Other considerations might include the possibility of a cardiac, pericardial or mediastinal mass. This is an unusual location for a cardiac abnormality. A left ventricular aneurysm is usually more inferior. Occasional but very rare corrected transposition of the great vessels can result in prominence of the left cardiac/mediastinal border, however, is associated with a narrow superior mediastinum due to superimposition of the aorta and pulmonary artery (so called "egg on a string" ).
Pericardial abnormalities include the possibility of masses and more commonly a pericardial cyst, for which this appearance is a possibility. Mediastinal abnormalities include most anterior mediastinal masses in particular in this location extending inferiorly over the cardiac right or left sided silhouette the possibility of a thymic mass.
CT imaging demonstrates that the epicentre of the lesion is higher within the anterior mediastinum and that the lesion lesion is uniformly of low attenuation. There is a comparable cystic appearance at MRI with low T1 and high T2 signal. No peripheral soft tissue is appreciated. Post gadolinium acquisitions (not demonstrated) demonstrated no enhancement within the lesion or at its margins.
The location of the lesion favours a mediastinal abnormality rather than a pericardial lesion. The diffuse cystic nature of the lesion is likely due to a thymic cyst. Thymic cysts may be cervical, mediastinal or a combination. Typically these demonstrate entirely cystic appearances, although the typical low T1/ high T2 signal may be affected by some variation in this signal according to variable turbidity and the often gelatinous nature of the contents of the cyst. Usually these are unilocular but multilocular lesions can occur too.
Thymic cysts are benign lesions which may be congenital or acquired, the latter usually occurring as the result of treatment of mediastinal lymphoproliferative disorders. They are also described as a postoperative complication although typically fluid collection in the anterior mediastinum are not within the thymus itself.
Thymic cysts are thought to reflect congenital cystic transformations of ductal epithelial formations of residual branchial pouch elements. Histological sampling may demonstrate walls with squamous, columnar or cuboidal epithelium with thymic resisual tissue. The walls demonstrate areas of inflammation, haemorrhage, fibrosis or granulation tissue. Typical Hassall's corpuscles in the cyst wall may be identified, more commonly in congenital lesions.
Thymic cysts require no specific treatment unless there are enlarging or causing mass effect. Reports of rapidly enlarging thymic cysts are usually due to haemorrhage within the lesion which would necessitate surgery for diagnosis and treatment. MRI may be of assistance in differentiating the rarer instance of a cystic thymoma or near complete cystic germ cell tumour. The absence of a peripheral rim of soft tissue or septation strongly and at the case against those diagnoses in this patient.