Katayose Tetsurou, Kawachi Toru, Miyasaka Ryohei, Kodama Takumi, Takase Nobuhiro, Iriyama Eri, Chang Weishan, Saitoh Hidetoshi
Department of Radiation Oncology, Chiba Cancer Center.
Graduate School of Human Health Sciences, Tokyo Metropolitan University.
Igaku Butsuri. 2016;36(2):79-84. doi: 10.11323/jjmp.36.2_79.
Flattening filter-free (FFF) beams generated by linear accelerators have been widely adopted in many hospitals recently for radiation therapy. FFF technology can provide higher dose rates so that shortening of the treatment time and less intra-fraction motion error are expected.In Japan, the current way of determining absorbed dose to water for FFF beams is to follow the Standard Dosimetry 12 protocol which was developed for flattened beams. Since it has been reported that the flattened beams and FFF beams have different beam properties, it is necessary to evaluate the usefulness of Standard Dosimetry 12 protocol for FFF beam dosimetry.This report reviews physical and dosimetric properties of FFF beams especially in terms of the effect on absorbed dose to water dosimetry using an ionization chamber. From the review, it became evident that the absorbed dose to water is underestimated by volume averaging effect of the ionization chamber. On the other hand, the absorbed dose to water is overestimated by using the beam-quality specifier TPR to predict the restricted mass collision stopping power ratio for FFF beams. Therefore, an alternative method was proposed for absorbed dose to water dosimetry of FFF beams based on Standard Dosimetry 12.
直线加速器产生的无均整器（FFF）射束近来已在许多医院广泛用于放射治疗。FFF技术能够提供更高的剂量率，因此有望缩短治疗时间并减少分次治疗期间的运动误差。在日本，目前确定FFF射束水吸收剂量的方法是遵循为均整射束制定的标准剂量学12协议。由于已有报道称均整射束和FFF射束具有不同的射束特性，因此有必要评估标准剂量学12协议用于FFF射束剂量测定的适用性。本报告回顾了FFF射束的物理和剂量学特性，特别是在使用电离室对水吸收剂量剂量测定的影响方面。通过回顾发现，电离室的体积平均效应会低估水吸收剂量。另一方面，使用射束质指标TPR预测FFF射束的受限质量碰撞阻止本领比会高估水吸收剂量。因此，基于标准剂量学12提出了一种用于FFF射束水吸收剂量剂量测定的替代方法。
