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. We have developed a tracking system by modifying an add-on collimator, the Siemens Moduleaf, for realtime applications in radiotherapy. As the add-on collimator works nearly completely independently 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 In order to reduce the complexity of the system, outdated electronic parts were replaced with newer components where practical.Verification was performed by measuring the latency of the system as well as the impact on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 leaf's travel direction. Latency measurements in the 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. Comparing the dose distribution on the film with a moving target between 'tracking disabled' towards 'tracking enabled' functions resulted in penumbra widths of 23 mm to 4 mm for 0.1 Hz sinusoidal movements with an amplitude of 32 mm, respectively. The maximum speed was therefore 20 mm s(-1). Latency was measured to be less than 50 ms for the signal runtimes. 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.