Melbourne Cancer Center, Melbourne, FL 32901, USA.
J Appl Clin Med Phys. 2012 Sep 6;13(5):3758. doi: 10.1120/jacmp.v13i5.3758.
The flattening filter free (FFF) X-rays can provide much higher dose rate at the treatment target compared to the conventional flattened X-rays. However, the substantial increase of dose rate for FFF beams may affect the ion recombination correction factor, which is required for accurate measurements using ionization chambers in clinical dosimetry. The purpose of this work is to investigate the ion recombination of three types of commonly used ion chambers (Farmer, PinPoint and plane-parallel) in the FFF photon radiation. Both 6 MV and 10 MV flattened and FFF beams were fully commissioned on a Varian TrueBeam linear accelerator. The ion recombination correction factor, P(ion), was determined using the two-voltage technique for a 0.6 cc Farmer chamber, a 0.015 cc PinPoint chamber, and a 0.02 cc parallel-plate chamber at different source-to-axis distances (SAD) in a solid water phantom or water tank phantom at a depth of 10 cm in a 10 × 10 cm(2) field. Good repeatability of measurements was demonstrated. Less than 1% difference in P(ion) between the flattened and FFF photons for all three ion chambers was observed. At a SAD of 100 cm and a depth of 10 cm for a 10 × 10 cm(2) field, P(ion) for the Farmer chamber was 1.004 and 1.008 for the 6 MV flattened and FFF beams, respectively. At the same setup using the Farmer chamber, P(ion) was 1.002 and 1.015 for the 10MV flattened and FFF beams, respectively. All P(ion) results for the Farmer, PinPoint, or parallel plate chamber in the 6 MV and 10 MV flattened and FFF beams were within 2% from the unity (1 ≤ P(ion) < 1.02). The P(ion) ratio of the FFF to flattened beams was 0.99~1.01 for both 6 MV and 10 MV photons. The ion recombination effect of the Farmer, PinPoint, and plane-parallel chamber in the FFF beams is not substantially different from that in the conventional flattened beams.
在与传统的平板 X 射线相比，使用不带准直器的（FFF）X 射线可以在治疗靶区提供更高的剂量率。然而，FFF 射线束的剂量率的显著增加可能会影响电离室临床剂量学中准确测量所需的离子复合修正因子。本工作旨在研究三种常用电离室（Farmer、PinPoint 和平行平板）在 FFF 光子辐射中的离子复合。6 MV 和 10 MV 的平野和 FFF 射线束都在瓦里安 TrueBeam 直线加速器上进行了全面的标定。使用双电压技术，在固体水模体或水箱模体中 10×10 cm2 射野的 10 cm 深处，在不同的源轴距（SAD）处，对 0.6 cc Farmer 室、0.015 cc PinPoint 室和 0.02 cc 平行平板室，确定了离子复合修正因子 P(ion)。测量结果具有良好的重复性。对于所有三种电离室，在平野和 FFF 光子下，P(ion) 的差异均小于 1%。在 10×10 cm2 射野、SAD 为 100 cm、深度为 10 cm 的情况下，对于 6 MV 平野和 FFF 射线束，Farmer 室的 P(ion) 分别为 1.004 和 1.008。在使用 Farmer 室的相同设置下，对于 10 MV 平野和 FFF 射线束，P(ion) 分别为 1.002 和 1.015。6 MV 和 10 MV 的平野和 FFF 射线束中，对于 Farmer、PinPoint 或平行平板电离室，所有 P(ion) 结果均在 1（1 ≤ P(ion) < 1.02）以内。6 MV 和 10 MV 光子的 FFF 与平野射线束的 P(ion) 比值为 0.99~1.01。在 FFF 射线束中，Farmer、PinPoint 和平行平板电离室的离子复合效应与传统的平野射线束没有明显的不同。
