Schmitt D, Nill S, Roeder F, Herfarth K, Oelfke U
German Cancer Research Center, Heidelberg, Germany.
Heidelberg University Hospital, Heidelberg, Germany.
Med Phys. 2012 Jun;39(6Part8):3686. doi: 10.1118/1.4734984.
Evaluation of different calculation methods for dose modification due to intrafraction prostate motion using film measurements as ground truth.
We acquired intrafraction motion data with the Calypso tumor tracking system by Varian Medical Systems Inc for 4 prostate IMRT patients treated with 35 fractions each. These motion data were transferred to a phantom platform which reproduces the observed motion and has a 20 cm diameter cylindrical solid water phantom mounted. For each patient all fractions were irradiated on one radiochromic MD-V2-55 film placed in the isocentric transversal slice of this phantom. These films serve as ground truth for three calculation Methods: 1) Recalculation of the plan with shifted target point for every segment with the segment's mean Calypso position. 2)+3) Convolution of the static dose distribution with a probability density function of the observed positions. For 2) only Calypso positions with activated beam on signal were used whereas for 3) all Calypso positions between the first and the last beam on signal for all fractions were employed. The comparisons between films and calculated dose distributions were made with the verification software VeriSoft 3.2 (PTW, Freiburg, Germany) where an 8×8 cm̂2 ROI around the isocenter was selected for gamma evaluation.
The segment shifted plans reach 3%/3mm gamma values above 90% against the films for all four patients. For both convolution methods three values are above 90%, only for the patient with the largest intrafraction motion they decrease to 89%.
Shifting of the target point for every segment is well suited to estimate the dosimetric consequences of intrafraction prostate motion. This may facilitate the evaluation of different margin sizes or dose prescribing recipes under different motion conditions. If such a lengthy calculation is not possible, a convolution with motion data can be used for acceptable results, too. Our work was partially supported by Siemens Healthcare and Varian Medical Systems Inc.
以胶片测量结果作为基准事实，评估因分次内前列腺运动而进行剂量修正的不同计算方法。
我们使用Varian Medical Systems公司的Calypso肿瘤追踪系统获取了4例接受前列腺调强放疗（IMRT）的患者的分次内运动数据，每位患者均接受35次分割治疗。这些运动数据被传输到一个体模平台，该平台可重现观察到的运动，并安装了一个直径为20 cm的圆柱形固体水模体。对于每位患者，所有分割均在放置于该体模等中心横向切片的一张放射变色MD-V2-55胶片上进行照射。这些胶片作为三种计算方法的基准事实：1）使用各射野段的平均Calypso位置对每个射野段的计划进行靶区移位重计算。2）+3）将静态剂量分布与观察位置的概率密度函数进行卷积。对于2），仅使用射野开启信号时的Calypso位置，而对于3），则采用所有分割中第一个和最后一个射野开启信号之间的所有Calypso位置。使用验证软件VeriSoft 3.2（德国弗莱堡PTW公司）对胶片和计算得到的剂量分布进行比较，在等中心周围选择一个8×8 cm²的感兴趣区（ROI）进行γ评估。
对于所有4例患者，射野段移位计划相对于胶片的γ值在3%/3 mm时均达到90%以上。对于两种卷积方法，三例患者的值均高于90%，仅对于分次内运动最大的患者，该值降至89%。
对每个射野段进行靶区移位非常适合于估计分次内前列腺运动的剂量学后果。这可能有助于评估不同运动条件下不同边界大小或剂量处方方案。如果无法进行如此冗长的计算，使用运动数据进行卷积也可得到可接受的结果。我们的工作部分得到了西门子医疗和Varian Medical Systems公司的支持。
