From a cat scan: what does "thymic gland tissue in lungs" mean?

Embryonic tissue: residual embryonic tissue from the thmus- an embryonic organ which usually disappears at or after birth.

It may be: just a poorly worded sentence.The thymus gland is not located in the lung, although it is in the thoracic cavity(anterior mediastinum). Residual thymic tissue in the mediastinum is normal at your age. An invasive thymic malignancy could involve the lungs, but otherwise it is impossible to have thymic tissue in the lungs, and even if it was, CT wouldnt be able to tell what it is.