Chung H, Yi B, Prado K
Universityof Maryland School of Medicine, Baltimore, MD.
Med Phys. 2012 Jun;39(6Part18):3829. doi: 10.1118/1.4735625.
In the process of measuring and validating fundamental dosimetry data prior to the clinical use of a treatment unit, it is prudent to compare measurements with previously published equivalent data. During the commissioning of an accelerator with flattening filter free (FFF) photon beams (Varian True Beam 6 MV FFF and 10 MV FFF) we compared measured Phantom Scatter Factors (Sp) with the Normalized Peak Scatter Factors (NPSFs) from the British Journal of Radiology Supplement 25 (BJR #25). The purpose of this work was to determine whether the energy independent NPSFs BJR#25 are valid for comparison with FFF photon beams.
All measurements were performed using a Varian TrueBeam linear accelerator with photon energies of 6-MV, 6-MV FFF and 10-MV FFF modes. For all measurements, a Scanditronix CC04 ionization chamber was used. Both water and in-air measurements were made to obtain NPSFs normalized to the 10 × 10 cm field size. For measurements in water, the chamber was positioned at 100 cm source-to-axis distance at the depth of dose maximum. For in-air measurements, the chamber was positioned at 100 cm source-to-axis distance with appropriate build-up cap. From BJR #25, NPSFs were obtained for comparison with the measurements.
The NPSF agreement between the 6-MV and 6-MV FFF with the BJR#25 were all within ±0.5%. The agreement ranged from 0.996 to 1.004 and 0.995 to 1.002 for 6-MV and 6-MV FFF, respectively. We also found that 10-MV FFF showed very similar trend.
The scatter factors reported in BJR #25 are valid for comparison for 6-MV, 6-MV FFF, and 10-MV. Additional investigation is needed to further understand the dosimetric characteristics of FFF mode.
在治疗单元临床使用前测量和验证基本剂量学数据的过程中，将测量结果与先前发表的等效数据进行比较是明智之举。在调试具有无均整器光子束（Varian TrueBeam 6 MV FFF和10 MV FFF）的加速器时，我们将测量的体模散射因子（Sp）与《英国放射学杂志增刊25》（BJR #25）中的归一化峰值散射因子（NPSFs）进行了比较。这项工作的目的是确定能量无关的NPSFs BJR#25是否适用于与FFF光子束进行比较。
所有测量均使用Varian TrueBeam直线加速器，采用6-MV、6-MV FFF和10-MV FFF模式的光子能量。对于所有测量，使用Scanditronix CC04电离室。进行了水中和空气中的测量，以获得归一化到10×10 cm射野大小的NPSFs。对于水中测量，电离室位于源轴距100 cm、剂量最大值深度处。对于空气中测量，电离室位于源轴距100 cm处，并配有合适的建成帽。从BJR #25中获取NPSFs，以便与测量结果进行比较。
6-MV和6-MV FFF与BJR#25之间的NPSF一致性均在±0.5%以内。6-MV和6-MV FFF的一致性范围分别为0.996至1.004和0.995至1.002。我们还发现10-MV FFF呈现出非常相似的趋势。
BJR #25中报告的散射因子对于6-MV、6-MV FFF和10-MV是有效的比较依据。需要进一步研究以深入了解FFF模式的剂量学特性。
