Grid technologies have proven their capabilities to settle challenging problems of medical data access. The grid ability to access distributed databases in a secure and reliable way while preserving data ownership opened new perspectives in medical data sharing and disease surveillance. This paper focuses on the implementation challenges of grid-powered sentinel networks within the e-sentinelle project. This initiative aims to create a lightweight grid dedicated to cancer data exchange and enable automatic disease surveillance according to definition of epidemiological alarms. Particularly, issues related to security, patient identification, databases integration, data representation and medical record linkage are discussed.