A sterilizer having a cassette for holding metal instruments or medical devices. The cassette includes a metallic container that defines a sterilization chamber. The container has at least one outer surface. A plurality of spaced-apart, elongated rails extend along the at least one outer surface of the container. Each of the rails includes a free, longitudinal edge wherein the edges of the rails lie in a first plane. An insulating shell encases the metallic container and the plurality of rails. An outer surface of the insulating shell lies in the first plane. A receiving unit includes a housing having an outer surface and an inner surface. The inner surface engages the edges of the rails and the outer surface of the insulating shell when the cassette is disposed within the inner housing. A plurality of spaced-apart elongated ribs extends along the outer surface of the housing.
