D'Souza W D, Meyer R R
Department of Medical Physics, University of Wisconsin, Madison, WI, USA.
Int J Radiat Oncol Biol Phys. 2001 Nov 15;51(4):1120-30. doi: 10.1016/s0360-3016(01)01802-8.
In conventional treatment planning for permanent I-125 prostate implants, it has been suggested that lower seed activities result in more homogeneous dose distributions and also less overdose of the critical structures. We sought to determine if this hypothesis holds by analyzing treatment plans constructed using an automated optimized approach.
We studied treatment plans for 10 patients using mixed-integer programming and the branch-and-bound method. Two mixed-integer models (that yielded somewhat different treatment plans) were developed: a "basic" model and a "dose homogeneity" model. For each resulting treatment plan, we examined dose homogeneity (by evaluating the dose non-uniformity ratio [DNR] and the full-width half-maximum [FWHM] of the differential dose-volume histogram [DVH]) as a function of three different source activities (0.35 mCi, 0.44 mCi, and 0.66 mCi). In addition, target coverage and critical structure dose distributions were evaluated. Plans using multiple source activities were also evaluated for resulting dose inhomogeneities.
The homogeneity model results in a more homogeneous dose distribution than the basic model. DNR is lowered by an average of 42% (standard deviation [SD] = 19%), 39% (SD = 21%), and 33% (SD = 21%) for the 0.35 mCi, 0.44 mCi, and 0.66 mCi seeds, respectively, when the homogeneity model is employed over the basic model. Corresponding average decreases in the FWHM of the DVH for 0.35 mCi, 0.44 mCi, and 0.66 mCi, respectively, are 29 Gy (SD = 28 Gy), 24 Gy (SD = 22 Gy), and 27 Gy (SD = 13 Gy). Seeds of 0.35 mCi and 0.44 mCi result in the lowest DNR and narrower FWHM of the DVH relative to 0.66 mCi seeds. In general, the 0.44 mCi seeds produce greater target coverage and require fewer seeds and needles than the 0.35 mCi seeds. Although 0.66 mCi seeds result in the greatest target coverage, they yield highest critical structure doses. They also yield solutions requiring the least number of seeds and needles. However, the dose distributions from 0.66 mCi seeds are highly inhomogeneous. Multiple source activities in the same treatment plan produce dose distributions that are comparable in homogeneity to 0.44 mCi seed implants.
Even when an optimization model that seeks to minimize dose inhomogeneity is employed, all factors involved in seed implants make 0.44 mCi the best activity choice in comparison with 0.35 mCi and 0.66 mCi.
在永久性碘 - 125前列腺植入物的传统治疗计划中，有人提出较低的籽源活度会导致更均匀的剂量分布，同时关键结构的过量照射也更少。我们试图通过分析使用自动优化方法构建的治疗计划来确定这一假设是否成立。
我们使用混合整数规划和分支定界法研究了10例患者的治疗计划。开发了两个混合整数模型（产生的治疗计划略有不同）：一个“基本”模型和一个“剂量均匀性”模型。对于每个生成的治疗计划，我们检查了剂量均匀性（通过评估剂量不均匀率[DNR]和微分剂量 - 体积直方图[DVH]的半高宽[FWHM]）作为三种不同源活度（0.35毫居里、0.44毫居里和0.66毫居里）的函数。此外，还评估了靶区覆盖情况和关键结构的剂量分布。还评估了使用多种源活度的计划所产生的剂量不均匀性。
与基本模型相比，均匀性模型产生的剂量分布更均匀。当使用均匀性模型而非基本模型时，对于0.35毫居里、0.44毫居里和0.66毫居里的籽源，DNR分别平均降低42%（标准差[SD]=19%）、39%（SD = 21%）和33%（SD = 21%）。对于0.35毫居里、0.44毫居里和0.66毫居里的籽源，DVH的FWHM相应的平均降低分别为29戈瑞（SD = 28戈瑞）、24戈瑞（SD = 22戈瑞）和27戈瑞（SD = 13戈瑞）。相对于0.66毫居里的籽源，0.35毫居里和0.44毫居里的籽源导致最低的DNR和更窄的DVH的FWHM。一般来说，0.44毫居里的籽源比0.35毫居里的籽源产生更大的靶区覆盖，并且所需的籽源和针更少。虽然0.66毫居里的籽源导致最大的靶区覆盖，但它们产生的关键结构剂量最高。它们还产生所需籽源和针数量最少的方案。然而，0.66毫居里籽源的剂量分布高度不均匀。同一治疗计划中的多种源活度产生的剂量分布在均匀性上与0.44毫居里籽源植入相当。
即使采用旨在最小化剂量不均匀性的优化模型，与0.35毫居里和0.66毫居里相比，籽源植入所涉及的所有因素使得0.44毫居里是最佳的活度选择。
