Chan Maria F, Chen Chin-Cheng, Shi Chengyu, Li Jingdong, Tang Xiaoli, Li Xiang, Mah Dennis
Dept. of Medical Physics, Memorial Sloan Kettering Cancer Center, New York, NY.
Dept. of Radiation Physics, ProCure Proton Center, Somerset, NJ.
Int J Med Phys Clin Eng Radiat Oncol. 2017 May;6(2):111-123. doi: 10.4236/ijmpcero.2017.62011. Epub 2017 May 16.
Radiochromic film for spot-scanning QA provides high spatial resolution and efficiency gains from one-shot irradiation for multiple depths. However, calibration can be a tedious procedure which may limit widespread use. Moreover, since there may be an energy dependence, which manifests as a depth dependence, this may require additional measurements for each patient. We present a one-scan protocol to simplify the procedure. A calibration using an EBT3 film, exposed by a 6-level step-wedge plan on a ProteusPLUS proton system (IBA, Belgium), was performed at depths of 18, 20, 24cm using Plastic Water (CIRS, Norfolk, VA). The calibration doses ranged from 65-250 cGy(RBE) (relative biological effectiveness) for proton energies of 170-200 MeV. A clinical prostate+nodes plan was used for validation. The planar doses at selected depths were measured with EBT3 films and analyzed using One-scan protocol (one-scan digitization of QA film and at least one film exposed to a known dose). The gamma passing rates, dose-difference maps, and profiles of 2D planar doses measured with EBT3 film and IBA MatriXX-PT, versus the RayStation TPS calculations were analyzed and compared. The EBT3 film measurement results matched well with the TPS calculation data with an average passing rate of ~95% for 2%/2mm and slightly lower passing rates were obtained from an ion chamber array detector. We were able to demonstrate that the use of a proton step-wedge provided clinically acceptable results and minimized variations between film-scanner orientation, inter-scan, and scanning conditions. Furthermore, for relative dosimetry (calibration is not done at the time of experiment) it could be derived from no more than two films exposed to known doses (one could be zero) for rescaling the master calibration curve at each depth. The sensitivity of the calibration to depth variations has been explored. One-scan protocol results appear to be comparable to that of the ion chamber array detector. The use of a proton step-wedge for calibration of EBT3 film potentially increases efficiency in patient-specific QA of proton beams.
用于点扫描质量保证（QA）的放射变色胶片具有高空间分辨率，并且通过对多个深度进行单次照射可提高效率。然而，校准可能是一个繁琐的过程，这可能会限制其广泛应用。此外，由于可能存在能量依赖性，表现为深度依赖性，这可能需要为每个患者进行额外测量。我们提出了一种单扫描方案来简化该过程。使用EBT3胶片进行校准，该胶片在比利时IBA公司的ProteusPLUS质子系统上通过六级楔形计划进行照射，在18、20、24厘米深度处使用塑料水（弗吉尼亚州诺福克市CIRS公司）。对于170 - 200 MeV的质子能量，校准剂量范围为65 - 250 cGy（相对生物效应）。使用临床前列腺+淋巴结计划进行验证。在选定深度处的平面剂量用EBT3胶片测量，并使用单扫描方案（QA胶片的单扫描数字化以及至少一张暴露于已知剂量的胶片）进行分析。分析并比较了用EBT3胶片和IBA MatriXX - PT测量的二维平面剂量的γ通过率、剂量差图和剖面图与RayStation治疗计划系统（TPS）计算结果。EBT3胶片测量结果与TPS计算数据匹配良好，对于2%/2毫米标准，平均通过率约为95%，从离子室阵列探测器获得的通过率略低。我们能够证明使用质子楔形提供了临床可接受的结果，并最大限度地减少了胶片扫描仪方向、扫描间和扫描条件之间的变化。此外，对于相对剂量测定（实验时未进行校准），可以从不超过两张暴露于已知剂量（一张可以为零）的胶片中推导出来，以便在每个深度重新缩放主校准曲线。已经探索了校准对深度变化的敏感性。单扫描方案结果似乎与离子室阵列探测器的结果相当。使用质子楔形对EBT3胶片进行校准可能会提高质子束患者特定QA的效率。
