The thyroid appears normal on these CT images.  study. There are no signs of calcifications or fat tissue in this mass to think in mediastinal germ cell tumors, also the age group would be atypical. The features suggesting the invasion of the thoracic wall would be atypical for lymphoma as well. At this age group and with this appearances differentials are favored among thymoma and, less likely, metastatic disease. 

Later proven to be a thymoma.