Reich P, Bezak E, Mohammadi M, Fog L
School of Chemistry and Physics, University of Adelaide, Australia.
Australas Phys Eng Sci Med. 2006 Mar;29(1):18-29. doi: 10.1007/BF03178824.
Patient dose verification is becoming increasingly important with the advent of new complex radiotherapy techniques such as conformal radiotherapy (CRT) and intensity-modulated radiotherapy (IMRT). An electronic portal imaging device (EPID) has potential application for in vivo dosimetry. In the current work, an EPID has been modelled using a treatment planning system (TPS) to predict transmitted dose maps. A thin slab of RW3 material used to initially represent the EPID. A homogeneous RW3 phantom and the thin RW3 slab placed at a clinical distance away from the phantom were scanned using a CT simulator. The resulting CT images were transferred via DICOM to the TPS and the density of the CT data corresponding to the thin RW3 slab was changed to 1 g/cm3. Transmitted dose maps (TDMs) in the modelled EPID were calculated by the TPS using the collapsed-cone (C-C) convolution superposition (C/S) algorithm. A 6 MV beam was used in the simulation to deliver 300 MU to the homogenous phantom using an isocentric and SSD (source-to-surface) technique. The phantom thickness was varied and the calculated TDMs in the modelled EPID were compared with corresponding measurements obtained from a calibrated scanning liquid-filled ionisation chamber (SLIC) EPID. The two TDMs were compared using the gamma evaluation technique of Low et al. The predicted and measured TDMs agree to within 2 % (averaged over all phantom thicknesses) on the central beam axis. More than 90 % of points in the dose maps (excluding field edges) produce a gamma index less than or equal to 1, for dose difference (averaged over all phantom thicknesses), and distance-to-agreement criteria of 4 %, 3.8 mm, respectively. In addition, the noise level on the central axis in the predicted dose maps is less than 0.1 %. We found that phantom thickness changes of approximately 1 mm, which correspond to dose changes on the central beam axis of less than 0.6 %, can be detected in the predicted transmitted dose distributions.
随着适形放疗（CRT）和调强放疗（IMRT）等新的复杂放疗技术的出现，患者剂量验证变得越来越重要。电子射野影像装置（EPID）在体内剂量测定方面具有潜在应用价值。在当前工作中，已使用治疗计划系统（TPS）对EPID进行建模，以预测透射剂量图。最初用一块薄的RW3材料代表EPID。使用CT模拟器扫描一个均匀的RW3体模以及放置在距体模临床距离处的薄RW3板。所得的CT图像通过DICOM传输到TPS，并将与薄RW3板对应的CT数据密度更改为1 g/cm³。TPS使用坍缩圆锥（C-C）卷积叠加（C/S）算法计算建模EPID中的透射剂量图（TDM）。在模拟中使用6 MV射线束，采用等中心和源皮距（SSD）技术向均匀体模输送300 MU。改变体模厚度，并将建模EPID中计算得到的TDM与从校准的扫描充液电离室（SLIC）EPID获得的相应测量值进行比较。使用Low等人的伽马评估技术比较这两个TDM。在中心射束轴上，预测的和测量的TDM在2%以内相符（对所有体模厚度求平均值）。对于剂量差异（对所有体模厚度求平均值）以及分别为4%、3.8 mm的距离一致性标准，剂量图中超过90%的点（不包括射野边缘）产生的伽马指数小于或等于1。此外，预测剂量图中心轴上的噪声水平小于0.1%。我们发现，在预测的透射剂量分布中，可以检测到体模厚度约1 mm的变化，这对应于中心射束轴上小于0.6%的剂量变化。
