FLUKA-based simulation analysis of induced radioactivity in proton therapy site
Objective To simulate and analyze the dose distribution from external exposure and its influencing factors of induced radioactivity in proton therapy site.Methods Referencing a domestically under-construction proton therapy facility,a geometric model of the proton therapy site was constructed,and the FLUKA program was used to simulate the distribution of the induced radioactive dose of the proton therapy site under the conditions of different energies,beam angles,irradiation time,cooling time and medium of the treatment site.Results For a 230 MeV proton beam with a current of 3.0 nA,directed along the negative Z-axis and irradiating a phantom for two minutes,at the shutdown moment,the ambient dose equivalent rates in air and vacuum 5,30,and 50 cm away from the phantom surface were(1 039.02±5.82)-(127.86±1.20)and(1 037.96±4.38)～(127.35±0.93)µSv/h,respectively.The mean difference was 0.51～1.06 µSv/h,and the air-immersed external irradiation accounted for<1%of the total irradiation,which rapidly decreased to 1/15 of the shutdown moment value after cooling for 10 minutes.Under the condition of 130～250 MeV,the ambient dose equivalent rates at the shutdown moments 5,30 and 50 cm away from the surface of the phantom were(427.49±3.12)-(1 058.41±4.66),(100.36±0.92)-(259.70±1.69)and(50.15±0.68)-(131.93±1.11)µSv/h,respectively.Irradiation for one-five minutes,and at the moment of shutdown at 5,30,and 50 cm from the surface of the phantom were(688.19±3.33)-(1 594.04±8.08),(167.60±1.35)-(388.24±2.96)and(84.73±0.69)-(195.94±1.56)µSv/h.The peripheral dose-equivalent rate of the sensed radioactivity decreases with the irradiation time,the energy of the beam,and the distance from the model.The peak dose equivalent rate around the induced radioactivity exists in the beam direction,which is significantly larger than that in the non-beam direction.Conclusion Proton therapy sites are characterized by relatively large levels of induced peripheral radioactive dose equivalent rates,mainly originating from patients.In actual practice,a suitable working position can be chosen according to the direction of the beam current,especially the direction of the final irradiation field beam current,in the non-beam current direction and as far away from the patient as possible.Within 10 minutes after the end of treatment,staff should try to avoid close contact with the patients.
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