If the selected node is the one whoose button you have clicked simply use the DataContext of the Button.
It should be the same as the DataContext of the Shape holding the Button. Is this what you need ? If for some reasone these DataContexts are different, you can access the Shape via the following code:
(sender as RadButton).ParentofType<
ParentOfType is extension method from Telerik.Windows.Control but we highly suggest you to avoid it if there is a possible way.
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