Neutron dose distribution in the treatment room for an accelerator in the flattening filter-free mode

Abstract

Objective: To provide specific data and to make recommendations for radiation protection for clinical applications, the neutron cumulative dose, and exposure time in the treatment room around an accelerator operating at 10 MV in the conditions of both a conventional dose rate (with a flattening filter [FF] mode) and an ultra-high dose rate (flattening filter-free [FFF] mode) were measured and analyzed. Methods: In the accelerator treatment room, the data were collected from four representative sites: the therapist's operating location, the distal end of the couch in the patient plane, the proximal maze, and the protective door. The field sizes were 5 × 5 cm2, 10 × 10 cm2, and 30 × 30 cm2; the output doses were 500 MU, 1000 MU, and 2000 MU; the solid water phantom simulating the patient's body was put on the isocenter; the dose rate in FF mode was a 10-MV X-ray at 600 MU/min; and the dose rate in FFF mode was 2400 MU/min. Under the same conditions, data were collected outside the selected area measuring three positions: the protective door, operating room, and center of the main protective wall position. Results: The neutron cumulative dose was negatively correlated with the field size; the closer to the target, the larger the neutron cumulative dose. The neutron cumulative dose was the largest at the distal end of the couch, and the neutron cumulative dose in FF mode was almost threefold as large as that in FFF mode. The neutron exposure time was positively correlated with the output dose. The FF mode takes slightly longer than the FFF mode. No neutron dose was read outside the accelerator room. Conclusions: Operating the accelerator in FFF mode will benefit both the patients and radiation therapists by reducing the neutron cumulative dose and exposure time. The current anti-neutron accelerator room design is equally applicable to an ultra-high-dose rate (FFF) mode accelerator.