HELICAL CT OF RENAL MASSES - THE VALUE OF DELAYED SCANS

OBJECTIVE. Routine scanning techniques used for helical CT of the abdo men result in dense cortical opacification of the kidney, whereas the medulla and collecting system are not well opacified, which potentiall y compromises detection of renal masses. The purpose of this retrospec tive study was to determine if additional delayed views (taken approxi mately 2-4 min after the start of injection of contrast material) are necessary for the detection and characterization of renal masses. MATE RIALS AND METHODS. Early (60-70 sec after the start of the injection o f contrast material) and delayed scans of 40 patients with suspected r enal masses were blindly evaluated by two observers. The patients harb ored a total of 187 renal masses (including 62 solid masses). Each reg ion of the kidney (upper, middle, and lower pole) was scored for the p resence of a mass. Scoring was done as a binary decision and also as a five-point confidence score for receiver operating characteristic ana lysis. RESULTS. We found 97 regions that contained renal masses and 11 4 regions that did not. Receiver operating characteristic analysis rev ealed the observers to have significantly greater confidence in detect ion of renal masses on the delayed scans. The binary data showed the t wo observers to have a sensitivity of 97% for delayed scans versus 77% (p = .0002) and 89% (p = .027), respectively, for the early scans. Fo r the first observer, early and delayed scans were of equal specificit y, but for the second observer, the delayed scans yielded greater spec ificity (94% versus 85%, p = .024). On the early scans, both observers were significantly more likely to miss a neoplastic lesion than a non neoplastic lesion. The less experienced of the two observers also tend ed to have greater difficulty in characterizing the lesions on the ear ly scans. CONCLUSION. Because of the significant risk of missing a ren al mass, especially a neoplasm, on early cortical-phase scans, additio nal delayed scans appear justified when a renal mass is suspected on t he basis of other imaging tests or clinical history.