I check the isocenter for every plan. If the User Origin in the Eclipse plan was not moved by dosimetry, the numbers should match exactly. If the User Origin was moved (i.e. it is not coincident with the DICOM origin), you must either add or subtract the DICOM offset from the isocenter values to reconcile them. Care must be taken when patient is prone, since the X-coordinate switches sign.
Jose C Pichardo, Ph.D., DABR