Ping Heng Siew, Kandaiya Sivamany
Radiotherapy Unit, Pantai Hospital Kuala Lumpur, Malaysia.
J Med Phys. 2012 Jul;37(3):155-8. doi: 10.4103/0971-6203.99239.
The objective of this work is to study the influence of the patient size and geometry on CBCT Hounsfield Unit and the accuracy of calibration Hounsfield Unit to electron density (HU-ED) using patient specific HU-ED mapping method for dose calculation. Two clinical cases, namely nasopharyngeal carcinoma (NPC) case and prostate case for 4 patients with different size and geometry were enrolled to assess the impact of size and geometry on CBCT Hounsfield Unit. The accuracy of the patient specific HU-ED mapping method was validated by comparing dose distributions based on planning CT and CBCT, dose-volume based indices and the digitally reconstructed radiograph (DRR) by analyzing their line profile plots. Significant differences in Hounsfield unit and line profile plots were found for NPC and prostate cases. The doses computed based on planning CT data sets and CBCT datasets for both clinical cases agree to within 1% for planning target volumes and 3% for organs at risk. The data shows that there are high dependence of HU on patient size and geometry; thus, the use of one CBCT HU-ED calibration curve made of one size and geometry will not be accurate for use with a patient of different size and geometry.
本研究的目的是使用患者特异性的HU-ED映射方法来计算剂量，研究患者体型和几何形状对CBCT亨氏单位（Hounsfield Unit，HU）的影响，以及校准HU到电子密度（HU-ED）的准确性。纳入了两个临床病例，即4例不同体型和几何形状的鼻咽癌（NPC）病例和前列腺病例，以评估体型和几何形状对CBCT HU的影响。通过比较基于计划CT和CBCT的剂量分布、基于剂量体积的指标以及通过分析其线轮廓图的数字重建射线照片（DRR），验证了患者特异性HU-ED映射方法的准确性。在NPC和前列腺病例中，发现亨氏单位和线轮廓图存在显著差异。对于两个临床病例，基于计划CT数据集和CBCT数据集计算的计划靶体积剂量在1%以内，危及器官剂量在3%以内。数据表明，HU高度依赖于患者的体型和几何形状；因此，使用由一种体型和几何形状制成的一条CBCT HU-ED校准曲线，对于不同体型和几何形状的患者来说是不准确的。
