Department of Radiation Oncology, VU University Medical Center, Amsterdam, The Netherlands.
Radiother Oncol. 2010 Dec;97(3):443-8. doi: 10.1016/j.radonc.2010.06.007. Epub 2010 Nov 1.
An internal target volume (ITV) is often used for incorporating tumor motion into radiotherapy planning but it overestimates the margins necessary for breathing motion. We describe a pragmatic approach using maximum- and minimum-intensity projections (MIP and Min-IP) only, for reducing ITVs in stereotactic radiotherapy by using dosimetric margins that compensate for motion-induced dose blurring.
We studied tumor motion characteristics from 26 repeat 4DCT scans derived from 10 patients. These were used to calculate the shift in cranio-caudal direction of the 80% isodose due to dose blurring of the time-averaged dose distribution caused by respiratory motion. The dosimetric margins necessary to compensate for dose blurring were calculated relative to the ITV, which can be determined efficiently using the MIP. Peak-to-peak motion amplitude was determined using the MIP and Min-IP. A programmable respiratory motion phantom was used to investigate imaging artifacts in determining the ITV for realistic motion patterns. Dose profiles were both calculated and measured in lung- and water-equivalent tissue.
Using margins for the 80% dose level permitted the use of smaller target volumes relative to the use of ITV-based volumes, with (i) greater reductions seen at the end-inspiration edge than at expiration side due to asymmetric breathing motion patterns and (ii) a linear relationship seen with breathing amplitude. The average reduction of the ITV at a 95% confidence level is given by 0.2×A(pp)-1.3 mm at expiration side, where A(pp) is the peak-to-peak breathing amplitude, and 0.3×A(pp)-2.2 mm at inspiration side. Dosimetric margins did not differ significantly between water-equivalent and lung tissue for 80% isodose.
A simple margin recipe for breathing motion linear with breathing amplitude can be used to calculate the ITV reductions achievable for stereotactic radiotherapy of lung tumors.
内靶区（ITV）常用于将肿瘤运动纳入放射治疗计划，但它高估了呼吸运动所需的边缘。我们描述了一种实用的方法，仅使用最大和最小强度投影（MIP 和 Min-IP），通过使用补偿运动引起的剂量模糊的剂量学边缘来减少立体定向放射治疗中的 ITV。
我们研究了来自 10 名患者的 26 次重复 4DCT 扫描的肿瘤运动特征。这些被用来计算由于呼吸运动引起的时间平均剂量分布的剂量模糊，导致 80%等剂量线在头脚方向的移位。为了补偿剂量模糊，需要计算相对于 ITV 的剂量学边缘，使用 MIP 可以有效地确定 ITV。使用 MIP 和 Min-IP 确定峰峰值运动幅度。使用可编程呼吸运动体模研究了在确定实际运动模式的 ITV 时的成像伪影。在肺和水等效组织中计算和测量了剂量分布。
使用 80%剂量水平的边缘可以相对于使用 ITV 体积使用更小的靶体积，（i）由于不对称的呼吸运动模式，在吸气末边缘的减少比在呼气侧更大，（ii）与呼吸幅度呈线性关系。95%置信水平的 ITV 的平均减少量由 0.2×A(pp)-1.3mm 给出，其中 A(pp)是峰峰值呼吸幅度，在吸气侧为 0.3×A(pp)-2.2mm。对于 80%等剂量线，水等效和肺组织之间的剂量学边缘没有显著差异。
与呼吸幅度成线性关系的简单呼吸运动边缘配方可用于计算立体定向放射治疗肺肿瘤的 ITV 减少量。
