Kry Stephen F, Titt Uwe, Pönisch Falk, Vassiliev Oleg N, Salehpour Mohammad, Gillin Michael, Mohan Radhe
Department of Radiation Physics, The University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, TX 77030, USA.
Int J Radiat Oncol Biol Phys. 2007 Jul 15;68(4):1260-4. doi: 10.1016/j.ijrobp.2007.04.002.
To measure and compare neutron fluence around an accelerator operating at 18 MV, both with the flattening filter present (FF mode) and absent (flattening-filter-free [FFF] mode).
The neutron fluence was measured at several locations in the patient plane using gold foil activation in neutron moderators. Differences in neutron fluence between the FF and FFF mode were assessed in three frameworks: (1) measured per monitor unit of machine-on time, (2) determined per dose on the central axis, and (3) determined for a complete course of prostate intensity-modulated radiotherapy.
Neutron fluence per monitor unit was approximately 20% lower when the accelerator was operated in the FFF mode than when it was in FF mode. The total amount of neutron fluence that would be obtained during the entire course of prostate intensity-modulate radiotherapy was 69% lower when the accelerator was operated in the FFF mode than when it was in the FF mode. This reduction in neutron fluence would correspond to a drastic reduction in the neutron dose equivalent received by the patient as a byproduct of high-energy radiotherapy. It would also correspond to a reduction in activation within the treatment vault and subsequent exposure to radiation therapists.
When feasible, operating the accelerator without a FF will benefit both patients and radiation therapists by reducing the number of unwanted neutrons and resultant exposure. This reduces the risk of negative effects from such exposure (e.g., second cancers).
测量并比较18兆伏加速器在有均整滤过器（FF模式）和无均整滤过器（无均整滤过器[FFF]模式）运行时周围的中子注量。
使用中子慢化剂中的金箔活化法，在患者平面的多个位置测量中子注量。在三个框架下评估FF模式和FFF模式之间中子注量的差异：（1）按机器开机时间的每个监测单位测量；（2）在中心轴上按每剂量确定；（3）针对前列腺调强放疗的完整疗程确定。
加速器在FFF模式下运行时，每个监测单位的中子注量比在FF模式下约低20%。加速器在FFF模式下运行时，前列腺调强放疗整个疗程中获得的中子注量总量比在FF模式下低69%。这种中子注量的减少将对应于患者作为高能放疗副产品所接受的中子剂量当量的大幅减少。这也将对应于治疗 vault 内活化的减少以及随后放射治疗师所受的辐射暴露减少。
在可行的情况下，不使用FF运行加速器将通过减少不需要的中子数量和由此产生的暴露，使患者和放射治疗师都受益。这降低了此类暴露产生负面影响（如二次癌症）的风险。
