A thymic cyst is an epithelium lined cyst involving the thymus. These cysts are often separated into congenital or acquired types. Congenital cysts are thin-walled, having thymic tissue in their lining and acquired cysts are multilocular and are usually formed as a result of an inflammatory process.
Most mediastinal cysts, including thymic cysts, are asymptomatic and incidentally found after chest imaging for other reasons. Approximately half of individuals with thymomas and thymic cysts are clinically asymptomatic. Symptoms are more commonly observed in infants and children, in which the cyst enlarges and compresses structures producing related symptoms . For example, a thymic cyst may enlarge to compress the trachea resulting in dyspnea. Adults may present with non-specific chest pain, cough, stridor, and/or dyspnea .
Workup consists of ensuring the proper diagnosis of a thymic cyst and ruling out associated benign and malignant conditions such as: cystic teratoma, lymphangioma, cystic degeneration of a seminoma, pericardial cyst, thymoma, and thymic cancer . Workup begins with patient history, physical exam, and imaging tests. The most common imaging modality currently used to evaluate a mediastinal mass is a chest computed tomography (CT) scan. Other imaging modalities used include magnetic resonance imaging and integrated positron emission tomography and CT (PET/CT) scan. A definitive diagnosis is established by pathological tissue examination (via biopsy or surgical resection).
On standard chest radiographs, thymic cysts may not be visible or may be indistinguishable from other non-lobulated thymic masses . Further workup should include a chest computed tomography (CT) scan and biopsy if clinically indicated. On CT scan, congenital thymic cysts usually appear as well-defined water-attenuated masses with translucent walls. Multilobular thymic cysts appear as well-defined, heterogeneous, cystic masses with a visible wall  . Some thymic cysts, in particular acquired cysts, may have an increased CT attenuation if infection or hemorrhage is present; these cysts may be erroneously diagnosed as solid masses. In some cases, there may be curvilinear calcification of the cyst wall .
Magnetic resonance imaging (MRI) depicts thymic cysts having typical characteristics of fluid (e.g., low signal intensity on T1-weighted images and uniform high signal intensity on T2-weighted images). If hemorrhage or infection is present, the cysts will show high signal intensity on both T1- and T2-weighted images. In summary, uncomplicated thymic cysts have the following characteristics on MRI :
A PET/CT scan provides a CT image as well as information about the avidity of the lesion for glucose (FDG uptake). Malignant cells tend to have higher glucose avidity, hence producing a high uptake value on the PET/CT scan. This information can provide clues about whether a thymic lesion is a benign cyst or malignant entity .
Definitive diagnosis is achieved by obtaining tissue for pathologic examination by way of either a surgical biopsy or surgical resection. Most surgical resections are performed using minimally invasive surgical technique called video-assisted thoracoscopy (VATS). VATS biopsy requires the patient to be placed under general anesthesia in the operating room. More recently, robotic surgical approach has been used to evaluate and resect mediastinal masses (including thymic cysts) .