Department of Radiation Oncology, Shandong Cancer Hospital and Institute, 440# Jiyan Road, Jinan, 250117, Shandong Province, People's Republic of China.
Medical Department, Affiliated Hospital of Shandong Academy of Medical Sciences, Jinan, 250031, People's Republic of China.
Radiat Oncol. 2018 Apr 27;13(1):80. doi: 10.1186/s13014-018-0989-y.
To evaluate the effect of pretreatment megavoltage computed tomographic (MVCT) scan methodology on setup verification and adaptive dose calculation in helical TomoTherapy.
Both anthropomorphic heterogeneous chest and pelvic phantoms were planned with virtual targets by TomoTherapy Physicist Station and were scanned with TomoTherapy megavoltage image-guided radiotherapy (IGRT) system consisted of six groups of options: three different acquisition pitches (APs) of 'fine', 'normal' and 'coarse' were implemented by multiplying 2 different corresponding reconstruction intervals (RIs). In order to mimic patient setup variations, each phantom was shifted 5 mm away manually in three orthogonal directions respectively. The effect of MVCT scan options was analyzed in image quality (CT number and noise), adaptive dose calculation deviations and positional correction variations.
MVCT scanning time with pitch of 'fine' was approximately twice of 'normal' and 3 times more than 'coarse' setting, all which will not be affected by different RIs. MVCT with different APs delivered almost identical CT numbers and image noise inside 7 selected regions with various densities. DVH curves from adaptive dose calculation with serial MVCT images acquired by varied pitches overlapped together, where as there are no significant difference in all p values of intercept & slope of emulational spinal cord (p = 0.761 & 0.277), heart (p = 0.984 & 0.978), lungs (p = 0.992 & 0.980), soft tissue (p = 0.319 & 0.951) and bony structures (p = 0.960 & 0.929) between the most elaborated and the roughest serials of MVCT. Furthermore, gamma index analysis shown that, compared to the dose distribution calculated on MVCT of 'fine', only 0.2% or 1.1% of the points analyzed on MVCT of 'normal' or 'coarse' do not meet the defined gamma criterion. On chest phantom, all registration errors larger than 1 mm appeared at superior-inferior axis, which cannot be avoided with the smallest AP and RI. On pelvic phantom, craniocaudal errors are much smaller than chest, however, AP of 'coarse' presents larger registration errors which can be reduced from 2.90 mm to 0.22 mm by registration technique of 'full image'.
AP of 'coarse' with RI of 6 mm is recommended in adaptive radiotherapy (ART) planning to provide craniocaudal longer and faster MVCT scan, while registration technique of 'full image' should be used to avoid large residual error. Considering the trade-off between IGRT and ART, AP of 'normal' with RI of 2 mm was highly recommended in daily practice.
为了评估预处理兆伏 CT（MVCT）扫描方法对螺旋断层放疗自适应剂量计算和摆位验证的影响。
使用 TomoTherapy 物理师工作站为人体异质胸部和骨盆体模虚拟靶区计划，并使用由 6 组选项组成的 TomoTherapy 兆伏图像引导放疗（IGRT）系统对其进行扫描：通过将 2 种不同的相应重建间隔（RI）相乘，实现了 3 种不同的采集螺距（AP）“精细”、“正常”和“粗糙”。为了模拟患者的摆位变化，每个体模分别在 3 个正交方向手动平移 5mm。分析了 MVCT 扫描选项对图像质量（CT 值和噪声）、自适应剂量计算偏差和位置校正变化的影响。
AP 为“精细”的 MVCT 扫描时间约为“正常”的两倍，是“粗糙”设置的 3 倍，所有这些都不会受到不同 RI 的影响。在具有不同密度的 7 个选定区域内，具有不同 AP 的 MVCT 提供了几乎相同的 CT 值和图像噪声。使用不同螺距获得的系列 MVCT 图像进行自适应剂量计算的 DVH 曲线重叠在一起，而模拟脊髓（p=0.761 和 0.277）、心脏（p=0.984 和 0.978）、肺（p=0.992 和 0.980）、软组织（p=0.319 和 0.951）和骨结构（p=0.960 和 0.929）之间截距和斜率的所有 p 值均无显著差异。此外，伽马指数分析表明，与在 MVCT 中“精细”计算的剂量分布相比，在 MVCT 中“正常”或“粗糙”计算的剂量分布中只有 0.2%或 1.1%的点不符合定义的伽马标准。在胸部体模中，所有大于 1mm 的配准误差都出现在上下轴上，即使使用最小的 AP 和 RI 也无法避免。在骨盆体模中，头脚方向的误差明显小于胸部，但“粗糙”AP 会导致较大的配准误差，通过“全图像”配准技术可以将其从 2.90mm 降低至 0.22mm。
在自适应放疗（ART）计划中，建议使用 AP 为“粗糙”、RI 为 6mm 的 MVCT 扫描，以提供更长和更快的头脚方向 MVCT 扫描，同时应使用“全图像”配准技术以避免较大的残余误差。考虑到 IGRT 和 ART 之间的权衡，在日常实践中强烈推荐使用 AP 为“正常”、RI 为 2mm 的方案。
