质子治疗场所感生放射性FLUKA模拟计算分析

FLUKA-based simulation analysis of induced radioactivity in proton therapy site

目的模拟并分析质子治疗场所感生放射性外照射剂量分布及其影响因素。方法参考国内某在建的质子治疗场所构建质子治疗场所几何模型,采用FLUKA程序模拟质子治疗场所在不同能量、束流角度、照射时间、冷却时间和治疗场所介质条件下,场所感生放射性剂量分布情况。结果230 MeV、3.0 nA、沿Z轴负方向质子束流照射模体2 min,停机时刻距模体表面5、30和50 cm处空气、真空介质的感生放射性周围剂量当量率分别为(1039.02±5.82)~(127.86±1.20)、(1037.96±4.38)~(127.35±0.93)μSv/h,均值差为0.51~1.06μSv/h,空气浸没外照射占比<1%,停机时刻感生放射性周围剂量当量率比停机10 min后高15倍以上。130~250 MeV条件下,停机时刻距模体表面5、30和50 cm处感生放射性周围剂量当量率依次为(427.49±3.12)~(1058.41±4.66)、(100.36±0.92)~(259.70±1.69)和(50.15±0.68)~(131.93±1.11)μSv/h。230 MeV、3.0 nA、沿Z轴负方向质子束流照射模体1~5 min,停机时刻距模体表面5、30和50 cm处感生放射性周围剂量当量率分别为(688.19±3.33)~(1594.04±8.08)、(167.60±1.35)~(388.24±2.96)和(84.73±0.69)~(195.94±1.56)μSv/h。感生放射性周围剂量当量率随照射时间、束流能量的增加而增加,随距模体表面距离的减少而降低。在束流方向存在感生放射性周围剂量当量率峰值,且明显大于非束流方向。结论质子治疗场所感生放射性周围剂量当量率水平相对较大,主要来源于患者。实际工作中可根据束流方向特别是末次照射野束流方向选择合适的工作位置,即非束流方向并尽可能远离患者。在结束治疗后的10 min内,工作人员应尽量避免与患者的近距离接触。

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(1039.02±5.82)-(127.86±1.20)and(1037.96±4.38)~(127.35±0.93)μSv/h,respectively.The mean difference was 0.51~1.06μSv/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)-(1058.41±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)-(1594.04±8.08),(167.60±1.35)-(388.24±2.96)and(84.73±0.69)-(195.94±1.56)μSv/h.The peripheral doseequivalent 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.