Vial Philip, Greer Peter B, Oliver Lyn, Baldock Clive
Department of Medical Physics, Liverpool and Macarthur Cancer Therapy Centres, Sydney 2170, Australia.
Med Phys. 2008 Oct;35(10):4362-74. doi: 10.1118/1.2975156.
Electronic portal imaging devices (EPIDs) integrated with medical linear accelerators utilize an indirect-detection EPID configuration (ID-EPID). Amorphous silicon ID-EPIDs provide high quality low dose images for verification of radiotherapy treatments but they have limitations as dosimeters. The standard ID-EPID configuration includes a high atomic number phosphor scintillator screen, a 1 mm copper layer, and other nonwater equivalent materials covering the detector. This configuration leads to marked differences in the response of an ID-EPID compared to standard radiotherapy dosimeters such as ion chambers in water. In this study the phosphor and copper were removed from a standard commercial EPID to modify the configuration to a direct-detection EPID (DD-EPID). Using solid water as the buildup and backscatter for the detector, dosimetric measurements were performed on the DD-EPID and compared to standard dose-in-water data for 6 and 18 MV photons. The sensitivity of the DD-EPID was approximately eight times less than the ID-EPID but the signal was sufficient to produce accurate and reproducible beam profile measurements for open beams and an intensity-modulated beam. Due to the lower signal levels it was found necessary to ensure that the dark field correction (no radiation) DD-EPID signal was stable or updated frequently. The linearity of dose response was comparable to the ID-EPID but with a greater under-response at low doses. DD-EPID measurements of field size output factors and beam profiles at the depth of maximum dose (dmax), and tissue-maximum ratios between the depths of 0.5 and 10 cm, were in close agreement with dose in water measurements. At depths beyond dmax the DD-EPID showed a greater change in response to field size than ionisation chamber measurements and the beam penumbrae were broader compared to diode scans. The modified DD-EPID configuration studied here has the potential to improve the performance of EPIDs for dose verification of radiotherapy treatments.
与医用直线加速器集成的电子射野影像装置（EPID）采用间接探测式EPID配置（ID-EPID）。非晶硅ID-EPID可为放射治疗的验证提供高质量的低剂量图像，但作为剂量仪存在局限性。标准的ID-EPID配置包括高原子序数的磷光闪烁体屏、一层1毫米厚的铜层以及覆盖探测器的其他非水等效材料。这种配置导致ID-EPID的响应与标准放射治疗剂量仪（如水介质中的电离室）相比存在显著差异。在本研究中，从标准商用EPID上去除了磷光体和铜，将配置修改为直接探测式EPID（DD-EPID）。以固体水作为探测器的建成和散射介质，对DD-EPID进行剂量测量，并与6和18兆伏光子的标准水中剂量数据进行比较。DD-EPID的灵敏度约为ID-EPID的八分之一，但信号足以对开放射野和调强射野进行准确且可重复的射野轮廓测量。由于信号水平较低，发现有必要确保暗场校正（无辐射）的DD-EPID信号稳定或频繁更新。剂量响应的线性与ID-EPID相当，但在低剂量时响应不足更明显。DD-EPID对射野尺寸输出因子、最大剂量深度（dmax）处的射野轮廓以及0.5至10厘米深度之间的组织最大剂量比的测量结果与水中剂量测量结果高度一致。在dmax深度之外，DD-EPID对射野尺寸的响应变化比电离室测量结果更大，并且与二极管扫描相比射野半值层更宽。此处研究的改进型DD-EPID配置有潜力提高EPID在放射治疗剂量验证方面的性能。
