Yue N, Dicker A P, Corn B W, Nath R, Waterman F M
Department of Therapeutic Radiology, Yale University School of Medicine, New Haven, CT 06510, USA.
Int J Radiat Oncol Biol Phys. 1999 Jan 15;43(2):447-54. doi: 10.1016/s0360-3016(98)00394-0.
PURPOSE/OBJECTIVE: The dosimetric evaluation of permanent 125I or 103Pd prostate implant is based on the assumption that both prostate and seeds are static throughout the entire treatment time which lasts months. However, the prostate is often edematous after the surgical implantation of seeds. Therefore, both the volume of the prostate and the seed locations change dynamically as the edema resolves. This effect has impact on the validity of postimplant analysis based upon a CT scan. If a CT scan is taken too early after implantation while there is edema in the prostate, the dose delivered by the implant may be underestimated. If the imaging is delayed too long, the dose may be overestimated. The magnitude of this effect depends on both of the half-life of the isotope used and the half-life and magnitude of the edema. This study describes a dynamic biomathematical model which takes edema into account in calculating the dose delivered by the implant and is used to investigate the optimum time to obtain the postimplant CT scan.
The dynamic biomathematical model is a numerical integration of the accumulated dose in which the prostate dimensions, the seed locations, and the source strength are all functions of time. The function which describes the change in prostate dimensions and seed locations as a function of time was determined in a separate study by analysis of serial postimplant CT scans. Dose-volume histograms (DVH) of the prostate for the total dose generated by the dynamic model are compared to DVHs generated by CT scans simulated for postimplant intervals ranging from 0 to 300 days after the implantation for 30 different combinations of the magnitude and duration of edema.
DVHs of the prostate calculated by taking edema into account show that the time of obtaining a CT scan for postimplant analysis is critical to the accuracy of dose evaluations. The comparison of the DVHs generated by the dynamic model to those generated by the CT scans simulated for a range of postimplant intervals show that obtaining the CT scan too early tends to underestimate the total dose while obtaining the CT scan after the edema is resolved tends to overestimate it. The results show that the optimum timing of the CT scan depends upon the duration of the edema and the half-life of the radioisotope used. It is almost independent of the magnitude of the edema. Thus, a unique optimum time window for the imaging study cannot be defined for either 125I or 103Pd implants. However, an optimum time window can be identified for which the calculated dose, on the average, will generally differ from the actual dose by less than 5%, with a maximum error not exceeding 15%. Such a window is 4 to 10 weeks after the implantation for an 125I implant, and 2 to 4 weeks for a 103Pd implant.
A dynamic biomathematical model to correct for the effects of edema in calculating the total dose delivered by an 125I or 103Pd seed implant has been developed. The model has been used to investigate the optimum time window during which the postimplant CT scans for analysis should be obtained.
目的/目标：永久性¹²⁵I或¹⁰³Pd前列腺植入的剂量学评估基于这样一种假设，即在持续数月的整个治疗期间，前列腺和植入粒子都是静止的。然而，在植入粒子的手术后，前列腺常常会出现水肿。因此，随着水肿消退，前列腺的体积和粒子位置都会动态变化。这种效应会影响基于CT扫描的植入后分析的有效性。如果在植入后不久，前列腺仍有水肿时就进行CT扫描，植入所给予的剂量可能会被低估。如果成像延迟过长，剂量可能会被高估。这种效应的大小取决于所使用同位素的半衰期以及水肿的半衰期和程度。本研究描述了一种动态生物数学模型，该模型在计算植入所给予的剂量时考虑了水肿，并用于研究获取植入后CT扫描的最佳时间。
动态生物数学模型是累积剂量的数值积分，其中前列腺尺寸、粒子位置和源强都是时间的函数。通过分析一系列植入后的CT扫描，在另一项研究中确定了描述前列腺尺寸和粒子位置随时间变化的函数。将动态模型生成的前列腺总剂量的剂量体积直方图（DVH）与针对植入后0至300天的不同植入后间隔模拟的CT扫描生成的DVH进行比较，水肿程度和持续时间有30种不同组合。
考虑水肿因素计算得到的前列腺DVH表明，获取植入后分析的CT扫描时间对于剂量评估的准确性至关重要。将动态模型生成的DVH与针对一系列植入后间隔模拟的CT扫描生成的DVH进行比较表明，过早获取CT扫描往往会低估总剂量，而在水肿消退后获取CT扫描则往往会高估总剂量。结果表明，CT扫描的最佳时机取决于水肿的持续时间和所使用放射性同位素的半衰期。它几乎与水肿的程度无关。因此，对于¹²⁵I或¹⁰³Pd植入物，无法定义一个独特的最佳成像研究时间窗口。然而，可以确定一个最佳时间窗口，在此窗口内计算得到的剂量平均而言通常与实际剂量的差异小于5%，最大误差不超过15%。对于¹²⁵I植入物，这样的窗口是植入后4至10周，对于¹⁰³Pd植入物是2至4周。
已开发出一种动态生物数学模型，用于校正¹²⁵I或¹⁰³Pd粒子植入所给予的总剂量计算中水肿的影响。该模型已用于研究获取用于分析的植入后CT扫描的最佳时间窗口。
