Dosimetric evaluation of neutron contamination caused by dental restorations during photon radiotherapy with a 15 MV Siemens Primus linear accelerator

The investigation of neutron contamination during radiotherapy by means of high-energy photon beams is a clinically important task. The present study aims to evaluate neutron contamination originating from the presence of various dental restorations, such as a tooth restored with amalgam, a tooth restored with Ni-Cr alloy, and a tooth restored with Ceramco. Using the MCNPX Monte Carlo code, we calculated the neutron contamination relative to the dose deposited in a water phantom at various penetration depths on the central axis of a photon beam provided by a 15 MV Siemens Primus linear accelerator operating in photon mode. Calculation of the energy spectrum of neutrons produced by the Siemens Primus linear accelerator revealed a peak at about 1 MeV. Dental restorations led to a maximum percentage neutron dose increase at a depth of 2.30 cm of 26.78%, 18.75%, and 10.71% for a tooth restored with amalgam, a tooth restored with Ceramco, and a tooth restored with Ni-Cr alloy, respectively, compared with 5.35% for a healthy tooth. Since the percentage neutron dose depends on the cross section of the photonuclear reaction with the restoration material and, hence, on the reaction energy threshold, underestimation of the dose in treatment planning may occur, especially if restoration materials of high atomic number are used. The fast-neutron equivalent dose for water decreases with increasing depth. For dental restorations with amalgam, Ceramco, and Ni-Cr alloy, it reaches maximum values beyond the dental phantom of 1.28, 1.15, and 0.94 mSv, respectively. The maximum values of the thermal-neutron equivalent dose beyond the dental phantom are 1.32, 1.23, and 1.16 mSv for amalgam, Ceramco, and Ni-Cr alloy, respectively.