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What is Thymoma?

Thymomas are tumors or masses which can be either benign (non-invasive) or malignant (invasive) and are found in older dogs, with the mean age being approximately 9 years old.  While there has been no particular breed predisposition for thymoma development, it seems that medium and large sized dogs are afflicted more often than smaller breeds.  

The thymus gland, in which the tumor develops, is located in the chest cavity, just in front of the heart.  The close proximity of the thymus gland to the heart and lungs makes a malignancy especially serious.

Thymomas are defined as rare tumors which grow in the lining (epithelium) of the thymus gland of both dogs and cats, usually being found in older animals.


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Symptoms of Thymoma in Dogs

The symptoms of thymoma in dogs will depend upon the size of the tumor.  Here are some of the symptoms you might notice in your pet:

  • Signs of respiratory distress - coughing or breathing difficulties
  • Difficulty swallowing
  • Exercise intolerance
  • Swelling of the face, neck or front limbs
  • Frequent regurgitation or vomiting
  • Muscle weakness
  • Increased thirst 
  • Increased urination
  • Weight loss


There are two types of thymoma in dogs:

  • Benign - This is a non-cancerous mass which is non-invasive, meaning that it doesn’t invade surrounding tissues and organs
  • Malignant - This is an invasive cancerous mass which does invade surrounding tissues and organs

Depending on the size of the tumor, its presence can cause problems like esophageal malfunction, certain types of anemia, neurological issues which can include myasthenia gravis and paralysis of the voice box.

Causes of Thymoma in Dogs

The thymus gland is part of the body’s immune system, reaching its maximum development during puberty and then continuing throughout the life of the host in a diminishing immunologic role.  Here is a simple description of how the tumor is created:

  • The gland develops neoplastic cells for which the etiology is unknown
  • This leads to the creation of nonfunctional tumor cells which eventually begin to develop immunodeficiencies
  • Or, it can lead to the development of functional tumor cells
  • Either type of these cells can proliferate through a complicated process involving suppression of the immune system to become the aforementioned tumor
  • The tumor begins small and, over time, grows large enough to begin to interfere with the normal functioning of the surrounding tissues and organs

As noted above, the reason why the neoplastic cells are created remains unknown as does the reason why some develop as benign while other develop as malignancies.

Diagnosis of Thymoma in Dogs

Your veterinary professional will need a good, thorough history from you that includes the symptoms you have noted, their duration and severity.  He’ll also need a physical health history about your pet which includes vaccinations, unless he already has that information.   Your veterinary professional will do a physical examination of your family pet and will likely need some additional testing to get to the root of the problem.  This testing could include:

  • CBC - complete blood count
  • Various chemistries
  • Urinalysis
  • Radiography (x-rays) of the chest
  • Ultrasound of the chest
  • CT scanning of the chest is also a possibility to further differentiate any masses found in the radiography and/or ultrasound
  • Cytology (aspiration) of the mass or of fluid to look for epithelial cells of the tumor but this doesn’t generally provide a sample of the tumor cells suitable for identification of the mass
  • Eventual biopsy of the mass - typically done through the surgical removal of the mass 

Once the results of the above testing, and any other testing which your vet feels is necessary, an appropriate treatment plan will be developed and initiated.

Treatment of Thymoma in Dogs

Treatment of your pet for thymoma in dogs will be dependent upon the type of mass suspected, the size of the mass and its location.  

  • A smaller benign mass which is not interfering with the normal function of the surrounding tissues and organs might not need to be removed until it gets larger but your vet will want to monitor it for any growth and metastatic changes
  • If a surgical option is recommended, and this is considered the best therapy for thymoma, the previously taken CT scan will be helpful to determine if it has invaded surrounding tissues and organs; ultimately, the tumor will need to be visualized during the surgical removal to ensure that it can be removed without unnecessary damage to surrounding tissue and organs
  • If removal of the tumor is not possible, or if it cannot be removed entirely, then radiation treatments will likely follow the surgery to treat the remaining portion(s) of the tumor - this treatment is likely to recommended whenever all of the tumor is not removed
  • Chemotherapy is an option but it is not the most widely utilized one
  • If myasthenia gravis is found secondary to the tumor, treatments will need to be initiated at some point for that disease

Recovery of Thymoma in Dogs

For dogs having a benign surgically resected thymoma who do not also suffer from megaesophagus (loss of tone and motility of the esophagus which allows regurgitation), the prognosis is good with long term remissions and cures being reported.  Research has reported that dogs having thymoma resected who are not suffering from myasthenia gravis have a greater than 80 percent survival rate at the one year mark.  As you would expect, for those dogs who have thymomas which are not resectable are reported to have a poorer prognosis as well as variable responses to the radiation and chemotherapy options.

While the above reported outcomes don’t paint a pretty picture, they do emphasize the importance of getting your canine family member to your veterinary professional sooner rather than later if you note any of the above symptoms in any degree of severity, or if you notice any unusual behaviors or habits in your pet.  It would be far better to over-protect your family pet than to deal with the results of waiting too long to seek medical care.  Treating your family pet as you would treat your children or anyone you love when something unusual is going on with them will go a long way toward ensuring a long and happy life with your doggy family member.

Thymoma Questions and Advice from Veterinary Professionals

Miniature Schnauzer
11 Years
Mild condition
0 found helpful
Mild condition

Our dog had a thymoma removed couole weeks ago. The way the surgeon told is that he was able to remove it all but duel its size it was stuck to other tissues in which they had to peel it away in order to remove the thymoma. My only question is given this is there still a very good chance this could grow back again? She had no symptoms before diagnosis, it was actually found by accident during a visit to her cardiologist.

Dr. Callum Turner, DVM
Dr. Callum Turner, DVM
3318 Recommendations
The problem with some thymomas is that the thymus is in a very delicate area and if the thymoma is pushing against adjacent structures it is not possible for us to take our usual safety margin of tissue which leave the possibility of cancerous cells being left behind. Another issue with thymomas is that they are generally unresponsive to chemotherapy; it is important to have histopathology done on the excised mass to confirm diagnosis. Regards Dr Callum Turner DVM

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