Yu Y, Waterman F M, Suntharalingam N, Schulsinger A
Department of Radiation Oncology, Thomas Jefferson University Hospital , Philadelphia, PA 19107-5097, USA.
Int J Radiat Oncol Biol Phys. 1996 Feb 1;34(3):717-25. doi: 10.1016/0360-3016(95)02153-1.
The objective of this work is to investigate whether the minimum peripheral dose is a practical parameter for dose specification in permanent 125I implants of the prostate.
The investigation was carried out by use of a computer model of ellipsoidal 125I implants in which the average dimension and elongation factor were varied to provide a wide range of geometries. Both ideal and nonideal implants were investigated. The 125I seeds were confined to the target volume except for a portion of the study in which the effect of placing seeds outside the target volume was investigated.
The minimum peripheral dose was found to be very sensitive to the seed placement. The irregularities in the seed spacing that inevitably occur in actual implants tend to lower the minimum peripheral dose. As a result, the minimum peripheral dose is generally significantly less than planned by an amount that is unpredictable, and often exceeds 25%. However, the percentage of the target volume that receives a dose less that the prescribed minimum peripheral dose is generally less than 10%. Implanting seeds outside the target volume improves the dose uniformity, but does not appear to offer any advantage in dose coverage, and increases the volume of normal tissue irradiated.
If a minimum peripheral dose is prescribed for a permanent 125I prostate implant, and the implant is planned using an idealized implant having precisely spaced seeds, the prescribed dose will rarely, if ever, be achieved. Reasonable agreement with the prescribed dose can be achieved only if the requirement for coverage is relaxed from 100 to 90%, or if the total source strength is increased by 20% to compensate for the anticipated imperfections in seed placement.
本研究的目的是探讨最小周边剂量是否是前列腺永久性¹²⁵I植入剂量规范的一个实用参数。
利用椭圆形¹²⁵I植入的计算机模型进行研究，其中平均尺寸和伸长因子有所变化以提供广泛的几何形状。研究了理想和非理想植入。¹²⁵I种子被限制在靶区内，除了一部分研究是调查将种子置于靶区外的影响。
发现最小周边剂量对种子植入位置非常敏感。实际植入中不可避免出现的种子间距不规则往往会降低最小周边剂量。结果，最小周边剂量通常比计划值显著低一个不可预测的量，且经常超过25%。然而，接受低于规定最小周边剂量的靶区体积百分比通常小于10%。在靶区外植入种子可改善剂量均匀性，但在剂量覆盖方面似乎没有任何优势，并且会增加受照射正常组织的体积。
如果为前列腺永久性¹²⁵I植入规定了最小周边剂量，并且使用种子精确间隔的理想化植入来进行计划，规定剂量很少（如果有的话）能够实现。只有将覆盖要求从100%放宽到90%，或者将总源强增加20%以补偿预期的种子植入缺陷，才能实现与规定剂量的合理一致。
