So whether the tumors are called malignant depends not on their appearance but on their behavior (whether they have invasive characteristics). In the absence of invasion, the presence of a clearly defined covering, or "capsule," tends to classify the tumor as benign . But since all thymomas can become invasive and extend beyond the capsule, they should all be considered potentially malignant.
How It Spreads Most malignant thymomas grow slowly and have a tendency to recur in the chest at the site of origin. They seldom metastasize elsewhere in the body. They can spread to the heart or lungs, especially to the coverings over the heart (pericardium) and lungs (pleura).
What Causes It The cause is unknown, but a number of systemic conditions, or syndromes, are associated with thymoma. Associated immune disorders include low red blood counts (pure red cell aplasia), low gamma globulins, signs and symptoms of connective tissue disorders, thyroiditis, ulcerative colitis, pernicious anemia , enteritis, sarcoid and, especially, a special type of muscle weakness called myasthenia gravis.
Various endocrine disorders are associated with thymoma, including hyperthyroidism, adrenal insufficiency (Addison's disease) and hypopituitarism.
