Wang Jian Z, Mayr Nina A, Nag Subir, Montebello Joseph, Gupta Nilendu, Samsami Nina, Kanellitsas Christos
Department of Radiation Medicine, The Ohio State University, Columbus, Ohio 43210, USA.
Med Phys. 2006 Apr;33(4):1025-32. doi: 10.1118/1.2181294.
Many factors influence response in low-dose-rate (LDR) brachytherapy of prostate cancer. Among them, edema, relative biological effectiveness (RBE), and dose heterogeneity have not been fully modeled previously. In this work, the generalized linear-quadratic (LQ) model, extended to account for the effects of edema, RBE, and dose heterogeneity, was used to assess these factors and their combination effect. Published clinical data have shown that prostate edema after seed implant has a magnitude (ratio of post- to preimplant volume) of 1.3-2.0 and resolves exponentially with a half-life of 4-25 days over the duration of the implant dose delivery. Based on these parameters and a representative dose-volume histogram (DVH), we investigated the influence of edema on the implant dose distribution. The LQ parameters (alpha=0.15 Gy(-1) and alpha/beta=3.1 Gy) determined in earlier studies were used to calculate the equivalent uniform dose in 2 Gy fractions (EUD2) with respect to three effects: edema, RBE, and dose heterogeneity for 125I and 103Pd implants. The EUD2 analysis shows a negative effect of edema and dose heterogeneity on tumor cell killing because the prostate edema degrades the dose coverage to tumor target. For the representative DVH, the V100 (volume covered by 100% of prescription dose) decreases from 93% to 91% and 86%, and the D90 (dose covering 90% of target volume) decrease from 107% to 102% and 94% of prescription dose for 125I and 103Pd implants, respectively. Conversely, the RBE effect of LDR brachytherapy [versus external-beam radiotherapy (EBRT) and high-dose-rate (HDR) brachytherapy] enhances dose effect on tumor cell kill. In order to balance the negative effects of edema and dose heterogeneity, the RBE of prostate brachytherapy was determined to be approximately 1.2-1.4 for 125I and 1.3-1.6 for 103Pd implants. These RBE values are consistent with the RBE data published in the literature. These results may explain why in earlier modeling studies, when the effects of edema, dose heterogeneity, and RBE were all ignored simultaneously, prostate LDR brachytherapy was reported to show an overall similar dose effect as EBRT and HDR brachytherapy, which are independent of edema and RBE effects and have a better dose coverage.
许多因素影响前列腺癌低剂量率（LDR）近距离放射治疗的反应。其中，水肿、相对生物效应（RBE）和剂量异质性以前尚未得到充分建模。在这项工作中，扩展后的广义线性二次（LQ）模型用于考虑水肿、RBE和剂量异质性的影响，并评估这些因素及其联合效应。已发表的临床数据表明，种子植入后前列腺水肿程度（植入后与植入前体积之比）为1.3 - 2.0，在植入剂量递送期间以4 - 25天的半衰期呈指数消退。基于这些参数和代表性剂量体积直方图（DVH），我们研究了水肿对植入剂量分布的影响。早期研究中确定的LQ参数（α = 0.15 Gy⁻¹和α/β = 3.1 Gy）用于计算相对于三种效应（水肿、RBE和剂量异质性）的2 Gy分次等效均匀剂量（EUD2），用于¹²⁵I和¹⁰³Pd植入。EUD2分析表明水肿和剂量异质性对肿瘤细胞杀伤有负面影响，因为前列腺水肿会降低对肿瘤靶区的剂量覆盖。对于代表性DVH，¹²⁵I和¹⁰³Pd植入的V100（100%处方剂量覆盖的体积）分别从93%降至91%和86%，D90（覆盖90%靶区体积的剂量）分别从处方剂量的107%降至102%和94%。相反，LDR近距离放射治疗的RBE效应[与外照射放疗（EBRT）和高剂量率（HDR）近距离放射治疗相比]增强了对肿瘤细胞杀伤的剂量效应。为了平衡水肿和剂量异质性的负面影响，¹²⁵I植入的前列腺近距离放射治疗的RBE确定为约1.2 - 1.4，¹⁰³Pd植入的为1.3 - 1.6。这些RBE值与文献中发表的RBE数据一致。这些结果可能解释了为什么在早期的建模研究中，当同时忽略水肿、剂量异质性和RBE的影响时，前列腺LDR近距离放射治疗据报道显示出与EBRT和HDR近距离放射治疗总体相似的剂量效应，而后两者不受水肿和RBE影响且具有更好的剂量覆盖。
