Sterilization methods for implantable prostheses are described where a polymeric stent may be sterilized, e.g., via ETO sterilization, at a temperature below a glass transition temperature of the stent. A separate delivery catheter may be sterilized separately and the stent and delivery catheter may then be combined in an aseptic or semi-aseptic environment and sterilized as an assembled system such that the requirements for sterilizing the system are relatively lower. Additionally and/or alternatively, valve and filter assemblies may be used with an optional mandrel assembly for maintaining sterility of the internal components of a catheter system.