Thymic Lesion
A thymic lesion is an abnormality of the thymus gland, a small organ in the front of the chest behind the breastbone that plays an important role in the developing immune system. Thymic lesions range from benign findings—such as thymic hyperplasia, thymic cysts, and small thymic remnants—to tumors such as thymoma, thymic carcinoma, and other rare cancers. Many are discovered incidentally on chest CT or MRI performed for other reasons. The likely cause depends on age, imaging features, and any associated medical conditions, and most thymic lesions can be characterized with imaging and, when needed, biopsy or surgical removal.
What is it?
The thymus is a small, two-lobed organ located in the front portion of the upper chest (anterior mediastinum), just behind the breastbone. It is most active during childhood, when it plays a key role in the development and maturation of T-cells, an important part of the immune system. After puberty, the thymus gradually shrinks and is progressively replaced by fat, but it remains present in adults and can be the site of various abnormalities.
A thymic lesion is any abnormality involving the thymus, and several different conditions fall under this term. Thymic hyperplasia refers to enlargement of the gland, sometimes seen after stress or treatment with chemotherapy (“rebound” hyperplasia), and may also be associated with autoimmune conditions such as Graves disease or myasthenia gravis. Thymic cysts are fluid-filled lesions that may be congenital or develop later in life; most are benign and asymptomatic. A small, residual amount of thymic tissue in adults can occasionally appear prominent and is sometimes labeled a thymic remnant.
Thymic tumors include thymoma, the most common tumor of the thymus, and the less common but more aggressive thymic carcinoma. Other tumors that can arise in or near the thymus include thymic neuroendocrine tumors, lymphomas involving the thymic region, and germ cell tumors. Thymomas are notably associated with several systemic conditions, the most well-known being myasthenia gravis—an autoimmune disorder that causes muscle weakness—so patients diagnosed with thymoma are usually evaluated for these associated diseases, and vice versa.
Symptoms depend on the type, size, and location of the lesion. Many thymic lesions cause no symptoms at all and are discovered incidentally on chest imaging done for other reasons. When symptoms occur, they may include vague chest discomfort, cough, shortness of breath, or, in larger lesions, signs of pressure on nearby structures such as facial or arm swelling from compression of major veins. Symptoms from associated conditions—such as muscle weakness, drooping eyelids, double vision, or fatigue from myasthenia gravis—can sometimes be the first clue that leads to a chest scan and discovery of a thymic tumor.
CT of the chest with intravenous contrast is the primary imaging test for evaluating a thymic lesion because it provides detailed information about size, shape, density, internal characteristics (such as fluid, fat, or calcification), and relationship to nearby structures. MRI is particularly useful when there is overlap on CT between thymic hyperplasia and tumor, since specific MRI techniques can detect microscopic fat within hyperplastic thymic tissue. PET/CT may be used in selected cases, especially for more aggressive lesions. Many thymic lesions ultimately require tissue sampling for definitive diagnosis, often through surgical removal rather than needle biopsy, since this both confirms the diagnosis and may be the definitive treatment.
Important to Know
Treatment of a thymic lesion depends on the specific diagnosis. Benign findings such as small thymic cysts and stable thymic remnants often require no treatment beyond reassurance and, in some cases, follow-up imaging. Thymic hyperplasia may regress on its own, particularly when related to a temporary cause such as recent illness or chemotherapy.
Thymomas are typically treated with complete surgical removal, often through a minimally invasive approach when feasible. Depending on the stage and findings, additional treatment with radiation or chemotherapy may be recommended. Thymic carcinoma and other more aggressive tumors are usually managed with a combination of surgery, radiation, and chemotherapy in specialized centers. Lymphomas involving the thymic region are generally treated with chemotherapy and sometimes radiation, with surgery used for biopsy rather than removal.
Because thymomas and other thymic tumors are associated with paraneoplastic syndromes such as myasthenia gravis, pure red cell aplasia, and certain autoimmune disorders, care is typically coordinated by a multidisciplinary team that may include thoracic surgeons, oncologists, neurologists, hematologists, and radiation oncologists. Imaging findings are always interpreted alongside the patient’s symptoms, medical history, and any associated conditions.
Red flag symptoms include rapidly worsening shortness of breath, severe chest pain, facial or upper-extremity swelling, new or worsening muscle weakness (especially around the eyes, face, throat, or limbs), difficulty swallowing, persistent cough, coughing up blood, or unintended weight loss. These warrant prompt medical evaluation.