The most common posterior mediastinal mass is
The posterior mediastinum contains the following structures: sympathetic ganglia, nerve roots, lymph nodes, parasympathetic chain, thoracic duct, descending thoracic aoa, small vessels and the veebrae. Most masses in the posterior mediastinum are neurogenicnature Ref Davidson 23rd edition pg 580
Neurogenic tumor
Lymph nodes
Neurogenic parasitic cyst
Teratoma
Respiratory system