Sensitivity of clinically relevant dosimetric parameters to contouring uncertainty in postimplant dosimetry of low-dose-rate prostate permanent seed brachytherapy

Abstract

PurposeThere is strong evidence relating postimplant dosimetry for low-dose-rate prostate seed brachytherapy to local control rates. The delineation of the prostate on CT images, however, represents a challenge due to the lack of soft-tissue contrast to identify the prostate borders. This study aims at quantifying the sensitivity of prostate V100 and D90 to contouring uncertainty as clinically relevant parameters for evaluation of target coverage in postimplant dosimetry. Methods and MaterialsCT images, postoperative plans, and contours of a cohort of patients (n = 43; low risk = 55.8%, intermediate risk = 39.5%, high risk = 4.7%), who had received prostate seed brachytherapy, were imported into MIM Symphony treatment planning system. The prostate contours in postimplant CT images were expanded or contracted uniformly in extents of ±1.00 mm, ±2.00 mm, ±3.00 mm, ±4.00 mm, and ±5.00 mm. The values for V100 and D90 were extracted from dose-volume histograms for each contour and compared. ResultsSignificant changes were observed in the values of D90 and V100 as well as the number of inacceptable plans for expansion or contraction of only few millimeters. Evaluation of a plan coverage based on D90 was found to be less sensitive to systematic contouring errors compared with V100. Number of plans incorrectly identified for lack or adequacy of coverage is lower using D90 compared with V100 for the same margin of error. ConclusionsEvaluation of a plan coverage based on V100 is too sensitive to systematic contouring errors of prostate. D90 increases the accuracy of CT-based postimplant quality assurance in identifying plans with insufficient coverage compared with V100.