Kawachi Toru, Saitoh Hidetoshi, Inoue Mitsuhiro, Katayose Tetsurou, Myojoyama Atsushi, Hatano Kazuo
Tokyo Metropolitan University Graduate School of Human Health Sciences, Tokyo, Japan.
Med Phys. 2008 Oct;35(10):4591-8. doi: 10.1118/1.2978228.
This article is intended to improve the certainty of the absorbed dose determination for reference dosimetry in CyberKnife beams. The CyberKnife beams do not satisfy some conditions of the standard reference dosimetry protocols because of its unique treatment head structure and beam collimating system. Under the present state of affairs, the reference dosimetry has not been performed under uniform conditions and the beam quality correction factor kQ for an ordinary 6 MV linear accelerator has been temporally substituted for the kQ of the CyberKnife in many sites. Therefore, the reference conditions and kQ as a function of the beam quality index in a new way are required. The dose flatness and the error of dosimeter reading caused by radiation fields and detector size were analyzed to determine the reference conditions. Owing to the absence of beam flattening filter, the dose flatness of the CyberKnife beam was inferior to that of an ordinary 6 MV linear accelerator. And if the absorbed dose is measured with an ionization chamber which has cavity length of 2.4, 1.0 and 0.7 cm in reference dosimetry, the dose at the beam axis for a field of 6.0 cm collimator was underestimated 1.5%, 0.4%, and 0.2% on a calculation. Therefore, the maximum field shaped with a 6.0 cm collimator and ionization chamber which has a cavity length of 1.0 cm or shorter were recommended as the conditions of reference dosimetry. Furthermore, to determine the kQ for the CyberKnife, the realistic energy spectrum of photons and electrons in water was simulated with the BEAMnrc. The absence of beam flattening filter also caused softer photon energy spectrum than that of an ordinary 6 MV linear accelerator. Consequently, the kQ for ionization chambers of a suitable size were determined and tabulated as a function of measurable beam quality indexes in the CyberKnife beam.
本文旨在提高射波刀束流参考剂量测定中吸收剂量确定的准确性。射波刀束流因其独特的治疗头结构和束流准直系统，不满足标准参考剂量测定协议的某些条件。在当前情况下，参考剂量测定并非在统一条件下进行，许多地方临时用普通6兆伏直线加速器的束流质量校正因子kQ替代射波刀的kQ。因此，需要以新的方式确定参考条件以及作为束流质量指标函数的kQ。分析了辐射场和探测器尺寸引起的剂量平坦度和剂量计读数误差，以确定参考条件。由于没有束流平坦化滤波器，射波刀束流的剂量平坦度低于普通6兆伏直线加速器。并且，如果在参考剂量测定中使用腔长为2.4厘米、1.0厘米和0.7厘米的电离室测量吸收剂量，计算得出在6.0厘米准直器射野的束轴剂量分别低估了1.5%、0.4%和0.2%。因此，建议将用6.0厘米准直器形成的最大射野以及腔长为1.0厘米或更短的电离室作为参考剂量测定条件。此外，为确定射波刀的kQ，用BEAMnrc模拟了水中光子和电子的实际能谱。没有束流平坦化滤波器也导致光子能谱比普通6兆伏直线加速器更软。因此，确定了合适尺寸电离室的kQ，并将其作为射波刀束流中可测量的束流质量指标的函数制成表格。
