Kaliyaperumal Venkatesan, Raphael C Jomon, Varghese K Mathew, Gopu Paul, Sivakumar S, Boban Minu, Raj N Arunai Nambi, Senthilnathan K, Babu P Ramesh
Department of Radiation Oncology, Amala Institute of Medical Sciences, Thrissur, Kerala, India.
Centre for Biomaterials, Cellular and Molecular Theranostics, VIT University, Vellore, Tamil Nadu, India.
J Med Phys. 2017 Jul-Sep;42(3):171-180. doi: 10.4103/jmp.JMP_24_17.
Cone-beam computed tomography (CBCT) images are presently used for geometric verification for daily patient positioning. In this work, we have compared the images of CBCT with the images of conventional fan beam CT (FBCT) in terms of image quality and Hounsfield units (HUs). We also compared the dose calculated using CBCT with that of FBCT. Homogenous RW3 plates and Catphan phantom were scanned by FBCT and CBCT. In RW3 and Catphan phantom, percentage depth dose (PDD), profiles, isodose distributions (for intensity modulated radiotherapy plans), and calculated dose volume histograms were compared. The HU difference was within ± 20 HU (central region) and ± 30 HU (peripheral region) for homogeneous RW3 plates. In the Catphan phantom, the difference in HU was ± 20 HU in the central area and peripheral areas. The HU differences were within ± 30 HU for all HU ranges starting from -1000 to 990 in phantom and patient images. In treatment plans done with simple symmetric and asymmetric fields, dose difference (DD) between CBCT plan and FBCT plan was within 1.2% for both phantoms. In intensity modulated radiotherapy (IMRT) treatment plans, for different target volumes, the difference was <2%. This feasibility study investigated HU variation and dose calculation accuracy between FBCT and CBCT based planning and has validated inverse planning algorithms with CBCT. In our study, we observed a larger deviation of HU values in the peripheral region compared to the central region. This is due to the ring artifact and scatter contribution which may prevent the use of CBCT as the primary imaging modality for radiotherapy treatment planning. The reconstruction algorithm needs to be modified further for improving the image quality and accuracy in HU values. However, our study with TG-119 and intensity modulated radiotherapy test targets shows that CBCT can be used for adaptive replanning as the recalculation of dose with the anisotropic analytical algorithm is in full accord with conventional planning CT except in the build-up regions. Patient images with CBCT have to be carefully analyzed for any artifacts before using them for such dose calculations.
锥束计算机断层扫描（CBCT）图像目前用于日常患者定位的几何验证。在这项工作中，我们在图像质量和亨氏单位（HU）方面比较了CBCT图像与传统扇形束CT（FBCT）图像。我们还比较了使用CBCT计算的剂量与FBCT的剂量。用FBCT和CBCT扫描均匀的RW3平板和Catphan体模。在RW3和Catphan体模中，比较了百分深度剂量（PDD）、剂量分布曲线、等剂量分布（用于调强放疗计划）以及计算的剂量体积直方图。对于均匀的RW3平板，HU差异在中心区域为±20 HU，在外围区域为±30 HU。在Catphan体模中，中心区域和外围区域的HU差异为±20 HU。在体模和患者图像中，从-1000到990的所有HU范围内，HU差异均在±30 HU以内。在使用简单对称和不对称射野的治疗计划中，两个体模的CBCT计划和FBCT计划之间的剂量差异（DD）均在1.2%以内。在调强放疗（IMRT）治疗计划中，对于不同的靶区体积，差异<2%。这项可行性研究调查了基于FBCT和CBCT的计划之间的HU变化和剂量计算准确性，并验证了CBCT的逆向计划算法。在我们的研究中，我们观察到与中心区域相比，外围区域的HU值偏差更大。这是由于环形伪影和散射贡献，这可能会妨碍将CBCT用作放射治疗计划的主要成像方式。重建算法需要进一步改进以提高图像质量和HU值的准确性。然而，我们对TG-119和调强放疗测试靶区的研究表明，CBCT可用于自适应重新计划，因为使用各向异性分析算法重新计算剂量与传统计划CT完全一致，除了剂量建成区。在将CBCT患者图像用于此类剂量计算之前，必须仔细分析是否存在任何伪影。
