In this thesis, a tracking system was developed by modifying an add-on collimator, the Siemens Moduleaf, for realtime applications in radiotherapy. As the add-on collimator works almost completely autonomously of the linear accelerator (LinAc), no modifications to the latter were necessary. The adaptations to the Moduleaf were mainly software-based. In order to reduce the complexity of the system, outdated electronic parts were replaced with newer components where practical. Verification was performed by measuring the latency of the system as well as the impact on applied dose to a predefined target volume, moving in the leafs travel direction. Latency measurements in software were accomplished by comparing the target and current positions of the leaves. For dose measurements, a Gafchromic EBT2 film was placed beneath the target 4D phantom, in between solid water plates, and moved alongside with it. Based on the results, a tracking-capable add-on collimator seems to be a useful tool for reducing the margins for the treatment of small, slow-moving targets. Radiotherapy is one of the most important methods used for the treatment of cancer. Irradiating a moving target is also one of the most challenging tasks to accomplish in modern radiotherapy.