Mediastinal cystic masses

Cysts are forte of ultrasonologists but when it comes to mediastinum, confusions prevail. We can very well characterize the cyst but we keep manoevring our transducer through the limited intercostal window to see the relations with mediastinal structures. Chest X-Rays are definitely helpful to guide us but in my view, CT scan can clear all the clouds when it comes to mediastinal cyst.
Differential diagnosis
1. Congenital benign cysts - Bronchogenic cyst, Esophageal duplication cyst, Neurenteric cyst, Pericardial cyst, Thymic cyst.
2. Meningocele
3. Mature cystic teratoma
4. Lymphangioma
5. Tumors with cystic degeneration - Thymomas, Hodgkin's disease, Germ cell tumor, Mediastinal carcinomas, Metastases to lymph nodes, Nerve root tumors.
6. Mediastinal abscess
7. Pancreatic pseudocyst
8. Hydatid cyst
Benign cysts : Fundamentals are same for cysts anywhere else in body. A smoothly marginated cyst with thin enhancing walls and homogenous low attenuation non-enhancing contents, with no infiltration of adjacent mediastinum are usually benign. MR will show T2 hyperintensity within cyst.
1. Bronchogenic cyst: Mostly situated near carina in middle or posterior mediastinum, these cysts are mostly detected incidentally during imaging. If on a CXR, you see a subcarinal homogenous round opacity, often towards right side, think of bronchogenic cyst. Confirmation can be done by CT scan revealing a low attenuation cyst. But, again it depends on whether the cyst is infected or got hemorrhage, attenuation may vary and even be more than 100 HU. Sometimes, air-fluid or fluid-fluid levels may also be seen.
2. Duplication cyst: CT/MR imaging appearance are identical to bronchogenic cyst except that they have more intimate contact with esophagus and walls are little bit thicker.
3. Pericardial cyst: You see a cyst at cardiophrenic angle, mostly on right side and if you can demonstrate a communication with pericardial sac, you will hit the bull's eye! But, you get misguided right from the CXR. Some may ignore it as mediastinal fat pad. While the sceptic ones may evaluate for morgagnian hernia. CT does the trick most of the time. (I even got confused with loculated pleural effusion today!)
4. Meningocele: CXR looks similar to a neurogenic tumor radiograph, where you see paraspinal mass in posterior mediastinum, with enlarged intervertebral foramina. CT shows a cyst and you are done with diagnosis.
5. Thymic cyst: Very very rare cyst. Anterior mediastinal cystic mass lesion (just like any other thymic mass lesion) with CT characteristics of cyst should be a thymic cyst.
6. Mature cystic teratoma: Mostly in anterior mediastinum, with soft tissue, fat and calcification. within. Hardly any doubt remain in most of the cases.
7. Lymphangioma: A lobulated cystic lesion insinuating here and there in the mediastinum, even extending to neck sometimes. Seldom they cause any compressive symptom.
Recommended article: http://radiographics.rsna.org/content/22/suppl_1/S79.abstract