Balvert Marleen, Gorissen Bram L, den Hertog Dick, Hoffmann Aswin L
Department of Econometrics and Operations Research/Center for Economic Research (CentER), Tilburg University, PO Box 90153, 5000 LE Tilburg, The Netherlands.
Phys Med Biol. 2015 Jan 21;60(2):537-48. doi: 10.1088/0031-9155/60/2/537. Epub 2014 Dec 30.
Inverse planning algorithms for dwell time optimisation in interstitial high-dose-rate (HDR) brachytherapy may produce solutions with large dwell time variations within catheters, which may result in undesirable selective high-dose subvolumes. Extending the dwell time optimisation model with a dwell time modulation restriction (DTMR) that limits dwell time differences between neighboring dwell positions has been suggested to eliminate this problem. DTMRs may additionally reduce the sensitivity for uncertainties in dwell positions that inevitably result from catheter reconstruction errors and afterloader source positioning inaccuracies. This study quantifies the reduction of high-dose subvolumes and the robustness against these uncertainties by applying a DTMR to template-based prostate HDR brachytherapy implants. Three different DTMRs were consecutively applied to a linear dose-based penalty model (LD) and a dose-volume based model (LDV), both obtained from literature. The models were solved with DTMR levels ranging from no restriction to uniform dwell times within catheters in discrete steps. Uncertainties were simulated on clinical cases using in-house developed software, and dose-volume metrics were calculated in each simulation. For the assessment of high-dose subvolumes, the dose homogeneity index (DHI) and the contiguous dose volume histogram were analysed. Robustness was measured by the improvement of the lowest D90% of the planning target volume (PTV) observed in the simulations. For (LD), a DTMR yields an increase in DHI of approximately 30% and reduces the size of the largest high-dose volume by 2-5 cc. However, this comes at a cost of a reduction in D90% of the PTV of 10%, which often implies that it drops below the desired minimum of 100%. For (LDV), none of the DTMRs were able to improve high-dose volume measures. DTMRs were not capable of improving robustness of PTV D90% against uncertainty in dwell positions for both models.
间质高剂量率（HDR）近距离放射治疗中用于驻留时间优化的逆向计划算法可能会产生导管内驻留时间变化较大的解决方案，这可能会导致出现不良的选择性高剂量子体积。有人建议通过扩展驻留时间优化模型，加入限制相邻驻留位置之间驻留时间差异的驻留时间调制限制（DTMR）来消除这个问题。DTMR还可以降低因导管重建误差和后装源定位不准确而不可避免产生的驻留位置不确定性的敏感性。本研究通过将DTMR应用于基于模板的前列腺HDR近距离放射治疗植入，量化了高剂量子体积的减少以及对这些不确定性的稳健性。三种不同的DTMR连续应用于从文献中获取的基于线性剂量的惩罚模型（LD）和基于剂量体积的模型（LDV）。这些模型在从无限制到导管内驻留时间均匀分布的不同DTMR水平下，以离散步骤求解。使用内部开发的软件在临床病例上模拟不确定性，并在每次模拟中计算剂量体积指标。为了评估高剂量子体积，分析了剂量均匀性指数（DHI）和连续剂量体积直方图。通过模拟中观察到的计划靶体积（PTV）最低D90%的改善来衡量稳健性。对于（LD），DTMR使DHI增加约30%，并将最大高剂量体积的大小减少2 - 5立方厘米。然而，这是以PTV的D90%降低10%为代价的，这通常意味着它降至所需的最低值100%以下。对于（LDV），没有一个DTMR能够改善高剂量体积测量。对于这两个模型，DTMR都无法提高PTV D90%对驻留位置不确定性的稳健性。
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