Fuss Martina, Sturtewagen Eva, De Wagter Carlos, Georg Dietmar
Division of Medical Radiation Physics, Department of Radiotherapy and Radiobiology, Medical University of Vienna, A-1090 Vienna, Austria.
Phys Med Biol. 2007 Jul 21;52(14):4211-25. doi: 10.1088/0031-9155/52/14/013. Epub 2007 Jun 18.
The suitability of radiochromic EBT film was studied for high-precision clinical quality assurance (QA) by identifying the dose response for a wide range of irradiation parameters typically modified in highly-conformal treatment techniques. In addition, uncertainties associated with varying irradiation conditions were determined. EBT can be used for dose assessment of absorbed dose levels as well as relative dosimetry when compared to absolute absorbed dose calibrated using ionization chamber results. For comparison, a silver halide film (Kodak EDR-2) representing the current standard in film dosimetry was included. As an initial step a measurement protocol yielding accurate and precise results was established for a flatbed transparency scanner (Epson Expression 1680 Pro) that was utilized as a film reading instrument. The light transmission measured by the scanner was found to depend on the position of the film on the scanner plate. For three film pieces irradiated with doses of 0 Gy, approximately 1 Gy and approximately 7 Gy, the pixel values measured in portrait or landscape mode differed by 4.7%, 6.2% and 10.0%, respectively. A study of 200 film pieces revealed an excellent sheet-to-sheet uniformity. On a long time scale, the optical development of irradiated EBT film consisted of a slow but steady increase of absorbance which was not observed to cease during 4 months. Sensitometric curves of EBT films obtained under reference conditions (SSD = 95 cm, FS = 5 x 5 cm(2), d = 5 cm) for 6, 10 and 25 MV photon beams did not show any energy dependence. The average separation between all curves was only 0.7%. The variation of the depth d (range 2-25 cm) in the phantom did not affect the dose response of EBT film. Also the influence of the radiation field size (range 3 x 3-40 x 40 cm(2)) on the sensitometric curve was not significant. For EDR-2 films maximum differences between the calibration curves reached 7-8% for X6MV and X25MV. Radiochromic EBT film, in combination with a flatbed scanner, presents a versatile system for high-precision dosimetry in two dimensions, provided that the intrinsic behaviour of the film reading device is taken into account. EBT film itself presents substantial improvements on formerly available models of radiographic and a radiochromic film and its dosimetric characteristics allow us to measure absorbed dose levels in a large variety of situations with a single calibration curve.
通过确定在高度适形治疗技术中通常改变的各种照射参数的剂量响应，研究了放射变色EBT膜用于高精度临床质量保证（QA）的适用性。此外，还确定了与不同照射条件相关的不确定性。与使用电离室结果校准的绝对吸收剂量相比，EBT可用于吸收剂量水平的剂量评估以及相对剂量测定。作为比较，纳入了代表当前胶片剂量测定标准的卤化银胶片（柯达EDR-2）。作为第一步，为用作胶片读取仪器的平板透明扫描仪（爱普生Expression 1680 Pro）建立了产生准确和精确结果的测量方案。发现扫描仪测量的光透射率取决于胶片在扫描仪板上的位置。对于照射剂量为0 Gy、约1 Gy和约7 Gy的三个胶片片，在纵向或横向模式下测量的像素值分别相差4.7%、6.2%和10.0%。对200个胶片片的研究显示出极好的片间均匀性。在长时间尺度上，照射后EBT胶片的光学显影包括吸光度缓慢但稳定的增加，在4个月内未观察到停止。在参考条件（源皮距=95 cm，射野尺寸=5×5 cm²，深度=5 cm）下获得的6、10和25 MV光子束的EBT胶片的感光度曲线未显示任何能量依赖性。所有曲线之间的平均间距仅为0.7%。模体中深度d（范围2-25 cm）的变化不影响EBT胶片的剂量响应。辐射野尺寸（范围3×3-40×40 cm²）对感光度曲线的影响也不显著。对于EDR-2胶片，X6MV和X25MV校准曲线之间的最大差异达到7-8%。放射变色EBT胶片与平板扫描仪相结合，提供了一个用于二维高精度剂量测定的通用系统，前提是考虑胶片读取设备的固有行为。EBT胶片本身在以前可用的射线照相和放射变色胶片模型上有实质性改进，其剂量测定特性使我们能够用单一校准曲线在多种情况下测量吸收剂量水平。
