Undesirable Radioisotopes Induced by Therapeutic Beams from Medical Linear Accelerators

Contemporary linear accelerators called often linacs, used in radiation medicine generate electrons and X-rays with energies up to over 20 MeV. Such energies are enough to induce nuclear reactions in which neutrons and radioisotopes are produced. These neutrons and radioisotopes are undesirable in therapy, because they are source of an additional dose to patients and to staff operating the medical accelerators. The therapeutic electrons and X-rays can induce electronuclear (e,e’n) and photonuclear (,n) reactions, respectively. These reactions take place inside the therapeutic beam in massive components of an accelerator head, mainly and in air. In the case of the X-rays the main neutron sources are the collimators of the beam, flattening filter giving the appropriate profile of the beam and the target in which electrons are converted into X-ray radiation. In the case of the electron beams the majority of neutrons are produced in the collimator system and in the scattered foils. The neutrons originated in both mentioned type of reactions have the broad energy spectrum with the high-energy end of more than ten MeV. Majority of the neutrons reach the concrete walls, ceiling and floor of the radiotherapy facility. Concrete is a good moderator. In this medium the neutrons undergo elastic collisions with nuclei of hydrogen, mainly. The slowed down neutrons may get out of concrete and return to air, contributing to the specific distribution of neutron energy inside the radiotherapy facility. Kinetic energies of the slowed down neutrons are distributed according to the Maxwell-Boltzmann distribution law. The neutrons can easy induce the simple capture (n,) reactions in the thermal and resonance energy range and radioisotopes are produced. The neutron field is almost uniform in whole accelerator room. Thus the radioisotopes originating from the neutron reactions can be produced in the accelerator components and accessories as well as in the wall, ceiling and floor of the radiotherapy facility. Moreover, the neutrons can induce simple capture reactions in the entrance door of the radiotherapy facility. The penetrative gammas are emitted as a result of these neutron reactions. Therefore, the gamma radiation can appear close to the radiotherapy facility door in the operator room during emission the high-energy therapeutic beams. In the paper the radioisotopes originating in the accelerator components and in the accessories as well as in the walls, ceiling, floor and door of the radiotherapy facility and in