Access control is the field of security which deals with permissions to access resources, where resources may be computing power, storage capacity and data. On the other hand computational grids are systems, where users share those resources in a mostly transparent way. Grid access control poses novel challenges, since the distributed nature of grids make it difficult to manage access control by a central authority. Numerous overlapping domains with different access control policies exist and the sharing of storage resources makes it possible that data leaves the domain of its owner. To enable the owner to enforce his access control policy in such cases, access control solutions adapted to grid environments are needed. In this article we introduce Semantic Access Certificates as an extension to existing access control solutions for grids, to solve some problems that arise when grids are used to process medical data.