Could you please tell us what controls do you have in your RadTileViewItems and how are you using them? We tried your scenario with ANTS memory profiler and everything looked fine - the RadTileView was clearing all of its old items when you change its ItemsSource and the memory was freed.
Also I'd like to mention that the CarbageCollector in Silverlight needs some time to clear the unused memory and Silverlight itself keeps some resources even if they are not needed at the moment.
If you have more questions please feel free to ask.
the Telerik team
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