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Prostate Cancer: Endorectal MR Imaging and MR Spectroscopic Imaging—Distinction of True-Positive Results from Chance-detected Lesions¹

¹ From the Departments of Radiology (J.J.H., F.V.C., A.Q., Y.L., J.K.), Anatomic Pathology (J.P.S., L.S.), and Urology (P.R.C.), University of California, San Francisco, 505 Parnassus Ave, Box 0628, M-372, San Francisco, CA 94143-0628. Received September 29, 2004; revision requested December 2; final revision received May 20, 2005; accepted June 13. Supported by NIH grants IRGICA76423-0IRI, R01 CA59897, and R01 CA79980. J.J.H. supported by UCSF Dean's Award for Summer Research. L.S. supported by NCI SPORE grant p50. Address correspondence to F.V.C. (e-mail: Fergus.Coakley{at}radiology.ucsf.edu).

Purpose: To retrospectively investigate size criteria for the identification of chance-detected lesions at endorectal magnetic resonance (MR) imaging and MR spectroscopic imaging of prostate cancer.

Materials and Methods: Approval of the committee on human research and written informed consent were obtained. This study was HIPAA compliant. Endorectal MR imaging and MR spectroscopic imaging were performed with a 1.5-T MR imager in 48 men with a mean age of 59 years (age range, 47–75 years) prior to radical prostatectomy. Two independent readers recorded the size and location of all suspected peripheral zone tumor nodules on MR images alone and on images obtained with combined MR imaging and MR spectroscopic imaging. Nodules detected at MR imaging were classified as matched lesions if tumor was present in the same location at step-section histopathologic review. For all matched lesions, values were calculated to examine agreement between measured and actual tumor size. Lesions that were overmeasured at MR imaging with a value of less than 0.2 were considered chance-detected lesions.

Results: At MR imaging, two of 27 and four of 35 matched lesions for readers 1 and 2, respectively, were chance-detected lesions. The corresponding numbers of lesions at combined MR imaging and MR spectroscopic imaging were one of 21 and one of 31, respectively. In all but two cases, the measured diameter of chance-detected lesions was more than twice that of the diameter at histopathologic analysis. By using this diameter threshold to distinguish true-positive results, the mean diameter of detected tumors at histopathologic analysis was 15 mm compared with 4 mm for both undetected and chance-detected tumors (P < .05).

Conclusion: To ensure uniformity in the comparison of scientific studies, peripheral zone tumors detected at MR imaging and MR spectroscopic imaging of the prostate that are in the same location as tumors detected at histopathologic review should be considered chance-detected lesions if the MR transverse diameter is more than twice the histopathologic transverse diameter.

© RSNA, 2006

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