INTRAOBSERVER AND INTEROBSERVER VARIABILITY OF MR IMAGING-DERIVED ANDCT-DERIVED PROSTATE VOLUMES AFTER TRANSPERINEAL INTERSTITIAL PERMANENT PROSTATE BRACHYTHERAPY

PURPOSE: To evaluate the relative accuracy and precision of magnetic r esonance (MR) imaging and computed tomography (CT) in the assessment o f postimplantation prostate volume by determining intraobserver, inter observer, and intermodality variations. MATERIALS AND METHODS: CT and MR images of 41 consecutive patients, after transperineal interstitial permanent prostate brachytherapy, were evaluated by two physicians to determine interobserver and intermodality variability in prostate vol ume measurements. Repeat evaluation in five randomly selected patients was used to determine intraobserver variability. RESULTS: Observer 1 versus 2 CT-determined mean prostate volume difference was statistical ly significant (-8.5 cm(3)+/-9.7[standard deviation],P <.001);observer 1 versus 2 MR-determined mean prostate volume difference was not sign ificant (1.9cm(3)+/-11.7,P = .492). CT intraobserver range of dimensio nal errors was 3.5 and 11.4 times that of MR imaging. Observer 1 CT an d MR volumes were significantly different (P = .001);observer 2 CT and MR volumes were not significantly different (P = .079). CONCLUSIONS: With both CT and MR imaging, variation is less when evaluations are co nducted by one observer. Variation in one observer may be further redu ced by using MR imaging in place of CT.