To implement a method for controlling the application of ionizing radiation to a body, provision is made for an instrument, which can apply therapeutic radiation sources to a body, with a part of a locating system being provided to locate the position of the therapeutic radiation sources on an instrument in the body. With the instrument inserted into the body of the patient, an assignment of the located position of the therapeutic radiation sources to the three-dimensional representation of the body is possible by using the locating system. The therapeutic radiation source can then be applied, when the located position corresponds to a position previously determined on the basis of the three-dimensional representation of the body.