MO‐F‐BRA‐04: Neutron Dose Equivalent at the Maze Entrance of GRID Therapy

Purpose: To evaluate the neutron and photon dose equivalent (Hn,D and HG) at the outer maze entrance with a grid block on a Varian Clinac 23EX accelerator; and to evaluate the neutron activation on the surface of the grid. Method and Materials:A Varian Clinac 23EX accelerator was used to produce 18 MV photons with a 7.62 cm brass grid. The neutron dose equivalent Hn,D was measured using an Andersson‐Braun neutron Rem meter, and the photon dose equivalent HG was measured using a Geiger Müller gamma‐ray survey meter at the outer maze entrance. The neutron activation dose on the surface of the grid was measured by the Geiger Müller survey meter after the irradiation. Results: With the gantry head tilted close to the inner maze entrance and with the jaws closed, the neutron dose equivalent reached its maximum, which is the same as the non‐grid beam. The difference between the grid and non‐grid measurement results was within the measurement uncertainty for different gantry angles. The measurement taken at 5 min after the irradiation shows the exposure rate at the surface of the grid can reach up to 360 mR/hr. The neutron activation activities have an average half life about 10 min. Conclusions: This work indicates that the neutron production in GRID therapy does not change from a conventional external beam therapy. However, the users of a grid should be aware of the possible high dose to the radiation worker from the neutron activation on the surface of the grid.