Tumor of the Thymus in Dogs

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

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A closer look: Tumor of the Thymus 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.

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Risk factors

Additional conditions associated with thymic tumors include myasthenia gravis syndrome and hypercalcemia.

Symptoms of myasthenia gravis syndrome

Difficulty swallowing

Regurgitation in Dogs Excessive Drooling (Ptyalism) in Dogs Weakness in Dogs Weight Loss in Dogs Muscle atrophy in dogs

Symptoms of hypercalcemia

Increased Thirst (Polydipsia) in Dogs Excessively Frequent Urination (Pollakiuria) in Dogs

Decreased appetite

Weakness in Dogs Lethargy in Dogs

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 in Dogs Coughing in Dogs Difficulty Breathing (Dyspnea) in Dogs

Dysphagia (difficulty swallowing)

Weight Loss in Dogs Excessive Drooling (Ptyalism) in Dogs

Invasive tumors of the thymus can present with the above signs, as well as cranial vena cava syndrome. The characteristic symptom of cranial vena cava syndrome is 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.

Prevention

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

Is Tumor of the Thymus in Dogs common?

Thymoma is rare in dogs.

Typical Treatment

Surgical removal of tumor
Radiation therapy
Immunosuppressants
Medical management of associated syndromes (immunosuppressants, anticholinesterase medications, motility drugs, histamine antagonists, and antibiotics)