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(PP191) Effect of edema, relative biological effectiveness and dose heterogeneity on prostate brachytherapy.
Mayr, Nina1, Nag, Subir1, Montebello, Joseph1, Gupta, Nilendu1, Kanellitsas, Christos1, Samsami, Nina1, Wang, Jian1, 1 Department of Radiation Medicine, Columbus, OH, USA
ABSTRACT- Purpose/Objective: To evaluate the combined effects of edema, relative biological effectiveness (RBE) and dose heterogeneity on modeling prostate cancer radiobiologically. Materials/Methods: The generalized linear-quadratic (LQ) model, extended to account for the effects of edema, RBE and dose heterogeneity, was employed to calculate tumor cell killing in permanent prostate brachytherapy. Earlier clinical studies have shown that prostate edema after seed implant has a magnitude (ratio of post- to pre-implant volume) of 1.3-2.0 and resolves exponentially with a half-life of 4-25 days over the implant period (1). These parameters and a representative dose-volume histogram (DVH) were used to study the influence of edema on the implant dose distribution. Based on the LQ parameters ( = 0.15 Gy-1, / = 3.1 Gy and repair half-time of 16 min.) determined in earlier studies (2), the equivalent uniform dose in 2 Gy fractions (EUD2) was calculated and changes in EUD2 were evaluated with respect to three effects: edema, RBE and dose heterogeneity for 125I and 103Pd implants. Results: The EUD2 analysis shows a negative effect of edema and dose heterogeneity on the tumor cell killing (see the dotted curves in the Figure). The prostate edema degrades the dose coverage to the tumor target significantly. For the representative DVH, the V100 (volume covered by 100% of prescription dose) is moved from 93% to 90% and 80%, and the D90 (dose covering 90% of target volume) from 107% to 100% and 88% of prescription dose for 125I and 103Pd implants, respectively. Conversely, as the dashed curves shown in the Figure, the RBE effect of permanent brachytherapy [vs. external-beam radiotherapy (EBRT) and high-dose-rate (HDR) brachytherapy] enhances tumor cell killing. The solid curves in the Figure show the combined effects of edema and RBE to achieve a target EUD2 of 70.5 Gy for 125I implant [see poster] and 73.1 Gy for 103Pd implant [see poster]. To balance the negative effects of edema and dose heterogeneity, the RBE of prostate brachytherapy was found to be approximately 1.2-1.4 for 125I and 1.4-1.7 for 103Pd implants. These results are consistent with the RBE values published in the literature. Conclusions: Edema and dose heterogeneity versus RBE have counteractive effects on permanent prostate brachytherapy. The results presented in this paper may explain why earlier studies of prostate modeling for permanent brachytherapy when the effects of edema, dose heterogeneity and RBE are ignored simultaneously, report the same overall dose effectiveness as found in clinical data from EBRT and HDR brachytherapy. 1. Waterman FM, Yue N, Corn BW, et al. Edema associated with I-125 and Pd-103 prostate brachytherapy and its impact on post-implant dosimetry: An analysis based on series CT acquisition. Int J Radiat Oncol Biol Phys 1998; 41:1069-1077. 2. Wang JZ, Guerrero M, Li XA. How low is the / ratio for prostate cancer ? Int J Radiat Oncol Biol Phys 2003; 55:194-203.
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