Department of Radiation Oncology, Universitätsklinikum Erlangen, Friedrich-Alexander-Universität Erlangen-Nürnberg, Universitätsstraße 27, 91054, Erlangen, Germany.
Department of Radiation Oncology and Image-applied Therapy, Graduate School of Medicine, Kyoto University, 54 Kawahara-cho, Shogoin, Sakyo-ku, Kyoto, 606-8507, Japan.
Med Phys. 2019 Jul;46(7):2969-2977. doi: 10.1002/mp.13566. Epub 2019 Jun 5.
Inter-fractional changes of the daily anatomy of the patient are a source of error in radiotherapy. This effect is especially present in the abdomen, where there are daily differences in the filling of the stomach or the intestines. This study investigated the impact of inter-fractional changes of pancreatic cancer patients who underwent breath-hold volumetric modulated arc therapy on the applied dose by using deformable image registration.
Deformable image registration was used to determine a displacement field which maps the computed tomography (CT) used for treatment planning onto daily breath-hold cone-beam CT images with limited field-of-view (FOV) which were acquired at the beginning of every treatment fraction. Different B-spline registration methods utilizing different metrics and regularizations were used and compared. The resulting deformed planning CT images were used to calculate the dose distributions on the anatomy of the day which can therefore be seen as a good estimation of the actually delivered dose during the treatment fractions. Additionally, the accumulated dose distribution was determined by accumulating all daily dose distributions using inverted deformation fields. The calculated dose distributions were then compared to the planned dose by comparison of dose volume histograms and dosimetric volumetric indices.
Best registration results were obtained using a gradient magnitude metric and a regularization of λ = 0.05. The accumulated dose distributions showed that the mean dose delivered to the clinical target volume was on median reduced by 0.61 Gy after 15 fractions from the planned dose. For the organs at risk, the accumulated dose distributions were still fulfilling all clinical goals for the patient cohort used in this study. It was found that rotating and shearing motions of the tumor led to a greater reduction in the mean target dose which correlated with the proximity of the target to the spine (r = 0.54, P = 0.038).
With the proposed method, it is possible to generate a deformed planning CT image which matches the anatomy of the day based on cone beam computed tomography images with a limited FOV and calculating the dose distribution of the day. The dosimetric impact of the inter-fractional variations was small for most patients but showed larger deviations if the tumor is located in close proximity to the spine.
患者的分次内解剖变化是放射治疗中的误差源。这种效应在腹部尤其明显，因为胃部或肠道的充盈程度每天都有差异。本研究通过形变图像配准，调查了接受屏气容积调制弧形治疗的胰腺癌患者分次内变化对应用剂量的影响。
采用形变图像配准，确定一个位移场，将用于治疗计划的计算机断层扫描（CT）映射到每天屏气锥形束 CT 图像上，锥形束 CT 图像的视场（FOV）有限，且在每个治疗分次的开始时采集。使用不同的 B 样条配准方法，利用不同的度量和正则化方法进行比较。使用生成的变形计划 CT 图像计算当天解剖结构的剂量分布，因此可以作为治疗分次中实际给予剂量的良好估计。此外，通过使用反转的变形场来累加所有的每日剂量分布，确定累积剂量分布。然后通过比较剂量体积直方图和剂量学体积指数来比较计算出的剂量分布与计划剂量。
使用梯度幅度度量和正则化参数λ=0.05 获得了最佳的配准结果。累积剂量分布表明，在 15 个分次后，临床靶体积接受的平均剂量中位数比计划剂量减少了 0.61 Gy。对于危及器官，累积剂量分布仍然满足本研究中使用的患者队列的所有临床目标。发现肿瘤的旋转和剪切运动导致靶区平均剂量的显著降低，这与靶区与脊柱的接近程度相关（r=0.54，P=0.038）。
通过所提出的方法，有可能基于视场有限的锥形束 CT 图像生成与当天解剖结构匹配的变形计划 CT 图像，并计算当天的剂量分布。对于大多数患者，分次内变化的剂量学影响较小，但如果肿瘤靠近脊柱，偏差会更大。
