Major T, Polgár C, Somogyi A, Németh G
Radiotherapy Department, National Institute of Oncology, 7-9 Rath Gy. u., H-1122, Budapest, Hungary.
Radiother Oncol. 2000 Mar;54(3):213-20. doi: 10.1016/s0167-8140(99)00170-x.
To investigate the influence of dwell time optimizations on dose uniformity characterized by dose values in dose points and dose non-uniformity ratio (DNR) and to analyze which implant parameters have influence on the DNR.
Double-plane breast implants with catheters arranged in triangular pattern were used for the calculations. At a typical breast implant, dose values in dose reference points inside the target volume and volumes enclosed by given isodose surfaces were calculated and compared for non-optimized and optimized implants. The same 6-cm treatment length was used for the comparisons. Using different optimizations plots of dose non-uniformity ratio as a function of catheter separation, source step size, number of catheters, length of active sections were drawn and the minimum DNR values were determined.
Optimization resulted in less variation in dose values over dose points through the whole volume and in the central plane only compared to the non-optimized case. At implant configurations consisting of seven catheters with 15-mm separation, 5-mm source step size and various active lengths adapted according to the type of optimization, the no optimization, geometrical (volume mode) and dose point (on dose points and geometry) optimization resulted in similar treatment volumes, but an increased high dose volume was observed due to the optimization. The dose non-uniformity ratio always had the minimum at average dose over dose normalization points, defined in the midpoints between the catheters through the implant volume. The minimum value of DNR depended on catheter separation, source step size, active length and number of catheters. The optimization had only a small influence on DNR.
In addition to the reference points in the central plane only, dose points positioned in the whole implant volume can be used for evaluating the dose uniformity of interstitial implants. The dose optimization increases not only the dose uniformity within the implant but also the high dose volume. The optimization on dose points and geometry provides the most uniform dose distribution. The dose non-uniformity ratio can be minimized by selecting the isodose line of the midpoints between the catheters in the whole volume for the dose prescription, but the dose coverage may not be adequate. For a clinically acceptable plan, a compromise should be made between dose non-uniformity and coverage.
研究驻留时间优化对以剂量点处剂量值和剂量不均匀率（DNR）表征的剂量均匀性的影响，并分析哪些植入参数对DNR有影响。
采用导管呈三角形排列的双平面乳腺植入物进行计算。对于典型的乳腺植入物，计算并比较了未优化和优化后的植入物在靶体积内剂量参考点以及给定等剂量面包围的体积内的剂量值。比较时使用相同的6厘米治疗长度。通过绘制不同优化情况下剂量不均匀率随导管间距、源步长、导管数量、活性段长度的变化图，确定最小DNR值。
与未优化情况相比，优化后整个体积以及仅中央平面内剂量点处的剂量值变化更小。在由七根导管组成的植入配置中，导管间距为15毫米，源步长为5毫米，根据优化类型调整了不同的活性长度，未优化、几何（体积模式）和剂量点（基于剂量点和几何形状）优化导致的治疗体积相似，但优化后观察到高剂量体积增加。剂量不均匀率在通过植入物体积的导管中点定义的剂量归一化点的平均剂量处总是最小。DNR的最小值取决于导管间距、源步长、活性长度和导管数量。优化对DNR的影响较小。
除了仅中央平面内的参考点外，整个植入物体积内的剂量点也可用于评估组织间植入物的剂量均匀性。剂量优化不仅增加了植入物内的剂量均匀性，还增加了高剂量体积。基于剂量点和几何形状的优化提供了最均匀的剂量分布。通过选择整个体积内导管中点的等剂量线进行剂量处方，可使剂量不均匀率最小化，但剂量覆盖可能不足。对于临床可接受的计划，应在剂量不均匀性和覆盖范围之间进行权衡。
