Effects of secondary neutron beam generated in radiotherapy on electronic medical devices

In recent years, the administration of radiotherapy to patients bearing an implantable electronic device, such as a pacemaker, has been increasing. Although the effects of primary radiation on electronic devices have been studied, research on the effects of the secondary radiation beam is limited. Because semiconductors constitute the main parts of an electronic device, the effects of the secondary neutrons in radiation should be considered. We performed a Monte Carlo simulation of neutron generation during radiotherapy, and considered the influence of neutrons on the electronic medical devices. Monte Carlo simulations were performed using the Particle and Heavy Ion Transport code System (PHITS), developed by the Japan Atomic Energy Agency. Nuclear data was taken from ENDF/B-VII. We irradiated a water phantom with a 10 MV X-ray beam and a 200 MeV proton beam. Then, the neutron dose was evaluated by using PHITS. Moreover, we also measured the dose with a CR-39 dosimeter. In the case of the 10 MV X-ray beam, although fast neutrons (> 100 keV) were generated in the upstream part of the beam, thermal neutrons were scarcely generated. Inside the water phantom, which simulated a human body, the generation of fast neutrons decreased rapidly as a result of thermalization. In the case of the 200 MeV proton beam, fast neutrons were distributed along the beam axis inside the water phantom, and were generated inside the water phantom. Thermal neutrons (< 0.5 eV) were uniformly distributed throughout the whole phantom. In the evaluation of secondary neutrons generated during radiotherapy, we consider that using Monte Carlo calculations in addition to experimental measurements will be useful for clinical applications. (author)