In order to approximately translate the dose at the reference point (expressed in the equipment coordinate system) to the dose in the patient anatomy, it is necessary to establish the relationship between the patient and the equipment.
This is accomplished by recording the distance in the table plane from the Equipment Landmark to each Patient Fiducial in TID 10002 (128756, DCM, "Equipment Landmark to Patient Fiducial Z Distance").
This distance is likely recorded by the operator who can measure it from the table top to the patient before starting the procedure. Note that the patient is assumed to be centered in the X axis of the X-Ray table (i.e., in the patient left-right axis).
For the creators of RDSR it's important to notice that the distance between the Equipment Landmark and each Patient Fiducial does not change within the RDSR across all Irradiation Events. In other words, if the distance should change during the scope of the RDSR because the patient moves and changes position during the XA procedure step, and this CONTAINER is included in the RDSR, an additional RDSR Instance should be created.
Also important to notice is that the same Patient Location Fiducial should not be repeated multiple times in the CONTAINER with different distances, and that the distances of all the Patient Location Fiducials should be consistently defined for a real patient shape.
Figure UUUU.4.5.3-1 illustrates the relationship between these concepts.
Table UUUU.4.5.3-2 shows an example of encoding patient position and patient location for a patient of 175 cm height laying on the table in head first supine orientation, with the top of the head at 15 cm from the table head in the direction of the table foot.
Table UUUU.4.5.3-2. Example of Encoding Patient Position, Equipment Landmark, and Patient Fiducial