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Effect of edema, relative biological effectiveness, and dose heterogeneity on prostate brachytherapy a)
Author(s) -
Wang Jian Z.,
Mayr Nina A.,
Nag Subir,
Montebello Joseph,
Gupta Nilendu,
Samsami Nina,
Kanellitsas Christos
Publication year - 2006
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.2181294
Subject(s) - brachytherapy , medicine , edema , nuclear medicine , prostate , relative biological effectiveness , dosimetry , effective dose (radiation) , implant , prostate cancer , radiation therapy , cancer , irradiation , surgery , physics , nuclear physics
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 ( α = 0.15Gy − 1and α ∕ β = 3.1 Gy ) determined in earlier studies were used to calculate the equivalent uniform dose in 2 Gy fractions ( EUD 2 ) with respect to three effects: edema, RBE, and dose heterogeneity forI125andPd103implants. TheEUD 2analysis 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, theV 100(volume covered by 100% of prescription dose) decreases from 93% to 91% and 86%, and theD 90(dose covering 90% of target volume) decrease from 107% to 102% and 94% of prescription dose forI125andPd103implants, 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 forI125and 1.3 – 1.6 forPd103implants. 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.