Digital libraries offer a vast source of very different information. To enable users to fruitfully browse through a collection of documents without necessarily having to state a complex query, advanced retrieval techniques have to be developed. Those methods have to be able to structure information in a semantic manner. This work presents some first steps in semantically organizing thematically pre-selected documents of a digital library. The semantic structure of the document collection will be expressively visualized by the proposed system. We illustrate our ideas using a database of medical abstracts from the field of oncology as a walking example.