The invention relates to a system for determining the position of radioactively marked target tissue of a patient in radiation therapy by means of a radiation therapy device. The system comprises at least one detector for the imaging detection of the radiation distribution of a radioactive radiation source that is located in target tissue region, a computing unit that is designed to calculate the pose of the target tissue region from measurement data of the detector and to calculate a correction variable as the difference between the calculated pose and comparison values, and an interface by means of which the data for the calculated pose of radioactively marked target tissue region and the correction variable can be transmitted by the computing unit to a radiation therapy device or a patient positioning device. The invention further relates to a corresponding method.
