Neutron dose equivalent inside and outside the treatment room of Varian Clinac 23EX accelerators.

摘要

This study investigated the characteristics of the neutron and photon dose equivalent to the operating personnel and the patient from a Varian Clinac 23EX medical accelerator working at 18 MV photon mode, with different configurations of gantry collimation system, shielding and treatment vault. The measurement results were used to study the effectiveness of semiempirical shielding calculation methods on this specific accelerator, which include photon dose equivalent calculation equations in NCRP report No. 151, the Kersey method, the Modified Kersey method and the Falcão method.;The neutron and photon dose equivalent rate at the treatment vault entrance (Hn,D and HG) and the adjacent treatment console area were evaluated on a Varian Clinac 23EX accelerator. Furthermore, the H n,D and the HG were evaluated after the flattening filter was removed. The Hn,D and HG were also evaluated in GRID therapy, with a grid block manufactured by .decimal (.decimal Inc., Sanford, FL, USA). An Andersson-Braun neutron Rem meter, a Geiger Müller and an ion chamber gamma ray survey were used in the measurements. The evaluation was based on measurements and comparison with the semiempirical calculations. The first semiempirical shielding calculation was performed to check the effectiveness of the semiempirical shielding calculation methods for a flattening filter free Varian Clinac 23EX accelerator. The effectiveness of borated polyethylene (BPE) boards, as a maze wall lining material, on neutron dose and photon dose reduction is also reported.;The secondary neutron and photon radiation to the patient in GRID therapy was measured using Luxel® dosimeters encapsulating a Tastrak solid state etched track neutron detector and an optically stimulated luminescence (OSL) photon detector on a RANDO® phantom. The radiation activation on the grid was also studied.;The results showed that the Hn,DD at the vault entrance was at its overall maximum when the gantry head was close to the inner maze entrance with closed jaws. However, the HG shows a more complicated pattern as a function of gantry angle and field size. The neutron capture gamma rays were a major component of the photon field in the maze. The scattering photons and leakage photons also played an important role in the overall photon dose equivalent.;Borated polyethylene is an efficient material in neutron dose reduction as well as neutron capture gamma dose reduction. It can be applied to retrofitting an existing vault as a result of linear accelerator energy upgrades, especially when shielding cost is a concern.;The HG and Hn,D normalized by monitor unit (MU) and patient dose will decrease from those of a flattening filtered beam. The HG and Hn,D in the GRID therapy do not have significant changes, compared with conventional external beam therapy. However, the neutron dose to the patient decreases when using the grid.;The photon dose equivalent calculation equations give very close estimation of the HG for both the flattening filter free (FFF) mode and the flattening filtered (FF) mode. However, it can either overestimate or underestimate the HG. The Kersey method overestimate the Hn,D of the Varian Clinac 23EX working in both the FFF mode and the FF mode. New H 0 values (0.24 – 0.37) were proposed for the FFF accelerator. The Falcão method overestimate the Hn,D of the Varian Clinac 23EX by 4 – 6 times. The widely used Modified Kersey method gives the closest estimation of the Hn,D for the shielding calculation of a Varian Clinac 23EX accelerator in a similar maze, in both the FFF mode and the FF mode. However, it should be used with caution to its potential to underestimate the Hn,D.