Instead of the current state (returning DataNode), using arbitrary function names, inherit from the interface AbstractFileReader as it is the desired way of IO in MITK. The reading is then done in a standardized way. Also, reading of RTDose is then possible by drag-n-drop.
Remember to also change the usages of RTDoseReader (i.e. in the DICOM Plugin org.mitk.gui.qt.dicom or the Dose visualizer plugin org.mitk.gui.qt.dosevisualization).