Abstract: Apparatus and method for the precision aiming of a radiation beam for treating an internal cancer tumor in a patient. The method of treatment includes introducing at least one reference marker into the patient in a defined and locatable position in relation to the cancer tumor. A defined point in the cross section of the beam is located by a sight that is compared to the position of the reference marker and adjusted to ensure that the treatment beam is directed precisely at the cancer tumor before the remaining doses of radiation are given to the patient. The device according to the invention includes equipment for emitting a treatment beam, a sight arranged in the path of the beam to define a point in the cross section of the beam, and a device to read the beam arranged to receive the beam that has passed through the patient and visualize the position of the at least one said reference marker and the position of the sight.