The influence of neutron contamination on pacemaker in photon beam radiotherapy by LINAC using the Monte Carlo method

Introduction: In radiation therapy with high-energy photon beams (E > 7 MeV) neutrons are generated mainly in LINACs head thorough (γ, n) interactions. These neutrons affect the shielding requirements in radiation therapy rooms. According to the protocol TG-34, photon absorbed dose of 10Gy can cause permanent damage to the pacemaker and the dose of 2Gy can make minor changes in the functioning of the pacemaker. So, in the radiotherapy of patients with pacemakers, the system should be designed to limit the dose of the pacemaker to 2Gy. In the current study, the Varian Clinac 2100C linear accelerator head and the pacemaker were fully simulated, the neutron and photon flux were evaluated by the FLUKA code. Materials and Methods: The linear accelerator Varian 2100c is simulated in 18 MeV energy using FLUKA code. A 70 × 40 × 20 cm3 water phantom simulated upper body of patient, at a distance of 100 cm from the source (SSD). According to the MIRD Phantom, the prostate is simulated at a depth of 4.5 cm from the phantom surface at a distance of 50 cm from the pacemaker. All components of the heart pacemaker, including battery and circuit parts, leads and connector were simulated according to real case. The results of the simulation were compared with measurements to verify the simulated model. Results: According to the distribution results of neutron flux around the head, the jaws despite of guide and control of photon beam toward the phantom, these components are not an effective barrier to neutron contamination, but also because of the heavy elements used in them, exacerbate the production of neutrons. The highest level of contamination is in target, due to photoneutron production in tungsten. Moreover, the distribution of neutron flux in the water phantom and the pacemaker shows a relative increase in neutron flux at a depth of 2 cm in the water phantom caused by the effect of build-up and neutron accumulation that is due to the neutron scattering within the phantom. Conclusion: For the treatment of prostate cancer where the pacemaker is located more than 40 cm from the treatment field, the neutron flux is observed in a wide range of thermal neutrons to fast. Therefore, in addition to the target tissue that receives the highest unwanted neutron dose during treatment, close organs and, subsequently, other organs receive a considerable dose of neutrons. The highest amount of flux on the surface of the titanium body of the pacemaker is for 100 keV photons and the most of the neutron flux is in the thermal region. The results of photon and neutron flux in the battery layers show that the maximum flux is in steel layer and then for lithium.