Purpose We analyzed the pathological outcomes of candidates for active surveillance according to tumor lesion diameter on diffusion weighted magnetic resonance imaging. Materials and Methods We retrospectively analyzed 188 candidates for active surveillance who had undergone diffusion weighted magnetic resonance imaging before radical prostatectomy between 2006 and 2012. We measured the diameter of the suspicious tumor lesion on diffusion weighted magnetic resonance imaging and stratified the cohort into 2 groups. Group 1 included patients with normal magnetic resonance imaging or a suspicious tumor lesion smaller than 1 cm and group 2 included patients with a suspicious tumor lesion larger than 1 cm. We compared pathological outcomes including insignificant prostate cancer in each group and analyzed whether different tumor diameters resulted in a change in insignificant prostate cancer rates. Results Group 1 consisted of 115 (61.2%) patients and group 2 included 73 (38.8%) patients. In group 1 magnetic resonance imaging was normal in 72 patients. Mean ± SD diameter of suspicious tumor lesions was 12.0 ± 5.58 mm. Tumor volume was significantly different between the groups (0.73 ± 0.86 vs 1.09 ± 1.07 cm3, p = 0.018), as was the rate of insignificant prostate cancer (48.7% vs 24.7%, p = 0.001). The rate of insignificant prostate cancer decreased as tumor diameter increased over 1 cm. On multivariate logistic regression analysis the diameter of suspicious tumor lesions was an important predictor of insignificant prostate cancer (OR 0.319, p = 0.014). Conclusions Our analysis demonstrates that the simple measurement of the diameter of suspicious tumor lesions on diffusion weighted magnetic resonance imaging could improve the prediction of insignificant prostate cancer in candidates for active surveillance.
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