Thymic Cyst

Clinical History

A 72-year-old patient was referred to the hospital’s outpatient department for evaluation worsening dyspnea.

Imaging Findings

A 72-year-old patient was referred to the hospital’s outpatient department for evaluation of a long-standing, slowly growing mediastinal mass occluding the left contour of the heart, pulmonary hilum and aortic arch. The patient had no history of previous thoracic surgery, radiation therapy or chemotherapy of a malignant tumor. The mass was evaluated with CT several years ago showing its cystic nature and was diagnosed as a pericardial cyst. The cyst was slowly growing. The patient developed worsening dyspnea during the last 11/2 years. Actually, the patient was referred to evaluate the possibility of surgical treatment. Because the last CT was performed several years ago and did not meet the current quality standards an MD-CT was performed (4x2.5 mm, 12.5 mm, 120 ml non-ionic contrast media with 370 mg/ml iodine i.v., 30 sec scan delay). CT showed a simple cyst occupying with its upper part the anterior mediastinum and with its lower part the left paracardial space displacing the left lung but not reaching the cardiophrenic angle. The cyst was not clearly separable from the pericardium. The density amounted 15-17 HE. No solid parts were seen. Conclusive diagnosis based only on CT criteria was not possible. Pericardial cyst, thymic cyst and lymphangioma have been discussed as differential diagnoses. The fact that the cyst was growing slowly and the possible impact on dyspnea due to lung compression where the reasons for thoracoscopic removal. Histopathologic assessment showed a benign thymic cyst.

Discussion

Cystic lesions in the anterior mediastinum may be pericardial cyst, thymic cyst, or lymphangioma. Differential diagnosis based only on imaging techniques is often not effective. All three may have varying densities on CT ranging from water to low soft tissue density. In case of larger cysts the location don’t serve as a useful criteria because the cyst may cover a range where all three kinds of cyst may occur with equal frequency. However, the fact that the cyst in the current case did not reach the cardiophrenic angle made a pericardial cyst less likely. On the other hand, growth along the pericardial reflection was more typical for a thymic cyst. Thymic cysts may be a part of cystic tumors such as cystic thymoma or thymic seminoma. They have been also reported in Hodgkin disease involving the thymus. Even when radiotherapy eradicates the tumor the cystic part may remain unchanged. In some cases the cysts may develop after irradiation of the mediastinum or chemotherapy and at least some of them are benign(1). Most of tumor-associated cysts are multiloculated (2). Hydatid cysts may occasionally present as thymic cysts (3). Multilocular thymic cysts with follicular lymphoid hyperplasia are found in HIV positive adults (4) and in 1 % of HIV positive newborn (5). MRI may provide useful additional information in order to differentiate real cysts from cystization of masses. In conclusion, the clearly cystic nature of the mass, its location and growth pattern along the pericardial reflection without occlusion of the cardiophrenic angle, the very slow growth over several years and the absence of any history of tumor or tumor treatment make the diagnosis of a thymic cyst most likely.

Differential Diagnosis List

Final Diagnosis

Thymic cyst

URL:	https://eurorad.org/case/822
DOI:	10.1594/EURORAD/CASE.822
ISSN:	1563-4086