WPF: How to stop the PriorityBinding's thread.

Question

Listbox's ItemsSource is TList

in XImage's Dispose method, How do I stop the thread which load the thumbnail?

<ListBox.ItemTemplate>
                <DataTemplate>
...
                                <Image.Source>
                                    <PriorityBinding>
                                        <Binding Path="Thumbnail" IsAsync="True" />
                                        <Binding Path="DefaultImage"/>
                                    </PriorityBinding>
                                </Image.Source>
....                                
</ListBox.ItemTemplate>