This paper presents a novel efficient symmetric encryption technique that can be applied to medical images. It uses genetic algorithm which makes it highly adaptive. Standard DI-COM images are segmented into a number of regions based on pixel intensity and entropy measurements. The novelty of the selective encryption method lies in the use of several encryption algorithms with variable key lengths to control the processing time required for the encryption process and the robustness quality. Encryption processing time, robustness of the encrypted image and the side information required for transmission of the decryption key are the main parameters for optimization. The trade-off among them stems from the variation in processing time with the key length of encryption algorithm, image size, number of regions and the side information to reduce processing time while maintaining a high level of robustness.