Spreeuw Hanno, Rozendaal Roel, Camargo Priscilla, Mans Anton, Wendling Markus, Olaciregui-Ruiz Igor, Sonke Jan-Jakob, Van Herk Marcel, Mijnheer Ben
The Netherlands Cancer Institute.
J Appl Clin Med Phys. 2015 May 8;16(3):5375. doi: 10.1120/jacmp.v16i3.5375.
Portal dosimetry using electronic portal imaging devices (EPIDs) is often applied to verify high-energy photon beam treatments. Due to the change in photon energy spectrum, the resulting dose values are, however, not very accurate in the case of wedged beams if the pixel-to-dose conversion for the situation without wedge is used. A possible solution would be to consider a wedged beam as another photon beam quality requiring separate beam modeling of the dose calculation algorithm. The aim of this study was to investigate a more practical solution: to make aSi EPID-based dosimetry models also applicable for wedged beams without an extra commissioning effort of the parameters of the model. For this purpose two energy-dependent wedge multiplication factors have been introduced to be applied for portal images taken with and without a patient/phantom in the beam. These wedge multiplication factors were derived from EPID and ionization chamber measurements at the EPID level for wedged and nonwedged beams, both with and without a polystyrene slab phantom in the beam. This method was verified for an EPID dosimetry model used for wedged beams at three photon beam energies (6, 10, and 18 MV) by comparing dose values reconstructed in a phantom with data provided by a treatment planning system (TPS), as a function of field size, depth, and off-axis distance. Generally good agreement, within 2%, was observed for depths between dose maximum and 15 cm. Applying the new model to EPID dose measurements performed during ten breast cancer patient treatments with wedged 6 MV photon beams showed that the average isocenter underdosage of 5.3% was reduced to 0.4%. Gamma-evaluation (global 3%/3 mm) of these in vivo data showed an increase in percentage of points with γ ≤ 1 from 60.2% to 87.4%, while γmean reduced from 1.01 to 0.55. It can be concluded that, for wedged beams, the multiplication of EPID pixel values with an energy-dependent correction factor provides good agreement between dose values determined by an EPID and a TPS, indicating the usefulness of such a practical solution.
使用电子射野影像装置（EPID）进行射野剂量测定常用于验证高能光子束治疗。然而，由于光子能谱的变化，如果使用无楔形板情况下的像素到剂量转换，对于楔形束，所得剂量值并不十分准确。一种可能的解决方案是将楔形束视为另一种光子束质量，需要对剂量计算算法进行单独的束流建模。本研究的目的是研究一种更实用的解决方案：使基于非晶硅EPID的剂量测定模型也适用于楔形束，而无需对模型参数进行额外的调试工作。为此，引入了两个能量相关的楔形乘法因子，分别应用于射野中有和没有患者/模体时采集的射野图像。这些楔形乘法因子是通过在EPID层面进行的EPID和电离室测量得出的，测量对象为有和没有聚苯乙烯平板模体的楔形束和非楔形束。通过将模体中重建的剂量值与治疗计划系统（TPS）提供的数据进行比较，作为射野大小、深度和离轴距离的函数，对用于三种光子束能量（6、10和18 MV）的楔形束的EPID剂量测定模型进行了验证。在剂量最大值和15 cm之间的深度处，通常观察到良好的一致性，偏差在2%以内。将新模型应用于10例使用楔形6 MV光子束进行乳腺癌患者治疗期间的EPID剂量测量，结果表明平均等中心剂量不足从5.3%降至0.4%。对这些体内数据进行γ评估（全局3%/3 mm）显示，γ≤1的点的百分比从60.2%增加到87.4%，而γ平均值从1.01降至0.55。可以得出结论，对于楔形束，将EPID像素值乘以能量相关的校正因子可使EPID和TPS确定的剂量值之间具有良好的一致性，表明这种实用解决方案的有效性。
