Study On Some Factors Affecting Neutron Contamination In Medical Linear Accelerator Treatment Room

Background and Purpose:Cancer is a serious threat to human life and health,and has a high incidence in recent years.With the development of technology,tumor radiotherapy has entered the era of precise treatment,almost any part of the tumor can be treated by radiotherapy.Accelerator is becoming one of the necessary equipment in large and medium-sized hospitals.At present,the energy of mainstream linac is above 6MV,which reaches the reaction threshold of some nuclides.Photons or electrons can interact with matter to produce photons.The most abundant elements in human tissues are H,C,N and O.neutrons can interact with these low atom nuclides to deposit energy into tissues.The ionizing effect of radiation can cause the damage of base,DNA protein cross-linking,single strand break and even double strand break.Neutrons will not only cause direct contact between patients and operators,but also make relevant personnel accept the induced radiation dose produced by neutrons.Many international organizations(such as NCRP,etc.)have studied the neutron dose in the treatment room,discussed the process of neutron generation and slowing down in the room,and introduced the use of neutron detection instruments in different occasions.The above data are all the experiments carried out on the old accelerator in the last century,while the new accelerator has been improved in structure and material composition.At present,there are many literatures about neutron dose level analysis in treatment room,which use MC simulation method.Because of many influencing factors,it is easy to miss some conditions,resulting in the simulation value inconsistent with the actual situation.In this paper,we measured the neutron dose level of a medical Keda linear accelerator in the radiotherapy room under different irradiation conditions,and provided basic data for the radiation room protection design and medical staff dose assessment.Method:(1)The effect of distance from the main beam on neutron dose was explored.The neutron dose at different positions was measured in the treatment room and labyrinth of the machine room by the neutron monitor,and the influence of the distance from the main beam to the neutron dose was observed.(2)To explore the effect of the irradiation field size on the neutron dose.In photon therapy mode,different irradiation fields were set up,and the neutron dose of1 m from the main beam was measured,and the relationship between the radiation field size and the neutron dose was observed.(3)The effect of ray energy on neutron dose was explored.In photon therapy mode,10 Me V and 6 Me V energy were selected to measure neutron dose,and the effect of ray energy on neutron dose was observed.(4)To explore the effect of radiation types on neutron dose.The photon therapy mode and electronic treatment mode were selected to measure the neutron dose under different treatment modes,and the relationship between the radiation type and the neutron dose was observed.Result:(1)In this experiment,we measured the change of neutron dose in the treatment room and the labyrinth.The difference of neutron dose in the treatment room is small.After scattering into the labyrinth through the wall,the neutron energy decreases significantly,and the dose inside the protective door is lower.(2)In IMRT,it should be noted that the leaked neutrons will irradiate the normal tissues outside the target area unnecessarily.(3)At the entrance of labyrinth,the neutron dose at 18 Me V is 24.7 times and6000 times higher than that at 10 Me V and 6 Me V,respectively.(4)The cross-sections of electron and photon are different,and the neutron dose produced by photon treatment mode is much higher than that of electron mode.Conclusion:In general,the labyrinth angle has a great attenuation on the neutron dose equivalent in the accelerator treatment room.According to the Z-type design,the shielding requirements of the protective door will be greatly reduced.The linac operating at 6 Me V can also produce neutrons,but the dose to the protective door is very small.In the shielding design of accelerator treatment room,the influence of high-grade energy rays should be considered.Neutron protection should be strengthened for the new type of mobile linear accelerator equipped with electron therapy mode.The neutron produced in the process of treatment with accelerator energy less than 10 Me V has little effect on the crowd in the waiting area outside the protective door,so the induced radioactivity caused by neutrons in the machine room should be considered.The half-life of most of the induced radionuclides was less than10 minutes,and the medical staff entered the computer room 10 minutes after the accelerator was shut down.