Department of Therapeutic Radiology, Yale University School of Medicine, New Haven, CT, USA.
Phys Med Biol. 2011 Aug 7;56(15):4895-912. doi: 10.1088/0031-9155/56/15/016. Epub 2011 Jul 19.
Previous studies have shown that procedure-induced prostate edema during permanent interstitial brachytherapy (PIB) can cause significant variations in the dose delivered to the prostate gland. Because the clinical impact of edema-induced dose variations strongly depends on the magnitude of the edema, the temporal pattern of its resolution and its interplay with the decay of radioactivity and the underlying biological processes of tumor cells (such as tumor potential doubling time), we investigated the impact of edema-induced dose variations on the tumor cell survival and tumor control probability after PIB with the (131)Cs, (125)I and (103)Pd sources used in current clinical practice. The exponential edema resolution model reported by Waterman et al (1998 Int. J. Radiat. Oncol. Biol. Phys. 41 1069-77) was used to characterize the edema evolutions previously observed during clinical PIB for prostate cancer. The concept of biologically effective dose, taking into account tumor cell proliferation and sublethal damage repair during dose delivery, was used to characterize the effects of prostate edema on cell survival and tumor control probability. Our calculation indicated that prostate edema, if not appropriately taken into account, can increase the cell survival and decrease the probability of local control of PIB. The magnitude of an edema-induced increase in cell survival increased with increasing edema severity, decreasing half-life of radioactive decay and decreasing photon energy emitted by the source. At the doses currently prescribed for PIB and for prostate cancer cells characterized by nominal radiobiology parameters recommended by AAPM TG-137, PIB using (125)I sources was less affected by edema than PIB using (131)Cs or (103)Pd sources due to the long radioactive decay half-life of (125)I. The effect of edema on PIB using (131)Cs or (103)Pd was similar. The effect of edema on (103)Pd PIB was slightly greater, even though the decay half-life of (103)Pd (17 days) is longer than that of (131)Cs (9.7 days), because the advantage of the longer (103)Pd decay half-life was negated by the lower effective energy of the photons it emits (∼21 keV compared to ∼30.4 keV for (131)Cs). In addition, the impact of edema could be reduced or enhanced by differences in the tumor characteristics (e.g. potential tumor doubling time or the α/β ratio), and the effect of these factors varied for the different radioactive sources. There is a clear need to consider the effects of prostate edema during the planning and evaluation of permanent interstitial brachytherapy treatments for prostate cancer.
先前的研究表明，永久性间质近距离放射治疗（PIB）过程中引起的前列腺水肿会导致前列腺内的剂量分布发生显著变化。由于水肿引起的剂量变化对肿瘤细胞存活和肿瘤控制概率的临床影响程度取决于水肿的严重程度、其消退的时间模式及其与放射性衰减和肿瘤细胞潜在倍增时间等生物学过程的相互作用，我们使用当前临床实践中使用的（131）Cs、（125）I 和（103）Pd 源研究了 PIB 过程中引起的水肿剂量变化对肿瘤细胞存活和肿瘤控制概率的影响。Waterman 等人报告的指数性水肿消退模型（1998 年，《国际放射肿瘤学、生物学和物理学杂志》，第 41 卷，第 1069-1077 页）用于描述先前在前列腺癌的临床 PIB 中观察到的水肿演变过程。我们使用了生物有效剂量的概念，该概念考虑了在剂量输送过程中肿瘤细胞的增殖和亚致死损伤修复，用于描述前列腺水肿对细胞存活和肿瘤控制概率的影响。我们的计算结果表明，如果不适当考虑前列腺水肿，可能会增加细胞存活并降低 PIB 的局部控制概率。细胞存活的水肿诱导增加量随水肿严重程度的增加、放射性衰变半衰期的减少和源发射的光子能量的减少而增加。在当前为 PIB 规定的剂量以及 AAPM TG-137 推荐的标称放射生物学参数表征的前列腺癌细胞中，由于（125）I 的放射性衰变半衰期较长，因此使用（125）I 源进行 PIB 比使用（131）Cs 或（103）Pd 源受水肿的影响更小。（131）Cs 或（103）Pd 源引起的水肿对 PIB 的影响相似。（103）Pd PIB 的水肿影响略大，尽管（103）Pd 的衰变半衰期（17 天）长于（131）Cs（9.7 天），但由于其发射的光子的有效能量较低（与（131）Cs 的 30.4keV 相比约为 21keV），这种优势被抵消了。此外，由于肿瘤特征（例如潜在肿瘤倍增时间或α/β 比）的差异，水肿的影响可能会降低或增强，这些因素的影响因不同的放射性源而异。在规划和评估前列腺癌的永久性间质近距离放射治疗治疗时，显然需要考虑前列腺水肿的影响。
