Thymoma In Dogs

dog at doctors officeSeveral body systems play important roles in a dog’s ability to fight off infections and disease. One of the most critical is the lymphatic system. An organ within the lymphatic system that produces antibodies when dogs are young is called the thymus gland. When cancer cells develop in the thymus, the cancer is called thymoma.

Canine thymoma is very uncommon, developing in maybe 1% of dogs, and typically not until they are 10 years of age or older. Because the thymus is located in the upper part of the chest, a thymoma that grows to be an enlarged mass can cause respiratory distress, extreme muscle weakness and other  symptoms. Fortunately, a thymoma tumor in dogs can be successfully treated through surgery and other options, often resulting in long disease-free intervals.

Types of Thymoma in Dogs

There are two variants of thymoma cancer in dogs. Lymphocytic-rich thymoma is just that — a tumor with many lymphocytes (a type of white blood cell). The other is an epithelial thymoma, which forms in the tissue that lines the organ.

Symptoms of Thymoma in Dogs 

Thymoma can present via a wide range of symptoms:

  • Respiratory distress/difficulty breathing
  • Coughing
  • Exercise intolerance
  • Swelling of the face/neck/front legs
  • Vomiting
  • Regurgitation
  • Excessive thirst and urination (often from high calcium levels caused by the cancer)
  • Muscle weakness
  • Esophagus loss of strength, becoming dilated
  • Orthopnea (altering the normal body stance — e.g., stretching or elevating the head and neck in order to aid breathing)Weight loss
  • Decreased appetite
  • Lethargy

Thymomas can also cause an autoimmune disease called myasthenia gravis, which causes profound muscle weakness. Myasthenia gravis can also occur due to other factors, such as genetic influences.  .

Workup to Diagnose Thymoma

Some symptoms of thymomas in dogs are similar to symptoms of lymphoma, which is cancer often affecting the lymph nodes including the mediastinum lymph nodes in the chest near the thymus. is Mediastinal lymphoma is a more common occurrence than  thymoma. A benign growth in the throat area or an infection also can cause some of the same symptoms as thymoma. If your dog is showing any of the symptoms mentioned above, a trip to your veterinarian is recommended to determine the cause.

If thymoma is suspected, the veterinarian will perform or order a series of tests to rule out other possibilities. These tests are also used to determine a treatment course if a thymoma diagnosis is confirmed.

  • It is likely your veterinarian will order bloodwork, including a complete blood count (CBC) and a blood chemistry profile. These tests reveal a lot about a dog’s overall condition, which could help determine which treatment course is recommended.
  • Since thymoma can cause myasthenia gravis, your veterinarian also may order an acetylcholine receptorantibody test (blood test).
  • A fine needle aspiration might be performed. This entails using a fine needle to withdraw cells or fluid from the tumor. The sample is then forwarded to a pathologist, who can determine whether cancer is present.
  • If the results of a fine needle aspiration indicate the tumor has many lymphocytes, it could be difficult to distinguish between a lymphocyte-rich thymoma and lymphoma. In this case, a biopsy to remove a piece of the mass may be recommended.
  • Since respiratory distress is a symptom, a veterinarian often willorder three-view chest X-rays as an initial screening tool, to rule out pneumonia or determine whether there is a mass present in the chest cavity. Chest x-rays can also help assess if there is spread (metastasis) to the lung tissue.  Fortunately, thymoma metastasis is uncommon.
  • An ultrasound may be recommended to get a better view of the mass and to attempt to assess the local thoracic lymph nodes. An ultrasound would be employed for an aspiration cytology and/or a biopsy.
  • If surgery to remove the thymoma is considered, a CT scan is highly recommended to help the surgical team determine the feasibility of removal, the best way to remove the tumor and to give a more detailed/specific view if the cancer has invaded into the lymph nodes in the chest cavity.

Thymoma Treatment and Prognosis in Dogs


If a thymoma is isolated within the thymus and can be removed, then open chest surgery to perform a thymectomy is the treatment of choice. The prognosis when surgery is successful is often good, with many dogs enjoying a long disease-free interval


If a tumor is infiltrative and has wrapped around vital structures, surgery is not an option. There can also be instances when a tumor is surgically removed but some cancer cells might remain. Lastly, sometimes surgery can be performed but is not elected for other reasons. In any of these circumstances, radiation therapy likely will be recommended to kill the malignancy. There are two primary types of radiation: conventionally fractionated radiation therapy (CFRT) and stereotactic radiation (SRS). The latter is more technologically advanced. SRS uses higher doses of radiation and precision targeting, meaning that fewer treatments are needed — 1 to 3, vs. 15 to 21 with CFRT. Since a dog must be anesthetized for radiation, SRS also results in far fewer anesthetic events, which can be a significant factor depending on a dog’s overall health. Thymoma often responds well to radiation, with the tumor(s) shrinking and staying in a “quiet” stage where there is no growth. Dogs that undergo radiation treatment can do well for a long time.


Prednisone, a corticosteroid, might be recommended for treating lymphocytic-rich thymoma. In many cases, treatment results in temporary tumor shrinkage.

Forgoing Treatment

Left untreated, a thymoma will continue growing. Thankfully, a canine thymoma often tends to grow slowly in general, but any symptoms displayed will persist and  eventually will worsen.

PetCure Oncology Treats Thymoma in Dogs

PetCure Oncology’s mission is to provide pets that have cancer with the best quality of life possible. To that end, we offer a wide range of options for treating thymoma and other types of cancer. Our available treatments include SRS radiation therapy, in which we specialize. You will find that we are caring and compassionate as we work with you and your beloved pet to extend your time together. For more information about our mission and treatment options, find a location near you and contact us today.

The contents of this article were provided in part by Dr. Renee Alsarraf, DVM, DACVIM (Oncology), a board-certified veterinary medical oncologist and member of PetCure Radiation Oncology Specialists (PROS).

Types of Cancers in Dogs

If your dog is displaying any symptoms of cancer or has been diagnosed with cancer, sort below by cancer type or tumor location to learn more about the most common types of cancer in dogs and available treatment options. Click on the links for more specific information on treatment and real patient stories.


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