If you need VTK-based robust DICOM RT dose, RT structure set, RT image, RT plan import (and all the other common DICOM IODs, such as CT, MRI, PET, US, deformable registration object, structured report, segmentation object, etc.) then you can use 3D Slicer and its SlicerRT extension.
You also get dose volume histogram computation and metrics, dose comparison (gamma analysis), external beam RT plan visualization (static and dynamic), isodose curve/surface display, dose warping, accumulation, RT structure set conversion to closed surface and binary labelmap, segmentation editing, dose computation using various engines (such as a simple proton dose engine; or orthovoltage treatment dose computation using EGSnrc), linac coordinate system visualization, patient collision analysis, etc.
Of course you can also use all standard Slicer features (many image registration, segmentation, visualization, quantification tools, one-click virtual reality display, etc.) for all these RT objects.
All these are VTK-based, open-source, completely free, with restriction-free license, free support, fully Python-scriptable and customizable and extensible in Python and C++.
If you have any questions then you can ask at discourse.slicer.org.