Bedford James L, Webb Steve
Joint Department of Physics, The Institute of Cancer Research and Royal Marsden NHS Foundation Trust, Downs Road, Sutton, Surrey, United Kingdom.
Med Phys. 2006 Apr;33(4):944-58. doi: 10.1118/1.2163832.
Previous studies have shown that, by optimizing segment shapes and weights directly, without explicitly optimizing fluence profiles, effective IMRT plans can be generated with fewer segments. This study proposes a method of direct-aperture optimization with aperture shape constraints, which is designed to provide segmental IMRT plans using a minimum of simple, regular segments. The method uses a cubic function to create smoothly curving multileaf collimator shapes. Constraints on segment dimension and equivalent square are applied, and each segment can be constrained to lie within the previous one, for easy generation of fluence profiles with a single maximum. To simply optimize the segment shapes and reject any shapes which violate the constraints is too inefficient, so an innovative method of feedback optimization is used to ensure in advance that viable aperture shapes are generated. The algorithm is demonstrated using a simple cylindrical phantom consisting of a hemi-annular planning target volume and a central cylindrical organ-at-risk. A simple IMRT rectum case is presented, where segments are used to replace a wedge. More complex cases of prostate and seminal vesicles and prostate and pelvic nodes are also shown. The algorithm produces effective plans in each case with three to five segments per beam. For the simple plans, the constraint that each segment should be contained within the previous one adds additional simplicity to the plan, for a small reduction in plan quality. This study confirms that direct-aperture optimization gives efficient solutions to the segmental IMRT inverse problem and provides a method for generating simple apertures. By using such a method, the workload of IMRT verification may be reduced and simplified, as verification of fluence profiles from individual beams may be eliminated.
先前的研究表明，通过直接优化射野形状和权重，而无需明确优化注量分布，就可以用更少的射野生成有效的调强放疗计划。本研究提出了一种具有射野形状约束的直接孔径优化方法，该方法旨在使用最少数量的简单、规则射野来提供分段调强放疗计划。该方法使用三次函数来创建平滑弯曲的多叶准直器形状。施加了对射野尺寸和等效方形的约束，并且每个射野可以被约束在前一个射野内，以便于生成具有单个最大值的注量分布。仅仅优化射野形状并拒绝任何违反约束的形状效率太低，因此使用一种创新的反馈优化方法来预先确保生成可行的孔径形状。使用一个简单的圆柱形体模来演示该算法，该体模由一个半环形的计划靶区和一个位于中心的圆柱形危及器官组成。给出了一个简单的调强放疗直肠病例，其中用射野来替代楔形板。还展示了前列腺和精囊以及前列腺和盆腔淋巴结等更复杂的病例。该算法在每种情况下都能产生有效的计划，每束射线有三到五个射野。对于简单计划，每个射野应包含在前一个射野内的约束为计划增加了额外的简单性，但计划质量略有下降。本研究证实，直接孔径优化为分段调强放疗逆问题提供了有效的解决方案，并提供了一种生成简单孔径的方法。通过使用这种方法，可以减少和简化调强放疗验证的工作量，因为可以省去对各个射野注量分布的验证。
