The word "mediastinum" refers to the central area in the chest, between the lungs, where important organs like the heart, major blood vessels, and other structures are located. Full definition
Feline Lymphoma Lymphoma is a cancer of the lymphocytes commonly originating in the gastrointestinal tract of mediastinum of cats. (vet.osu.edu)
Stage IV: all peripheral lymph nodes plus the spleen, liver, and / or anterior mediastinum in the chest involved (marvistavet.com)
On rare occasions ectopic thyroid tissue (abnormal migration of thyroid tissue during embryonic development) can give rise to lesions in the tongue, ventral neck, cranial mediastinum and heart base. (wearethecure.org)