These are specific DICOM standards, in the form of RT DICOM objects, to address the specific needs for Radiation Therapy.

RT Image:  Specifies radiotherapy images obtained by radiation therapy devices, such as conventional simulators and portal imaging devices.  These are often referred to as the "portal films" in the analogue film world.  These are also used for calculated images using the same geometry as the treatment system, such as digitally reconstructed radiographs (DRRs).