Canine Thymoma

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Thymoma is a tumor of the thymus gland, which is part of your dog's lymph system. The gland is located next to the heart and lungs, an area particularly sensitive to abnormal growth. Thymomas don't produce unique symptoms and are difficult to diagnose. Fortunately, many cases don't involve malignant cancer.


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The Thymus

If you've never heard of the thymus, you are certainly not alone. This tiny gland is present in humans as well as dogs, cats and many other animals, but it's not exactly the most well-known organ in the body. Its sole function is to produce lymphocytes, or T-cells, that fight infections and regulate immune system activity. The organ starts to shrink as your pet gets older because its purpose has been realized when he reaches maturity, according to Vet Surgery Central.

Development and Risk Factors


Thymoma is a rare condition in dogs. It's most often diagnosed in larger canines and is most common among the Labrador retriever and German shepherd breeds, according to the Animal Medical Center of Southern California. This abnormal growth tends to appear in older dogs and the average age of canine patients is 11 years. Males and females are equally likely to develop these tumors. Thymoma develops when cells within the thymus gland become malignant and reproduce at an accelerated rate. However, veterinary researchers have not identified a cause associated with the emergence of the tumors in either dogs or humans.



Some cases of thymoma have no symptoms at all, while others produce general symptoms that could be caused by dozens of different health conditions. If your dog has trouble breathing, has a diminished capacity for exercise or has a lingering cough, then he could be suffering from a tumor of the thymus. A large thymoma may press against your pet's throat, causing him to regurgitate after eating or frequently drool due to his inability to swallow. You may also notice a change in the sound of your dog's barks as the tumor impacts his voice box.



It's impossible to diagnose thymoma based on the clinical symptoms alone. An X-ray of your dog's chest can reveal the tumor to confirm your vet's suspicions. He may use a needle to take a sample of the mass before making a definitive diagnosis. Malignant tumors, also called invasive thymoma, can spread throughout the lymph system as well as the nearby heart and lungs. Benign growths are rarely fatal. Roughly 50 percent of canine thymoma cases involve a benign tumor, so don't start panicking over the vet's diagnosis just yet. Exploratory surgery to gather a sample of the tumor may be required to tell if it's invasive or benign.



Non-invasive tumors are usually easy to remove with surgery because they're not physically attached to surrounding vital structures. Cases of invasive thymoma are difficult to treat with surgery. Radiation is an effective tool for combating thymoma in dogs, and roughly 75 percent of patients respond to this type of therapy, according to the Veterinary Society of Surgical Oncology. Chemothrapy has a low success rate at fighting these tumors, but it has been used as a treatment in conjunction with steroids. If your pet has been diagnosed with thymoma, discuss all of your options with your veterinarian to determine the best course of action for you and your dog. Depending on his health condition, treatment measures may carry a high risk of fatal side-effects.


By Quentin Coleman

The Veterinary Cancer Center: Thymoma in Dogs
Veterinary Society of Surgical Oncology: Cranial Mediastinal Thymoma
Vet Surgery Central: Thymoma in Dogs and Cats
Animal Medical Center of Southern California: Thymoma

About the Author
Quentin Coleman has written for various publications, including All Pet News and Safe to Work Australia. He spent more tan 10 years nursing kittens, treating sick animals and domesticating semi-feral cats for a local animal shelter. He graduated from the University of Delaware with a bachelor's degree in journalism.