Background-Use of computed tomography (CT) for diagnostic evaluation has increased dramatically over the past two decades. Even though CT is associated with substantially higher radiation exposure than conventional x-rays, typical clinical doses are not known. We sought to estimate the radiation dose associated with common CT studies in clinical practice; assess variation in dose across types of studies, patients, and institutions; and quantify the potential cancer risk associated with these examinations. Methods-Retrospective cross-sectional study describing radiation dose associated with the 11 most common types of diagnostic CT studies performed on 1,119 consecutive adult patients at four San Francisco Bay Area institutions between January 1 and May 30, 2008. We estimated lifetime attributable risks of cancer by study type from these measured doses.

Purpose:To determine whether magnetic resonance (MR) imaging and MR spectroscopic imaging fi ndings can improve predictions made with the Kattan nomogram for radiation therapy. Materials and Methods:The institutional review board approved this retrospective HIPAA-compliant study. Ninety-nine men who underwent endorectal MR and MR spectroscopy before externalbeam radiation therapy for prostate cancer (January 1998 to June 2007) were included. Linear predictors were calculated with input variables from the study sample and the Kattan original coeffi cients. The linear predictor is a single weighted value that combines information of all predictor variables in a model, where the weight of each value is its association with the outcome. Two radiologists independently reviewed all MR images to determine extent of disease; a third independent reader resolved discrepancies. Biochemical failure was defi ned as a serum prostate-specifi c antigen level of 2 ng/mL (2 m g/L) or more above nadir. Cox proportional hazard models were used to determine the probabilities of treatment failure (biochemical failure) in 5 years. One model included only the Kattan nomogram data; the other also incorporated imaging fi ndings. The discrimination performance of all models was determined with receiver operating characteristics (ROC) curve analyses. These analyses were followed by an assessment of net risk reclassifi cation. Results:The areas under the ROC curve for the Kattan nomogram and the model incorporating MR imaging fi ndings were 61.1% (95% confi dence interval: 58.1%, 64.0%) and 78.0% (95% confi dence interval: 75.7%, 80.4%), respectively . Comparison of performance showed that the model with imaging fi ndings performed signifi cantly better than did the model with clinical variables alone ( P , .001).Overall, the addition of imaging fi ndings led to an improvement in risk classifi cation of about 28%, ranging from approximately a minimum of 16% to a maximum of 39%, depending on the risk change considered important.