The invention relates to a storage phosphor plate assembly (1, 20) comprising at least one storage phosphor plate (1) having a storage phosphor layer for storing X-ray information and a substrate layer on which the storage phosphor layer is provided, the substrate layer containing a permanent magnetic material, and a support plate (20, 22) containing a magnetic material, the at least one storage phosphor plate (1) being held on the support plate (20, 22) due to magnetic forces. The invention further relates to an according X-ray cassette and system for reading out storage phosphor plates (1). By means of the invention, an easy and safe handling of storage phosphor plates of various sizes and/or forms, in particular during the read-out process in the read-out system, is achieved.