Findings: PA radiograph shows a rounded well-defined mass at the costophrenic angle in the lower left hemithorax (A, arrow). We can infer that the mass is anterior because is displacing the heart towards the right and it does not obliterate the para-aortic line.
In my opinion, when a large mass if adjacent to the midline it is difficult to determine whether it is mediastinal or pulmonary.

Coronal and axial enhanced CT show a solid mediastinal mass with areas of necrosis (B-C, arrows). The most common solid lesion in the cardiophrenic angle are enlarged lymph nodes, which usually are multiple and not very large. In big soft-tissue tumors of this area a thymic origin should be suspected. Although thymic tumors originate in the anterior superior mediastinum, they may slide down along the mediastinal planes and appear at the cardiophrenic angle in the lower mediastinum. Biopsy confirmed the diagnosis of thymoma.

Final diagnosis: mediastinal thymoma

Congratulations to Mohamed Abdulghaffarand MK who were the first to suggest the correct diagnosis

Teaching point: Remember that CT is very helpful in diagnosing cardiophrenic angle masses according to their radiographic density: fat (pericardial fat pad, Morgagni’s hernia); fluid (pericardial cyst) and soft-tissue ( lymphadenopahy and the occasional thymic tumour).