A structure that can be partially assembled at one location, transported to a site in modules, and then fully assembled into a structure suitable for housing a therapeutic radiation source is disclosed. The modules include reinforced walls that are filled with radiation shielding fill material to form a barrier around a central treatment area. Additional modules are placed over the first set of modules and form a barrier above the treatment room. A door with a selectively retractable threshold provides access to the treatment room. The module including the radiation equipment is included in removable portions so that the radiation equipment can be removed and replaced. The modules also form a patient waiting room and control area separated from the treatment by the barrier.