SU-FF-T-44: Dosimetric Analysis of the Effect of Edema in 131Cs Prostate Permanent Seed Implants

Abstract

Purpose: To investigate the effect of edema on different dosimetric parameters in 131 Cs prostate permanent seed implants. Materials and Methods: Transrectal ultrasound(US) and computertomography(CT)images were used to determine pre‐ and post‐needle implant volumes, and post‐implant prostate volumes in 31 patients who had received 131 Cs implant. Dose volume histograms(DVH) were generated to determine the prostate volumes that received 100%, 150% and 200% of prescribed dose to calculate quality indices(QIs) and fractional D90(FD90) for each set of volumes for all patients. Results: No statistical differences were found between post‐needle and post‐implant(day 0) volumes obtained by US and CTimages(p=0.56). The half life of the edema was found to be 9.72days. The change in mean values of coverage index(CI), dose non‐uniformity ratio(DNR), overdose index(ODI), relative dose‐homogeneity index(DHI) and FD90 can be described by CI ( t )= CI (0)+[1− CI (0)][1− exp (−λ CI t )] (1) DNR ( t )= DNR (0)+[1− DNR (0)][1− exp (−λ DNR t )] (2) ODI ( t )= ODI (0)+[1− ODI (0)][1− exp (−λ ODI t )] (3) DHI ( t )= DHI (0) exp (−λ DHI t ) (4) and D 90( t )=( R 0 /λ) exp (−λ t ){ FD 90(0)+[ FD 90(0)− a ][1− exp (−λ FD t )]} (5) differential equations. The values of correlation coefficients and time constants obtained from least square fit of these equations were found to be 0.9867 & 0.0316 for CI, 0.9988 & 0.0148 for DNR, 0.9958 & 0.00148 for ODI, and 0.9983 & 0.0125 for DHI, respectively. Conclusions: CI, DHI and FD90 decreased, while DNR and ODI increased from pre‐implant plans to post‐implant plans at day 0 due to edema formation. CI, DNR, ODI and FD90 increased and DHI decreased with increasing post‐implant time and attained optimal values in 4 weeks. During this period 85% dose is delivered due to short half life of 131 Cs seeds. Therefore, it is important to account for the effect of edema at the time of implant while defining seed positions. Implants performed based on pre‐implant volume study only result in poor dosimetric results in 131 Cs implants.