Leiomyosarcoma in Dogs

Neoplasia or abnormal tissue growths form all over the body; some are benign while others are malignant cancer tumors. Of all the malignant neoplasia found in dogs, 1 to 3% occur in the gastrointestinal tract. Leiomyosarcoma is the second most common type of GI cancer, making it still rather rare in dogs. It usually affects older dogs, with a median onset of around 10 years. It is characterized by a tumor which forms in the smooth muscle cells along the walls of the GI tract, usually in the stomach or small intestine. It commonly metastasizes at a rate of 16-30% which is not as high as some cancers, but still indicates that the tumors will spread, often to other organs like the liver or spleen. Tumors can also lead to perforation of the walls of the stomach or bowels. Even with surgery, most dogs survive only a matter of months.

There are several different types of gastrointestinal cancer in dogs. Tumors that form in the involuntary muscles along the walls of the stomach and intestine are called leiomyosarcoma. This type of cancer develops relatively slowly, but it will spread to other areas of the body once metastasis begins, and it can often be fatal.

Finding the tumor early will give your dog the best chance of survival. If you notice any of the following signs in your dog discuss them with a veterinarian immediately:

• Vomiting (often with blood)
• Weight loss
• Diarrhea
• Lethargy
• Abdominal pain or swelling
• Palpable mass
• Constipation
• Tenesmus (tendency to clear the bowls often)
• Blood in the stool (hematochezia)
• Black tar-like stool with dried blood (melena)

Types

Neoplasia are defined by the type of cells on which they grow, as well as the place in the body where they are found, and whether the tumor is malignant (cancerous) or benign. There are several types of GI tumors which could be relevant to your dog’s diagnosis:

• Adenocarcinoma – malignant tumor that forms on glandular cells and is the most common form of GI cancer found in dogs, making up 42 to 72% of the cases. It metastasizes very quickly.
• Leiomyosarcoma – malignant tumor that forms in involuntary smooth muscles cells. As the second most common form of GI cancer found in dogs it makes up about 20 to 30% of cases. It normally metastasizes somewhat slower than adenocarcinoma.
• Leiomyoma - the benign form of leiomyosarcoma. This is also a tumor of involuntary, smooth muscle cells, but it will rarely grow or migrate to other parts of the body. It is rare in dogs.
• GISTs (GI Stroma Tumors) - another type of tumor which forms on the walls of the GI tract. It is based on nervous system cells called “interstitial cells of Cajal.” These cells control the involuntary muscles of the intestinal tract and are sometimes called the pace-makers of the GI. GISTs look a lot like leiomyosarcoma and historically the two have been confused; however recent studies in humans and dogs have shown differences which can be relevant to treatment and many more leiomyosarcoma are now diagnosed as GISTs. In dogs, GISTs are generally found in the large intestine rather than the stomach. GIST may refer to a malignant or benign tumor.

It’s not known what causes leiomyosarcoma in dogs. The condition is more likely to develop in older dogs. As with humans, family medical history could indicate a predisposition, and environmental factors may play a part, but this is even more difficult to track with dogs. Among breeds, Belgian Shepherd’s are considered predisposed to form GI carcinoma such as adenocarcinoma, but leiomyosarcoma has not been identified as being especially common in any particular breed. 

Based on the symptoms, your veterinarian may suspect cancer of the stomach. Various imaging techniques will indicate the presence of a tumor including radiography with contrast, ultrasound, and endoscopy. An endoscopy involves inserting a tool down the throat, so your dog will need to fast for several hours and receive an anesthetic if this test is ordered.

Before recommending a treatment plan, the veterinarian will endeavor to find out as much as possible about the tumor. Standard procedure includes several radiographs of the thorax, an abdominal ultrasound, and full blood and urine tests. Hypoglycemia commonly develops with leiomyosarcoma, and blood or urine samples may also show if the cancer has spread to the liver. Occasionally the veterinarian may order a laparotomy or exploratory surgery to further analyze the tumor.

Surgery is the most common treatment for leiomyosarcoma. Of course, this will depend on the size of the tumor and whether or not it has spread. Depending on the results of your dog’s tests, the veterinarian may decide that the cancer has already progressed beyond the level of treatment and recommend euthanasia. Surgery can be risky, and if the cancer has spread more than was anticipated, it’s possible the dog may end up having to be euthanized in the process.

Surgery commonly involves removing part of the stomach or intestinal wall with the tumor, and reconnecting the two parts back together. Your dog will need to spend at least several days in a veterinary hospital after the surgery. Fluids will be administered through an IV as well as antibiotics to control infection and medication for vomiting.

Additional biopsies are usually performed during surgery and the removed tumor is analyzed to determine its level of aggressiveness more accurately.  This information can help to determine if further measures are necessary. Tumors which are removed cleanly with no signs of metastasis are usually not treated further. If the tumor was only partially removed, or there were signs of metastasis the veterinarian may recommend chemotherapy.

In the best case scenario, your dog will need to recover from major surgery. This can take two to three weeks. Your dog will need to limit activity. He will most likely be prescribed a pain reliever and often an antacid as well. You will need to check the incision daily for signs of tearing or infection, and he should also have a check-up with the veterinarian after several weeks.

If chemotherapy is prescribed, this usually will include a number of injections administered at two or three week intervals. Expect at least a 90 minute appointment each time for additional tests and monitoring. Dogs do not typically suffer hair loss, but they may have gastrointestinal side effects.

There are a number of studies tracking the post-surgery survival times for groups of dogs with leiomyosarcoma. Figures range from an average survival time of 10 months to close to 2 years. One dog was recorded as surviving for 7 years after surgery. The chances are higher if the surgery is completed before metastasis has begun. There aren’t a lot of statistics on the effectiveness of chemotherapy with leiomyosarcoma. There are a few successful cases, but other dogs have died from tumor related issues only a few months after treatment. There is a better chance of recovery from leiomyosarcoma than from more aggressive cancers like adenocarcinoma, but you should remain guarded about your dog’s prospects.