Imaging of invasive thymoma in the costophrenic recess presenting as thickening of arcuate ligaments of the diaphragm

Imaging findings in a patient with invasive thymoma in the costophrenic recess are presented, in whom computed tomography (CT) and MRI revealed lateral arcuate ligament thickening. Increased fluoro-deoxy-glucose (FDG) uptake on subsequent positron emission tomography (PET)/CT was helpful in suggesting the correct diagnosis. A second patient with much more obvious invasive thymoma occurring in the costophrenic recess is presented for comparison. It is a well-known fact that thymic malignancies can metastasize to the pleura even years after resection. Rarely, they may present as focal thickening of the diaphragmatic lateral arcuate ligament.