I know VTK is used in many different scientific and corporate applications, and it is extremely reliable. However, I would like to know if and how VTK has been validated with DICOM data.
What I mean is, after loading an DICOM image into a vtkImageData and passing in on to a renderer with a picker interactor, how do I know for sure that the hounsfield value at x,y,z really corresponds to the correct pixel.
VTK is used in many FDA-approved medical applications, most of them read images from DICOM. How and what you validate depends on the intended use, what you claim your system to be capable of, what are the potential risks, etc. Since these are all very specific to your particular system, it hard to give relevant advice in general.
If you are not sure what to do then probably you need to hire a medical device consulting company to help with your filing. It may also help if you build your software on higher-level software frameworks, such as 3D Slicer or MITK, which are free, open-source medical image viewer and analysis applications that are used as basis of medical devices. You still need to do your own validation but you can cut down on implementation time of features and automatic tests.