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

PURPOSE: There 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 V-100 and D-90 to contouring uncertainty as clinically relevant parameters for evaluation of target coverage in postimplant dosimetry. METHODS AND MATERIALS: CT 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 V-100 and D-90 were extracted from dose-volume histograms for each contour and compared. RESULTS: Significant changes were observed in the values of D-90 and V-100 as well as the number of inacceptable plans for expansion or contraction of only few millimeters. Evaluation of a plan coverage based on D-90 was found to be less sensitive to systematic contouring errors compared with V-100. Number of plans incorrectly identified for lack or adequacy of coverage is lower using D-90 compared with V-100 for the same margin of error. CONCLUSIONS: Evaluation of a plan coverage based on V-100 is too sensitive to systematic contouring errors of prostate. D-90 increases the accuracy of CT-based postimplant quality assurance in identifying plans with insufficient coverage compared with V-100. 2016 American Brachytherapy Society. Published by Elsevier Inc. All rights reserved.