Detect WPF focus reaching end of focus scope

Question

In WPF I can specify that a control container is a Focus Scope and that tab navigation should cycle through the controls (i.e. when I tab out of the final control, focus will return to the first):

<Border FocusManager.IsFocusScope="True" KeyboardNavigation.TabNavigation="Cycle">
<ItemsControl ItemsSource="{Binding}">
        <ItemsControl.ItemTemplate>
            <DataTemplate>
                <TextBox x:Name="Editor" Text="{Binding}" />
            </DataTemplate>
        </ItemsControl.ItemTemplate>
    </ItemsControl>
</Border>

What I am trying to do is to detect when focus leaves the final field. Without knowing precisely the number of controls within the focus scope, does anyone know if this is possible?