When VTK and ITK are used together, the
\#define ThreadInfoStruct vtkMultiThreader::ThreadInfo
from VTK/Common/Core/vtkMultiThreader.h

would clobber the ThreadInfoStruct definition from
ITK/Modules/Core/Common/include/itkMultiThreaderBase.h:
104   struct ThreadInfoStruct
105     {
106     ThreadIdType ThreadID;
107     ThreadIdType NumberOfThreads;
108     void* UserData;
109     ThreadFunctionType ThreadFunction;
110     enum { SUCCESS, ITK_EXCEPTION, ITK_PROCESS_ABORTED_EXCEPTION, STD_EXCEPTION, UNKNOWN } ThreadExitCode;
111     };

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using ThreadInfoStruct=vtkMultiThreader::ThreadInfo;

avoids the pre-processor conflicts.

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See https://discourse.itk.org/t/vtk9-and-itk5-conflicts/802
for discussion.