Department of Oncology Physics, Edinburgh Cancer Centre, Western General Hospital, Crewe Road South, Edinburgh EH4 2XU, United Kingdom.
Med Phys. 2011 Feb;38(2):632-45. doi: 10.1118/1.3525841.
As traditional film is phased out in most radiotherapy centers, computed radiography (CR) systems are increasingly being purchased as a replacement. CR plates can be used for patient imaging, but may also be used for a variety of quality assurance (QA) purposes and can be calibrated in terms of dose. This study looks at their suitability for verification of intensity modulated radiation therapy (IMRT) dose distributions.
A CR plate was calibrated in terms of the relative dose and the stability of response over 1 year was studied. The effect of exposing the CR plate to ambient light and of using different time delays before scanning was quantified. The CR plate was used to verify the relative dose distributions for ten IMRT patients and the results were compared to those obtained using a two dimensional (2D) diode array.
Exposing the CR plate to 10 s of ambient light between irradiation (174 cGy) and scanning erased approximately 80% of the signal. Changes in delay time between irradiation and scanning also affected the measurement results. The signal on the plate was found to decay at a rate of approximately 3.6 cGy/min in the first 10 min after irradiation. The use of a CR plate for IMRT patient-specific QA resulted in a significantly lower distance to agreement (DTA) and gamma pass rate than when using a 2D diode array for the measurement. This was primarily due to the over-response of the CR phosphor to low energy scattered radiation. For the IMRT QA using the CR plate, the average gamma pass rate was 97.3%. For the same IMRT QA using a diode array, the average gamma pass rate was 99.7%. The gamma criteria used were 4% dose difference and 4 mm DTA for head and neck treatments and 3% dose difference and 3 mm DTA for prostate treatments. The gamma index tolerance was 1. The lowest 10% of the dose distribution was excluded from all gamma and DTA analyses.
Although the authors showed that CR plates can be used for patient specific IMRT QA, the practical problems such as the over-response to low energy scatter and signal fading with light exposure and time mean that alternative detectors such as radiochromic film or diode arrays will be a more sensible choice for most radiotherapy departments.
随着传统胶片在大多数放射治疗中心逐步淘汰，计算机射线摄影（CR）系统越来越多地被购买作为替代品。CR 板可用于患者成像，但也可用于各种质量保证（QA）目的，并可根据剂量进行校准。本研究探讨了其用于验证调强放射治疗（IMRT）剂量分布的适用性。
根据相对剂量对 CR 板进行校准，并研究了其在 1 年内响应稳定性的变化。量化了将 CR 板暴露在环境光下以及在扫描前使用不同延迟时间的影响。使用 CR 板验证了 10 名 IMRT 患者的相对剂量分布，并将结果与二维（2D）二极管阵列的结果进行比较。
将 CR 板在照射（174 cGy）和扫描之间暴露于 10 秒的环境光中，会消除约 80%的信号。照射和扫描之间延迟时间的变化也会影响测量结果。在照射后 10 分钟内，板上的信号以大约 3.6 cGy/min 的速率衰减。使用 CR 板进行 IMRT 患者特定 QA 的结果导致距离一致性（DTA）和伽马通过率显著低于使用二维二极管阵列进行测量的结果。这主要是由于 CR 磷光体对低能散射辐射的过度响应。对于使用 CR 板进行的 IMRT QA，平均伽马通过率为 97.3%。对于使用二极管阵列进行相同的 IMRT QA，平均伽马通过率为 99.7%。使用的伽马标准为头颈部治疗的 4%剂量差异和 4 mm DTA，前列腺治疗的 3%剂量差异和 3 mm DTA。伽马指数容限为 1。从所有伽马和 DTA 分析中排除剂量分布的最低 10%。
尽管作者表明 CR 板可用于患者特定的 IMRT QA，但由于对低能散射的过度响应以及光暴露和时间引起的信号衰减等实际问题，对于大多数放射治疗部门而言，替代探测器（如光致变色胶片或二极管阵列）将是更明智的选择。
