Grønborg Caroline, Vestergaard Anne, Høyer Morten, Söhn Matthias, Pedersen Erik M, Petersen Jørgen B, Agerbæk Mads, Muren Ludvig P
a Department of Medical Physics , Aarhus University/Aarhus University Hospital , Aarhus , Denmark.
b Department of Oncology , Aarhus University/Aarhus University Hospital , Aarhus , Denmark.
Acta Oncol. 2015;54(9):1461-6. doi: 10.3109/0284186X.2015.1062138. Epub 2015 Aug 27.
The bladder is a tumour site well suited for adaptive radiotherapy (ART) due to large inter-fractional changes, but it also displays considerable intra-fractional motion. The aim of this study was to assess target coverage with a clinically applied method for plan selection ART and to estimate population-based and patient-specific intra-fractional margins, also relevant for a future re-optimisation strategy.
Nine patients treated in a clinical phase II ART trial of daily plan selection for bladder cancer were included. In the library plans, 5 mm isotropic margins were added to account for intra-fractional changes. Pre-treatment and weekly repeat magnetic resonance imaging (MRI) series were acquired in which a full three-dimensional (3D) volume was scanned every second min for 10 min (a total of 366 scans in 61 series). Initially, the bladder clinical target volume (CTV) was delineated in all scans. The t = 0 min scan was then rigidly registered to the planning computed tomography (CT) and plan selections were simulated using the CTV_0 (at t = 0 min). To assess intra-fractional motion, coverage of the CTV_10 (at t = 10 min) was quantified using the applied PTV. Population-based margins were calculated using the van Herk margin recipe while patient-specific margins were calculated using a linear model.
For 49% of the cases, the CTV_10 extended more than 5 mm outside the CTV_0. However, in 58 of the 61 cases (97%) CTV_10 was covered by the selected PTV. Population-based margins of 14 mm Sup/Ant, 9 mm Post and 5 mm Inf/Lat were sufficient to cover the bladder. Using patient-specific margins, the overlap between PTV and bowel-cavity was reduced from 137 cm(3) with the plan selection strategy to 24 cm(3).
In this phase II ART trial, 5 mm isotropic margin for intra-fractional motion was sufficient even though considerable intra-fractional motion was observed. In online re-optimised ART, population-based margin can be applied although patient-specific margins are preferable.
膀胱是一个非常适合自适应放疗（ART）的肿瘤部位，因为其分次间变化较大，但它也表现出相当大的分次内运动。本研究的目的是评估一种临床应用的计划选择ART方法对靶区的覆盖情况，并估计基于人群和患者特异性的分次内边界，这对于未来的重新优化策略也具有相关性。
纳入了9例在膀胱癌每日计划选择的临床II期ART试验中接受治疗的患者。在库计划中，添加了5mm的各向同性边界以考虑分次内变化。在治疗前和每周重复进行磁共振成像（MRI）系列检查，其中每秒扫描一个完整的三维（3D）体积，持续10分钟（61个系列中总共366次扫描）。最初，在所有扫描中勾画膀胱临床靶区（CTV）。然后将t = 0分钟的扫描与计划计算机断层扫描（CT）进行刚性配准，并使用CTV_0（在t = 0分钟时）模拟计划选择。为了评估分次内运动，使用应用的计划靶区（PTV）对CTV_10（在t = 10分钟时）的覆盖情况进行量化。基于人群的边界使用范·赫克边界公式计算，而患者特异性边界使用线性模型计算。
在49%的病例中，CTV_10在CTV_0之外扩展超过5mm。然而，在61例中的58例（97%）中，CTV_10被选定的PTV覆盖。基于人群的上/前边界为14mm、后边界为9mm、下/侧边界为5mm足以覆盖膀胱。使用患者特异性边界，PTV与肠腔之间的重叠从计划选择策略时的137cm³减少到24cm³。
在这项II期ART试验中，尽管观察到相当大的分次内运动，但5mm的各向同性分次内运动边界就足够了。在在线重新优化的ART中，可以应用基于人群的边界，尽管患者特异性边界更可取。
