Hunt Margie A, Hsiung Ching-Yeh, Spirou Spirodon V, Chui Chen-Shou, Amols Howard I, Ling Clifton C
Department of Medical Physics, Memorial Sloan-Kettering Cancer Center, New York, NY 10021, USA.
Int J Radiat Oncol Biol Phys. 2002 Nov 1;54(3):953-62. doi: 10.1016/s0360-3016(02)03004-3.
To evaluate and develop optimum inverse treatment planning strategies for the treatment of concave targets adjacent to normal tissue structures.
Optimized dose distributions were designed using an idealized geometry consisting of a cylindrical phantom with a concave kidney-shaped target (PTV) and cylindrical normal tissues (NT) placed 5-13 mm from the target. Targets with radii of curvature from 1 to 2.75 cm were paired with normal tissues with radii between 0.5 and 2.25 cm. The target was constrained to a prescription dose of 100% and minimum and maximum doses of 95% and 105% with relative penalties of 25. Maximum dose constraint parameters for the NT varied from 10% to 70% with penalties from 10 to 1000. Plans were evaluated using the PTV uniformity index (PTV D(max)/PTV D(95)) and maximum normal tissue doses (NT D(max)/PTV D(95)).
In nearly all situations, the achievable PTV uniformity index and the maximum NT dose exceeded the corresponding constraints. This was particularly true for small PTV-NT separations (5-8 mm) or strict NT dose constraints (10%-30%), where the achievable doses differed from the requested by 30% or more. The same constraint parameters applied to different PTV-NT separations yielded different dose distributions. For most geometries, a range of constraints could be identified that would lead to acceptable plans. The optimization results were fairly independent of beam energy and radius of curvature, but improved as the number of beams increased, particularly for small PTV-NT separations or strict dose constraints.
Optimized dose distributions are strongly affected by both the constraint parameters and target-normal tissue geometry. Standard site-specific constraint templates can serve as a starting point for optimization, but the final constraints must be determined iteratively for individual patients. A strategy whereby NT constraints and penalties are modified until the highest acceptable PTV uniformity index is achieved is discussed. This strategy can be used, in simple patient geometries, to ensure the lowest possible normal tissue dose. Strategies for setting the optimum dose constraints and penalties may vary for different optimization algorithms and objective functions. Increasing the number of beams can significantly improve normal tissue dose and target uniformity in situations where the PTV-NT separation is small or the normal tissue dose limits are severe. Setting unrealistically severe constraints in such situations often results in dose distributions that are inferior to plans achieved with more lenient constraints.
评估并制定针对毗邻正常组织结构的凹陷靶区治疗的最佳逆向治疗计划策略。
使用理想化几何结构设计优化剂量分布，该结构由一个带有凹陷肾形靶区（计划靶体积，PTV）的圆柱形模体以及距离靶区5 - 13毫米放置的圆柱形正常组织（NT）组成。曲率半径为1至2.75厘米的靶区与半径在0.5至2.25厘米之间的正常组织配对。靶区被限制在处方剂量为100%，最小和最大剂量分别为95%和105%，相对惩罚因子为25。NT的最大剂量约束参数从10%变化到70%，惩罚因子从10到1000。使用PTV均匀性指数（PTV D(max)/PTV D(95)）和正常组织最大剂量（NT D(max)/PTV D(95)）评估计划。
在几乎所有情况下，可实现的PTV均匀性指数和正常组织最大剂量都超过了相应的约束。对于小的PTV - NT间距（5 - 8毫米）或严格的NT剂量约束（10% - 30%）尤其如此，此时可实现的剂量与要求的剂量相差30%或更多。应用于不同PTV - NT间距的相同约束参数产生不同的剂量分布。对于大多数几何结构，可以确定一系列能导致可接受计划的约束。优化结果在相当程度上与射束能量和曲率半径无关，但随着射束数量的增加而改善，特别是对于小的PTV - NT间距或严格的剂量约束。
优化的剂量分布受到约束参数和靶区 - 正常组织几何结构的强烈影响。标准的特定部位约束模板可作为优化的起点，但最终的约束必须针对个体患者进行迭代确定。讨论了一种策略，即修改NT约束和惩罚因子，直到达到最高可接受的PTV均匀性指数。在简单的患者几何结构中，这种策略可用于确保尽可能低的正常组织剂量。设置最佳剂量约束和惩罚因子的策略可能因不同的优化算法和目标函数而异。在PTV - NT间距小或正常组织剂量限制严格的情况下，增加射束数量可显著改善正常组织剂量和靶区均匀性。在这种情况下设置不切实际的严格约束通常会导致剂量分布比采用更宽松约束所实现的计划更差。
