Lewis David, Chan Maria F
RCF Consulting, LLC, 54 Benedict Road, Monroe, Connecticut 06468.
Memorial Sloan-Kettering Cancer Center, 136 Mountain View Boulevard, Basking Ridge, New Jersey 07920.
Med Phys. 2015 Jan;42(1):416-29. doi: 10.1118/1.4903758.
A known factor affecting the accuracy of radiochromic film dosimetry is the lateral response artifact (LRA) induced by nonuniform response of a flatbed scanner in the direction perpendicular to the scan direction. This work reports a practical solution to eliminate such artifacts for all forms of dose QA.
EBT3 films from a single production lot (02181401) cut into rectangular 4 × 5 cm(2) pieces, with the long dimension parallel to the long dimension of the original 20.3 × 25.4 cm(2) sheets, were exposed at a depth of 5 cm on a Varian Trilogy at the center of a 20 × 20 cm(2) open field at seven doses between 50 and 1600 cGy using 6 MV photons. These films together with an unexposed film from the same production lot were lined one next to the other on an Epson 10000 XL or 11000 XL scanner in portrait orientation with their long dimension parallel to the scan direction. Scanned images were then obtained with the line of films positioned at seven discrete lateral locations perpendicular to the scan direction. The process was repeated in landscape orientation and on three other Epson scanners. Data were also collected for three additional production lots of EBT3 film (11051302, 03031401, and 03171403). From measurements at the various lateral positions, the scanner response was determined as a function of the lateral position of the scanned film. For a given color channel X, the response at any lateral position L is related to the response at the center, C, of the scanner by Response(C, D, X) = A(L,X) + B(L,X) ⋅ Response(L, D, X), where D is dose and the coefficients A(L,X) and B(L,X) are determined from the film measurements at the center of the scanner and six other discrete lateral positions. The values at intermediate lateral positions were obtained by linear interpolation. The coefficients were determined for the red, green, and blue color channels, preserving the ability to apply triple-channel dosimetry once corrections were applied to compensate for the lateral position response artifact. To validate this method, corrections were applied to several films that were exposed to 15 × 15 cm(2) open fields and large IMRT and VMAT fields and scanned at the extreme edges of the scan window in addition to the central location. Calibration and response data were used to generate dose maps and perform gamma analysis using single- or triple-channel dosimetry with FilmQAPro 2014 software.
The authors' study found that calibration curves at the different lateral positions could be correlated by a simple two-point rescaling using the response for unexposed film as well as the response of film exposed at high doses between 800 and 1600 cGy. The coefficients A(L,X) and BL,X for each color channel X were found to be independent of dose at each lateral location L. This made it possible to apply the relationship Response(C, D, X) = A(L,X) + B(L,X) ⋅ Response(L, D, X), to the raw film responses, permitting correction of the response values at any lateral position to an equivalent response, as if that part of the film was located at the center of the scanner. This correction method was validated for several films exposed to open as well as large IMRT and VMAT fields.
The work reported elaborates on the process using the correction procedures to eliminate the lateral response artifact and demonstrates improvements in the accuracy of radiochromic film dosimetry for the radiation therapy quality assurance applications.
影响放射变色胶片剂量测定准确性的一个已知因素是平板扫描仪在垂直于扫描方向上的非均匀响应所引起的横向响应伪影（LRA）。本文报道了一种切实可行的解决方案，可消除所有形式剂量质量保证中的此类伪影。
将来自单个生产批次（02181401）的EBT3胶片切成4×5 cm²的矩形片，其长边尺寸与原始20.3×25.4 cm²片材的长边尺寸平行，在瓦里安Trilogy直线加速器上5 cm深度处，于20×20 cm²开放射野中心，使用6 MV光子，以50至1600 cGy之间的七个剂量进行照射。这些胶片与来自同一生产批次的未曝光胶片一起，在爱普生10000 XL或11000 XL扫描仪上纵向排列，长边尺寸平行于扫描方向。然后在垂直于扫描方向的七个离散横向位置获取扫描图像。该过程在横向方向以及另外三台爱普生扫描仪上重复进行。还收集了另外三个EBT3胶片生产批次（11051302、03031401和03171403）的数据。根据在各个横向位置的测量结果，确定扫描仪响应作为扫描胶片横向位置的函数。对于给定的颜色通道X，在任何横向位置L处的响应与扫描仪中心C处的响应之间的关系为Response(C, D, X) = A(L,X) + B(L,X) ⋅ Response(L, D, X)，其中D是剂量，系数A(L,X)和B(L,X)由在扫描仪中心和其他六个离散横向位置的胶片测量确定。中间横向位置的值通过线性插值获得。针对红色、绿色和蓝色颜色通道确定系数，以便在应用校正以补偿横向位置响应伪影后，仍能够应用三通道剂量测定法。为验证此方法，对几张暴露于15×15 cm²开放射野以及大型调强放疗（IMRT）和容积调强弧形放疗（VMAT）射野的胶片进行校正，并除了在中心位置扫描外，还在扫描窗口的极端边缘进行扫描。使用校准和响应数据生成剂量图，并使用FilmQAPro 2014软件通过单通道或三通道剂量测定法进行伽马分析。
作者的研究发现，不同横向位置的校准曲线可以通过简单的两点重新缩放相关联，使用未曝光胶片的响应以及在800至1600 cGy之间高剂量下曝光的胶片的响应。发现每个颜色通道X的系数A(L,X)和B(L,X)在每个横向位置L处与剂量无关。这使得可以将关系Response(C, D, X) = A(L,X) + B(L,X) ⋅ Response(L, D, X)应用于原始胶片响应，从而将任何横向位置处的响应值校正为等效响应，就好像该部分胶片位于扫描仪中心一样。这种校正方法在几张暴露于开放射野以及大型IMRT和VMAT射野的胶片上得到了验证。
本文报道的工作详细阐述了使用校正程序消除横向响应伪影的过程，并证明了在放射治疗质量保证应用中放射变色胶片剂量测定准确性的提高。
