Tumor of the Thymus in Cats

The thymus is an organ responsible for the maturation of a special immune cell called a T-lymphocyte. This unique organ sits just in front of the heart, or more specifically, in the cranial anterior mediastinum of the chest cavity. A tumor of the thymus is known as a thymoma and arises from these immune cells (T-lymphocytes) the organ continuously makes. A thymoma tumor is commonly seen in older cats approximately ten years of age, but because clinical signs do not usually appear until the tumor has had time to grow, the tumor could be found inside felines of a younger age.

A tumor of the thymus in cats can be difficult to pinpoint, as most clinical signs associated with a thymus tumor are non-specific. Some of the most common ailments you may note in your feline with this type of tumor include a sudden decrease in weight, difficulties swallowing, exercise intolerance and in some cases, coughing. However, other conditions such as a throat obstruction could cause the same symptoms, which is why immediate care by a veterinary professional is a must.

When a tumor of the thymus arises in a feline, the clinical symptoms are rather non-specific and difficult to differentiate from other common cat ailments. It is not until the tumor evolves over time that symptoms start to become more prevalent in your cat. The most commonly noted symptoms in cats with a tumor of the thymus include: 

• Weight loss
• Difficulty swallowing
• Coughing
• Difficulty breathing 
• Intolerance of exercise 

Thymomas can also cause secondary conditions of the immune system such as: 

• Hypercalcemia: A high level of blood calcium that presents lethargy, weakness, a decrease in appetite, polyuria and polydipsia.
• Autoimmune Myasthenia Gravis: A chronic autoimmune disease of the neuromuscular system with muscle weakness as its main symptom.

Types

Tumor of the thymus in cats can either be non-invasive or invasive. Both thymoma tumors can cause the previously stated clinical symptoms, but additional symptoms have been reported that are specific to the tumor type. 

Non-Invasive Thymoma

A thymus tumor that is classified as a non-invasive is confined to the thymus. Symptoms of non-invasive thymomas include: 

• Dyspnea and coughing are often noted in a non-invasive thymoma due to the fact that the tumor is pushing on the trachea. 
• Drooling and dysphagia are also signs of a non-invasive tumor as the esophagus is also being pushed by the large tumor. 
• Laryngeal paralysis
• Thymoma-associated Paraneoplastic syndrome (hypercalcemia & Autoimmune Myasthenia Gravis) 

Invasive Thymoma

With a thymus tumor that is classified as an invasive thymoma, the cancer has spread to its surrounding structures. Symptoms of invasive thymoma include:

• Thymoma-associated Paraneoplastic syndrome (hypercalcemia & Autoimmune Myasthenia Gravis)
• Cranial vena cava syndrome (swelling of the throat, head, and limbs)

The cause for a tumor of the thymus to occur in cats and all other mammals in unknown. Thymomas are very rare in cats and males are thought to have a predisposition to the disease.

Due to the unspecific clinical signs of a tumor of the thymus, diagnosing a cat with this disease ultimately begins with a chest x-ray, finding a tumor just in front of the heart. The chest x-ray, or thoracic radiograph, will not be enough to determine the type of the tumor and a biopsy will be performed. The veterinarian will likely choose to perform an aspiration cytology, or fine needle aspiration biopsy, to remove a small portion of the mass to examine. Aspiration cytology is a very useful diagnostic technique when it comes to thymomas, however, a thymoma tumor can be easily confused with a lymphoma tumor, causing need for additional tests. 

The veterinarian may request the feline to have an ultrasound taken of the chest, an MRI or a CT scan to get a better look at the exact location of the tumor. A urinary analysis and blood analysis are also commonly used as diagnostic tools for a tumor of the thymus in cats. 

The primary treatment for a tumor of the thymus in cats is surgery, removing the thymoma tumor. However, invasive tumors that have spread to surrounding structures cannot be removed safely and the veterinarian may advise one of the following treatment options:

Radiation

Radiation treatments are administered daily, usually five days a week and have proven to be very effective. 75% of felines respond to radiation treatments. 

Chemotherapy

Chemotherapy is a combination of drugs infused to the patient over a select period of time. Chemotherapy is not usually the first choice for veterinarians, as a complete response to the drug is uncommon.

The overall prognosis for cats recovering from a tumor of the thymus is excellent. The majority of cats that have had the thymoma tumor surgically removed are cured of the disease and only 20 percent of cats have had a reoccurrence of the disease. The average survival time of a cat previously diagnosed and treated for thymoma cancer is between two and five years. Pet owners can improve their feline’s survival rate by following up with the veterinarian and completing tests, treatments, etc. as directed.