The effect of optimization on surface dose in intensity modulated radiotherapy (IMRT)
The effect of optimization on surface dose in intensity modulated radiotherapy (IMRT)
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DOI：
10.1088/0031-9155/49/21/005
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发表时间：
2004-11-07
影响因子：
3.5
通讯作者：
Hoole, ACF
中科院分区：
文献类型：
--
作者：
Thomas, SJ;Hoole, ACF
尽管调强放射治疗已在临床上被证明会增加一些患者的皮肤剂量，但也已表明调强放疗本身不会增加皮肤剂量。造成这种明显差异的原因是，逆向规划可能会产生对皮肤表面附近的切向光束段给予高通量的解决方案，以试图对抗堆积区域。如果临床靶区 (CTV) 停止于皮肤表面附近，但计划靶区 (PTV) 未到达皮肤表面，则没有临床理由需要治疗皮肤。 CTV-PTV 裕度的存在纯粹是为了确保场足够大以允许几何不确定性。通过基于 PTV 剂量的目标函数，向皮肤提供过量注量的计划可能具有较低的目标函数，因此在优化中是优选的。我们描述了计划评估的技术，该技术基于通过重新计算多个计划来对计划进行分析，其中等中心已偏移了等于 CTV-PTV 边距的距离。我们证明，当考虑到系统和随机设置误差时，减少基于 PTV 的目标的计划变更可能会给 CTV 带来更差的剂量分布，并增加皮肤剂量。讨论了避免该问题的几种可能策略，包括使用皮肤作为危险器官、修改 PTV 以避免皮肤，以及在计划中而不是在治疗中使用“假装推注”。后者给出了最好的结果。讨论了使用评估方法本身作为优化目标函数的基础的可能性。
Although IMRT has been shown clinically to increase skin doses for some patients, it has also been shown that intensity modulated delivery does not, of itself, increase skin doses. The reason for this apparent difference is that inverse planning can result in solutions that give high fluence to tangential beam segments near the skin surface, in an attempt to counter the build-up region. In cases where the clinical target volume (CTV) stops short of the skin surface, but the planning target volume (PTV) does not, there is no clinical reason to treat the skin. The CTV-PTV margin exists purely to ensure that fields are large enough to allow for geometrical uncertainties. With an objective function based on the doses to the PTV, it is possible for a plan that gives excess fluence to the skin to have a lower objective function, and hence to be preferred in an optimization. We describe I technique of plan evaluation, based on analysis of a plan by recalculating several plans in which the isocentre has been offset by a distance equal to the CTV-PTV margin. We demonstrate that changes to a plan that reduce a PTV-based objective can give a worse dose distribution to the CTV when systematic and random set-up errors are accounted for, and increase skin dose. Several possible strategies for avoiding this problem are discussed, including the use of the skin as an organ at risk, modification of the PTV to avoid the skin, and the use of 'pretend bolus' applied in planning but not in treatment. The latter gave the best results. The possibility of using the evaluation method itself, as the basis of an objective function for optimization, is discussed.