Tumor of the Thymus in Dogs

Key Takeaways

Thymoma in dogs is a rare cancerous tumor of the thymus, a glandular organ located in the upper chest.

• The cause of thymoma is unknown

• Thymoma can be classified as invasive or non-invasive

• Symptoms can be non-specific, such as exercise intolerance, coughing, dyspnea (difficulty breathing), dysphagia (difficulty swallowing), and weight loss

• Associated conditions include cranial vena cava syndrome (with associated swelling of the jaw, neck, and forelimbs), myasthenia gravis, which is often associated with megaesophagus (an abnormally dilated esophagus), and hypercalcemia (resulting in increased thirst, increased urination, decreased appetite, weakness, and lethargy)

• Diagnosis is by imaging (x-rays, CT, MRI) and biopsy

• For non-invasive tumors, surgical removal is usually curative and the prognosis is excellent

• Radiation therapy is very effective for tumors that cannot be removed surgically

• For those dogs with megaesophagus or aspiration pneumonia, prognosis is guarded to poor

A Closer Look: What is Thymoma in Dogs?

The thymus is an organ located in the chest cavity in front of the heart. It acts as a part of the immune system to produce mature T cells used to fight disease in the body. Thymomas are often associated with myasthenia gravis.

Thymoma is rare in dogs and has a good prognosis with surgical removal. Secondary myasthenia gravis is commonly associated with aspiration pneumonia, which is a life-threatening emergency.

Risk Factors

Additional conditions associated with thymic tumors include

Myasthenia gravis syndrome:

• Difficulty swallowing




Weight loss

• Poor body condition (muscle wasting)

• Secondary aspiration pneumonia symptoms including fever and rapid breathing


Increased thirst

Increased urination

• Decreased appetite



Possible Causes

The cause of thymoma is unknown. It is most common in medium and large breeds, with Labrador Retrievers and German Shepherds over-represented.

Main Symptoms

Non-invasive thymic tumors are sometimes asymptomatic. Other symptoms may include: 

Exercise intolerance


Dyspnea (difficulty breathing)

• Dysphagia (difficulty swallowing)

Weight loss


Invasive tumors of the thymus can present with the above signs, as well as cranial vena cava syndrome: 

• Swelling of submandibular area, neck, thoracic inlet, and thoracic limbs

Testing and Diagnosis

The first steps to diagnose a case of possible thymoma are physical exam, blood work, urinalysis, and x-rays. Tumors are usually identified in x-rays, but other imaging techniques may also be recommended (ultrasound, CT, MRI). From there, ultrasound guided or surgical biopsy may be performed. Exploratory thoracotomy (exploring the upper chest area surgically) may be necessary to differentiate invasive and non-invasive tumors.

Steps to Recovery

Treatment is surgical removal of the tumor, if possible. If the tumor has invaded the vital organs of the chest and excision is not possible, radiation therapy is the treatment of choice.

In dogs with secondary myasthenia gravis, treatment also requires intensive care to ensure they are receiving adequate nutrition, medical intervention with immunosuppressants,  anticholinesterase medications, motility drugs, histamine antagonists, and antibiotics for management of megaesophagus.

Prognosis with complete surgical removal is excellent and usually curative. In about 1/3 of cases, the tumor recurs locally, and a second surgery is likely to be effective. In cases where the tumor has metastasized (rare), cannot be removed, and/or is associated with aspiration pneumonia, prognosis is guarded to poor.


As the cause is not known, specific prevention is not identified. Thymoma is not contagious. Regular veterinary care can help detect symptoms early.

Is Thymoma Common in Dogs?

Tumor of the thymus in dogs is rare.

Typical Treatment

• Surgical removal of tumor

• Radiation therapy

• Immunosuppressants

• Medical management of associated syndromes (immunosuppressants,  anticholinesterase medications, motility drugs, histamine antagonists, and antibiotics)

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