Klein E E, Low D A, Meigooni A S, Purdy J A
Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, MO 63108.
Int J Radiat Oncol Biol Phys. 1995 Feb 1;31(3):583-92. doi: 10.1016/0360-3016(94)00369-V.
Wedge-shaped isodoses are desired in a number of clinical situations. Physical wedge filters have provided nominal angled isodoses with dosimetric consequences of beam hardening, increased peripheral dosing, nonidealized gradients at deep depths, along with the practical consequences of filter handling and placement problems. Dynamic wedging uses a combination of a moving jaw and changing dose rate to achieve angled isodoses. The clinical implementation of dynamic wedge and an accompanying quality assurance program are discussed in detail.
The accelerator at our facility has two photon energies (6 MV and 18 MV), currently with dynamic wedge angles of 15 degrees, 30 degrees, 45 degrees, and 60 degrees. The segmented treatment tables (STT) that drive the jaw in concert with a changing dose rate are unique for field sizes ranging from 4.0 cm to 20.0 cm in 0.5 cm steps, resulting in 256 STTs. Transmission wedge factors were measured for each STT with an ion chamber. Isodose profiles were accumulated with film after dose conversion. For treatment-planning purposes, dmax orthogonal dose profiles were measured for open and dynamic fields. Physical filters were assigned empirically via the ratio of open and wedge profiles.
A nonlinear relationship with wedge factor and field size was found. The factors were found to be independent of the stationary field setting or second order blocking. Dynamic wedging provided more consistent gradients across the field compared with physical filters. Percent depth doses were found to be closer to open field. The created physical filters provided planned isodoses that closely resembled measured isodoses. Comparative isodose plans show improvement with dynamic wedging.
Dynamic wedging has practical and dosimetric advantages over physical filters. Table collisions with physical filters are alleviated. Treatment planning has been solved with an empirical solution. Dynamic wedge is a positive replacement for physical filters, and a first step for commercial introduction of dynamic conformal therapy.
在许多临床情况下都需要楔形等剂量线。物理楔形滤过器能提供标称角度的等剂量线，但存在束硬化、周边剂量增加、深部非理想梯度等剂量学后果，以及滤过器操作和放置问题等实际后果。动态楔形技术利用移动准直器和改变剂量率的组合来实现角度等剂量线。本文详细讨论了动态楔形技术的临床应用及相关质量保证程序。
我们机构的加速器有两种光子能量（6兆伏和18兆伏），目前动态楔形角度为15度、30度、45度和60度。与剂量率变化协同驱动准直器的分段治疗床（STT）对于0.5厘米步长、范围从4.0厘米到20.0厘米的射野尺寸是独特的，共产生256个STT。用离子室测量每个STT的透射楔形因子。剂量转换后用胶片累积等剂量曲线。为进行治疗计划，测量了开放野和动态野的dmax正交剂量曲线。通过开放野和楔形野曲线的比值凭经验确定物理滤过器。
发现楔形因子与射野尺寸呈非线性关系。这些因子与固定野设置或二级挡块无关。与物理滤过器相比，动态楔形技术在整个射野上提供了更一致的梯度。百分深度剂量更接近开放野。生成的物理滤过器提供的计划等剂量线与测量的等剂量线非常相似。对比等剂量计划显示动态楔形技术有所改进。
与物理滤过器相比，动态楔形技术具有实际和剂量学优势。避免了治疗床与物理滤过器的碰撞。凭经验解决了治疗计划问题。动态楔形技术是物理滤过器的理想替代品，也是动态适形治疗商业推广的第一步。
