Broderick Maria, Leech Michelle, Coffey Mary
Division of Radiation Therapy, School of Medicine, Trinity College Dublin, Ireland, UK.
Radiat Oncol. 2009 Feb 16;4:8. doi: 10.1186/1748-717X-4-8.
Intensity Modulated Radiation Therapy (IMRT) is a means of delivering radiation therapy where the intensity of the beam is varied within the treatment field. This is done by dividing a large beam into many small beamlets. Dose constraints are assigned to both the target and sensitive structures and computerised inverse optimization is performed to find the individual weights of this large number of beamlets. The computer adjusts the intensities of these beamlets according to the required planning dose objectives. The optimized intensity patterns are then decomposed into a series of deliverable multi leaf collimator (MLC) shapes in the sequencing step. One of the main problems of IMRT, which becomes even more apparent as the complexity of the IMRT plan increases, is the dramatic increase in the number of Monitor Units (MU) required to deliver a fractionated treatment. The difficulty with this increase in MU is its association with increased treatment times and a greater leakage of radiation from the MLCs increasing the total body dose and the risk of secondary cancers in patients. Therefore one attempts to find ways of reducing these MU without compromising plan quality. The design of inverse planning systems where the beam is divided into small beamlets to produce the required intensity map automatically introduces complexity into IMRT treatment planning. Plan complexity is associated with many negative factors such as dosimetric uncertainty and delivery issues A large search space is required necessitating much computing power. However, the limitations of the delivery technology are not taken into consideration when designing the ideal intensity map therefore a further step termed the sequencing step is required to convert the ideal intensity map into a deliverable one. Many approaches have been taken to reduce the complexity. These include setting intensity limits, putting penalties on the cost function and using smoothing filters Direct Aperture optimization (DAO) incorporates the limitations of the delivery technology at the initial design of the intensity map thereby eliminating the sequencing step. It also gives control over the number of segments and hence control over the complexity of the plan although the design of the segments is independent of the person preparing the plan.
调强放射治疗（IMRT）是一种实施放射治疗的方法，其中射束强度在治疗野内是变化的。这是通过将一个大射束分割成许多小射束单元来实现的。剂量约束被分配给靶区和敏感结构，并进行计算机化的逆向优化，以确定这大量射束单元各自的权重。计算机根据所需的计划剂量目标调整这些射束单元的强度。然后在排序步骤中，将优化后的强度模式分解为一系列可交付的多叶准直器（MLC）形状。IMRT的一个主要问题，随着IMRT计划复杂性的增加变得更加明显，就是在进行分次治疗时所需的监测单位（MU）数量急剧增加。MU增加带来的困难在于它与治疗时间增加以及来自MLC的辐射泄漏增加相关，从而增加了患者的全身剂量和继发癌症的风险。因此，人们试图找到在不影响计划质量的情况下减少这些MU的方法。逆向计划系统的设计是将射束分割成小射束单元以自动生成所需的强度图，这给IMRT治疗计划引入了复杂性。计划复杂性与许多负面因素相关，如剂量学不确定性和交付问题，需要一个大的搜索空间，因此需要大量计算能力。然而，在设计理想强度图时没有考虑到交付技术的局限性，所以需要进一步的步骤，即排序步骤，将理想强度图转换为可交付的强度图。已经采取了许多方法来降低复杂性。这些方法包括设置强度限制、对成本函数施加惩罚以及使用平滑滤波器。直接孔径优化（DAO）在强度图的初始设计中纳入了交付技术的局限性，从而消除了排序步骤。它还能控制段的数量，进而控制计划的复杂性，尽管段的设计与准备计划的人员无关。
