This cases illustrates a stable well-defined anterior mediastinal mass with regular calcified walls and low density content. 

The patient brought a previous CT scan (not shown) performed two years ago, in which contrast media was used, demonstrating the lesion with the same characteristics and without contrast enhancement. 

Due the patient age, location and image stability the differential diagnosis list is small, being the hypothesis of a thymic cyst being strongly considered. 

Calcification may be seen in cystic thymic tumors and has been described in mediastinal hydatid although very rare.

Although being a benign lesion, thymic cyst can degenerate into a carcinoma. 

The patient refused further invasive investigation. A follow-up CT study was proposed as the lesion seems to be stable for at least 5 years.