Thymomas are rare tumors that arise from the epithelium (lining) of the thymus gland in the dog and cat. They are typically diagnosed in older animals, with the median age in dogs being 9 years and in cats it is 10 years. There is no breed predilection but medium and large dogs are overrepresented. Thymomas are considered benign or malignant based on their clinical features rather than on histologic (under the microscope) features. Benign thymomas do not invade into adjacent structures within the chest cavity while malignant thymomas do invade adjacent structures.

Common signs of thymoma

  • Patients may show signs of respiratory distress such as coughing or difficulty breathing
  • Facial, neck, or front limb swelling may occur due to the obstruction of the blood vessels or the lymphatic vessels -a condition known as precaval syndrome
  • Up to 40% of dogs with thymomas (and occasionally cats) will have signs of myasthenia gravis? This syndrome is characterized by muscle weakness and megaesophagus (distension of the esophagus which can cause chronic regurgitation)
  • Immune-mediate anemia can be seen in patients with thymomas
  • A severe form of dermatitis (skin disease) has been associated with thymomas in cats

How is thymoma diagnosed?

  • Staging tests include CBC / biochemistry profile / urinalysis, chest radiographs (x-rays), chest ultrasound, and in some cases a CT scan.
  • Thoracic (chest) radiographs typically show an anterior mediastinal mass (mass in the front portion of the chest cavity), pleural effusion (fluid around the lungs), and occasionally megaesophagus (distended esophagus).
  • Cytology (aspirate) of the mass or of the pleural fluid can be performed but typically does not yield a diagnostic sample (the epithelial cells of the tumor).
  • Typically lymphocytes are the cells seen on cytology which can make differentiating a thymoma from another cancer, such as lymphoma, difficult.
  • With a thymoma mature lymphocytes should be visualized on cytology whereas with lymphoma lymphoblasts (immature lymphocytes) would be seen.
  • Flow cytometry (an advanced type of test that evaluates every cell in a sample using lasers) can be used to help differentiate between thymoma and lymphoma.
  • A biopsy is needed to confirm the diagnosis of the thymoma. Biopsy is usually obtained at the time of surgical removal of the tumor / mass.
How is thymoma treated?
  • The best therapy for thymoma is surgical removal (a CT scan may be performed prior to surgery to determine how invasive the tumor is but ultimately, visualization of the tumor while in surgery is necessary to determine if a particular mass can be removed surgically).
  • Treatment for myasthenia gravis must also be provided in patients suffering from it. 
  • If a thymoma cannot be removed or if the entire tumor cannot be removed with surgery, radiation therapy can be used to treat the remaining mass.

     Will chemotherapy be recommended?

Sometimes chemotherapy can be recommended, but it is not the most common therapy for this type of cancer.

     Will radiation therapy be recommended?

Radiation therapy will be recommended in many cases of this tumor when the entire mass has not be widely resected by surgery.


What is the prognosis for dogs and cats with thymomas?

  • The prognosis for dogs with surgically resected benign thymomas without megaesophagus is good, with long term remissions and cures seen
  • In dogs whose tumors are surgically resectable and who do not have myasthenia gravis there is a greater than 80% survival at 1 year
  • In cats the median survival time was almost 2 years for surgically resected thymomas
  • Patients with non-resectable thymomas generally have a poorer prognosis and show variable responses to radiation and chemotherapy

Contributed by: