With regards to imaging of primary extra nodal lymphomas, in which of the following sites does extra nodal Hodgkin's disease most commonly occur:

At presentation, Hodgkin disease is usually supra diaphragmatic which means thymus is involved due to contiguous spread from one nodal group to the next along the lymphatic pathways. Hence thymus is involved before spleen and other abdominal organs. Thyroid gland is rarely involved in Hodgkin lymphoma.