Tumor of the Thymus in Dogs

Thymoma is a rare tumor that most commonly affects medium to large breed dogs and is most often found in the Labrador and German Shepherd breeds. Most thymomas occur in older dogs, with the median age of 11 years. Thymomas may cause cranial caval syndrome, hypercalcemia, aspirational pneumonia, and/or myasthenia gravis.

The thymus is an organ within the lymphatic system located inside the chest cavity just in front of the heart. The thymus produces a subset of lymphocytes, or white blood cells, called T-cells, which are integral in killing foreign cells, activating other immune cells to fight infection, and ceasing the immune response once a threat has been eliminated. A thymoma is a rare tumor originating from the epithelium of the thymus, or the layer of tissue surrounding the thymus.

• Coughing
• Labored breathing
• Rapid breathing
• Cranial caval syndrome: swelling of the head, neck, and forelimbs, fluid build-up in the chest
• Myasthenia gravis: muscle weakness, enlarged esophagus, frequent regurgitation
• Hypercalcemia (high calcium level in the blood): increased thirst, increased urination, decreased appetite, weakness, and lethargy
• Decreased exercise tolerance
• Difficulty swallowing
• Weight loss
• Paralysis of voice box
• Aspirational pneumonia
• Abnormal heartbeat

Types

The cause of thymoma is completely unknown.

The veterinarian who you've brought your dog to will conduct a thorough physical examination of your dog and ask you to recount the onset of symptoms in order to aid diagnosis. A chemical blood profile to measure levels of proteins and calcium in the blood will reveal hypercalcemia, which affects 5% of dogs with thymoma. A complete blood count will measure red and white blood cell and platelet levels in order to detect any abnormalities and anemia, which can be found in some dogs with thymomas. Your dog’s blood will also be tested for the presence of antibodies to acetylcholine receptors, which would be a positive indicator of myasthenia gravis. Another indicator of myasthenia gravis is a Tensilon test, which for which your dog will be injected with tensilon (also called edrophonium) and his muscle movements will be tested. If your dog has myasthenia gravis, muscle strength will be improved during the test. Further, a urinalysis will measure mineral content, glucose and protein levels in your dog’s urine.

X-rays are an important diagnostic tool, and will indicate a build-up of fluid in the lungs, as well as the presence of a mass which may be in between the functioning lungs, on the thymus or in front of the heart, and the presence of an enlarged esophagus, which is a symptom of myasthenia gravis. Definitive diagnosis depends upon a biopsy of the mass, which is most commonly conducted once the mass has been entirely surgically removed and is most likely the case with thymoma. If other tumors are present, they will more likely be diagnosed through a fine-needle aspiration biopsy prior to surgery. CT scans, MRIs or chest ultrasounds may be conducted.

Surgery is the primary treatment for thymoma. Tumors may be removed through open surgery, during which the ribs are spread, or the breastbone is cut with a saw. However, some veterinary surgeons may offer a procedure known as VATS, or video-assisted thoracoscopic surgery, in which a camera on a thin tube, or thoracoscope, is inserted into the chest through a small incision and instruments that can remove the tumor are inserted into the chest cavity through two other small incisions. VATS is less invasive and can reduce in-hospital recovery time; however, it may not be available in your area.

Another treatment option only recommended in cases where the tumor cannot be removed surgically, is radiation, which is effective in 75% of cases. Radiation side effects include inflammation of the heart sac and lungs. Radiation treatments are administered daily for a period of 18 to 21 treatments. Chemotherapy is found to be ineffective and is not considered a viable treatment option for thymoma.

The prognosis for invasive thymoma is guarded, as is that of dogs receiving radiation therapy. Dogs receiving radiation therapy have a median survival time of 248 days following treatment. The prognosis for thymic carcinoma is very poor. Prognosis is also negatively affected by concurrent myasthenia gravis and aspiration pneumonia.

The prognosis for noninvasive thymoma is excellent, particularly in dogs whose thymoma is completely removed.

Regardless of the method of treatment, follow-up appointments will be scheduled in order to review x-rays and monitor the tumor’s size, and/or recurrence you're your dog does undergo surgery, management of surgical sites is incredibly important, as is easing your dog into home life after surgery. Be sure to follow all post-surgery instructions, paying special attention to guidance on diet, pain management, and activity.