The Characteristic of Dose Equivalent Rates of Photon and Neutron Outside the Treatment Room under High-energy Electron Mode of Linac

Objective:The study aimed to explore the characteristic of dose equivalent rates(DERs)of photon and neutron outside the treatment room under high-energy electron mode(dose rate 1000 MU/min)of linac and to evaluate the effectiveness of treatment room protection.Methods:A 451 P pressurized ion chamber and a thermo neutron detector were used to measure the DERs of the photon and neutron at selected points.The effects of field size,applicator size,applicator,anthropomorphic phantom(CDP)and lead block on DER were investigated respectively.Results:The DERs of a photon at the center of shielding door(point A),control console(point B),primary shielding walls(point C,D)and roof of treatment room(point E)increased with increasing electron energy,but decreased with the increasing field size.The DERs of a photons at points A and B are smaller than 2.5μSv/h for all scenarios,while those at point D greater than 2.5μSv/h when irradiated by 18-22 Me V electron.In addition,CDP may change the DERs of a photon at points C and D about 5% to 30%.On the other hand,the DERs of neutron increase with increasing electron energy but decrease with the increase of field size and applicator size,however,the lead block and the applicator itself will change the DERs of leak neutron at point A,but its amplitude is less than 0.5μSv/h.The maximum DER of neutron at point A is 6.18μSv/h irradiated by 22 Me V electron.For other scenarios,they are all in the range of national standards limits.Conclusion:The DERs of a photon and neutron outside the treatment room mainly depend on the energy,field size and irradiation direction of the electron beam.If high-energy 18 Me V and 22 Me V electron beams will be used,the primary shielding walls and shielding doors need reconstructing or increasing thickness.