Thymic cyst arising in the middle mediastinum posterior to the left atrium--a peculiar location. 
While masses of thymic origin are commonly located within the anterior mediastinum, abnormal migration of thymic tissue during development can result in thymic lesions in other intrathoracic locations.  Ectopic thymic lesions in the middle mediastinum are extremely rare.  We present a case of a calcified thymic cyst located posterior to the left atrium entirely within the caudal aspect of the middle mediastinum, a location not previously reported.