Thermal neutron fluence in a treatment room with a Varian linear accelerator at a medical university hospital

摘要

The indium foil activation technique has been employed to measure thermal neutron fluences (Φ_(th)) among various locations in the treatment room with a 20 x 20 cm2 field size and a 15 and 10 MV X-ray beam. Spatial Φ_(th) are visualized using colored three-dimensional graphical representations; intensities are up to (1.97 + 0.13) x 10~5 and (1.46 ± 0.13) x 10~4 n cm~(-2)/Gy-X at isocenter, respectively. The Φ_(th) is found to increase with the X-ray energy of the LINAC and decreases as it moves away from the beam center. However, thermal neutron exposure is not assessed in routine dosimetry planning and radiation assessment of patients since neutron dose contributes < 1% of the given therapy dose. However, unlike the accelerated beam limited within the gantry window, photoneutrons are widely spread in the treatment room. Distributions of Φ_(th) were measured in water phantom irradiated with 15 MV X-ray beams. The shielding effect of the maze was also evaluated. The experimentally estimated Φ_(th) along the maze distance was fitted explicate and the tenth-value layer (TVL) was calculated and discussed. Use of a 10 cm-thick polyethylene door placed at the maze was suitable for radiation shielding.