Canine splenic lesions: epidemiological patterns and the predictive value of hemoperitoneum for malignancy

In the world of veterinary medicine, the spleen is a vital organ for dogs, acting as a reservoir for blood and a key player in the immune system. When this organ develops lumps or masses, it presents a difficult puzzle for veterinarians. These growths can be harmless overgrowths of normal tissue, or they can be aggressive cancers that spread quickly. Because the symptoms of a benign lump and a malignant tumor often look identical on an ultrasound, and because the organ itself is prone to bleeding, the standard approach has often been to remove the spleen surgically to get a definitive answer. For decades, a widely accepted guideline known as the "two-thirds rule" has shaped how vets think about these cases. This rule suggests that roughly two out of every three splenic masses in dogs are cancerous, and of those cancers, two out of three are a specific, deadly type called hemangiosarcoma. However, this rule was built on data from different regions and populations, leaving a gap in knowledge about whether it holds true for dogs in South America and whether there are other clues available before surgery to predict the outcome.

A team of researchers at a veterinary school in Brazil set out to examine this question by looking back at the medical history of hundreds of dogs. They gathered records from 228 dogs that had undergone spleen removal surgery between 2008 and 2024. For each animal, they reviewed the dog's age, breed, and size, and they carefully analyzed the tissue samples under a microscope to determine exactly what kind of lesion was present. They also paid close attention to a specific condition often seen in these patients: the presence of free fluid in the abdomen. In many cases, this fluid was blood, a condition known as hemoperitoneum. The researchers wanted to see if the presence of this bloody fluid could serve as a reliable warning sign that the mass was cancerous, specifically hemangiosarcoma, rather than a benign issue.

The study revealed a population of dogs that was predominantly middle-aged to older, with a median age of ten years. While large breeds and mixed-breed dogs made up the majority of the cases, the most surprising finding was the actual breakdown of the diseases found. Contrary to the long-held "two-thirds rule," the researchers found that the masses were almost evenly split between cancerous and non-cancerous causes. Of the nearly 300 lesions identified, 49.5% were malignant tumors, while 50.5% were benign conditions. The most common cancer found was hemangiosarcoma, followed by lymphoma, but benign issues like blood clots, tissue death due to lack of blood flow, and non-cancerous growths were just as frequent. This suggests that the old rule of thumb may not apply universally and that a dog with a splenic mass has a significantly better chance of having a benign condition than previously thought.

The researchers then turned their attention to the fluid found in the abdomen of these dogs. About one-third of the animals had free fluid visible on ultrasound. When they analyzed the fluid from the dogs where testing was possible, a clear pattern emerged. Dogs that had bloody fluid in their belly were much more likely to have a cancerous spleen than those without it. Specifically, the presence of this bloody fluid made a dog more than four times as likely to have a neoplastic lesion compared to a non-neoplastic one. The connection was even stronger when looking specifically for hemangiosarcoma. If a dog presented with bloody abdominal fluid, the odds that the mass was hemangiosarcoma were more than eleven times higher than if the mass was any other type of splenic disease. In the cases of hemangiosarcoma that did have fluid, the vast majority of that fluid was blood, whereas other types of lesions were more often associated with different types of fluid or no fluid at all.

These findings offer a clearer picture for veterinarians and pet owners navigating the uncertainty of splenic disease. While the presence of a mass in the spleen remains a serious concern that often requires surgery, the study indicates that the likelihood of the mass being benign is higher than the traditional "two-thirds rule" suggests. Furthermore, the presence of bloody fluid in the abdomen serves as a powerful, clinically relevant indicator. It does not guarantee a cancer diagnosis, but it significantly raises the probability, particularly for hemangiosarcoma. This information allows vets to better assess the risks before an operation, helping them prepare for the possibility of a malignant outcome and offering a more grounded perspective on the nature of these common canine health issues. The study confirms that while the spleen can be a site of serious disease, it is also frequently the location of non-cancerous conditions, and the specific nature of the fluid surrounding the organ can provide a vital clue to the diagnosis.