Hello Monika Kogler,
Please accept my apology for the delayed response. You can remove the selected / checked RadTreeViewItems while iterating through the RadTreeView SelectedItems/ CheckedItems collection. If the current item has already been removed as a child of already removed item - continue to the next selected/checked item, else - remove it.
I attached a sample solution and let me know if it satisfies your needs.
I will be glad if I can further assist you.
All the best,
the Telerik team
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