Multimodal diagnostics of gastrointestinal tumours with MRI, CT and video-endoscopy is a rapidly changing domain. The education at our universities should overcome the obstacles of traditional learning based on paper media and oral lectures with retention rates of 10-30% only. The paper presents the objectives and the results of the design phase of the project ODITEBl - Open Distributed TExt Book, for Computer-Assisted Instruction in the domain mentioned above. The main objective is to produce an electronic interactive textbook in order to shift education to more efficient learning settings with higher retention rates. The main concepts are 1) three-layer architecture (dynamic case layer, intermediate query layer, static instruction layer) 2) case pool distribution 3) active learners experience (interactive exploration of original image data).