancer has spread. That changes the decision a great deal.

Our note: That a benign mass can look like cancer on ultrasound, and that the answer usually comes after removal, is from general knowledge, not a source we quote. Ask your vet about other ways to sample the mass, and their risks.

5. The decision owners face

The choice is often between surgery to remove the spleen, palliative care, and euthanasia, sometimes in an emergency.

A study surveyed owners of dogs who came in as emergencies with bleeding into the belly. “Owners reported QOL as the most important factor influencing their decision-making (92%)” (QOL means quality of life). Dogs who had surgery lived longer on average: median survival was 213 days with surgery and 39 days with palliative care. But for dogs whose mass was cancer, the survival benefit of surgery disappeared. And “Thirty-four owners (26%) second-guessed or were unsure of their decision” (Menard, Sylvester & Lopez, J Am Vet Med Assoc 2023).

That last figure matters. There is often no clear right answer, and owners who choose either way may later wonder. → How to Know When It’s Time to Say Goodbye

Our note: The survival figures in section 5 come from one hospital and from owners who answered a survey.

6. Questions to ask your vet

  • How was the mass found? Was there any bleeding?
  • Is there one mass, or more?
  • Has anything been seen in the chest, liver or heart?
  • What would surgery involve, and what does it cost?
  • If it’s benign, is removing the spleen a cure?
  • If it’s cancer, what are the options after surgery?
  • What happens if we wait?

Our note: You can ask your vet what waiting would mean for your dog. A mass may bleed later, and your vet can explain how likely that is.

7. What we could not verify

Commonly saidWhat we found
“Two-thirds of splenic masses are cancer, and two-thirds of those are haemangiosarcoma”Too simple. It depends on how the mass was found. We found no study that originated the rule.
“A benign result means the dog is safe”Usually, not always. 9 of 53 dogs with benign results in one study died early of a suspected cancer.
“Where the mass is on the spleen tells you if it’s cancer”Not in one 436-dog study. More than one mass did raise the odds.
Survival with haemangiosarcomaNot stated here.

Sources

Always confirm with your vet before acting on anything on this page.