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Diffusion-weighted MRI of the Prostate: Limits of Cancer Detection
Jennifer K. Yates, MD, Courtney Woodfield, M.D,, Gyan Pareek, M.D,, Joseph Renzulli, M.D,, George Haleblian, M.D,.
Brown University, Providence, RI, USA.

BACKGROUND:
Endorectal MRI has been shown to be an effective adjunct for evaluation of clinically organ- confined prostate cancer. Studies have shown that it can effectively demonstrate extracapsular extension and seminal vesicle invasion in the hands of an experienced radiologist. MRI with diffusion-weighted imaging (DWI) is a new technique which when applied to the prostate can allow for clearer delineation of normal prostate tissue from malignant tissue; however, limits of detection are unknown. We evaluate the sensitivity of DWI in detecting prostate cancer.
METHODS:
125 consecutive patients with a biopsy proven adenocarcinoma of the prostate underwent endorectal MRI with DWI on a 1.5 T magnet with calculation of estimated tumor volumes as part of a preoperative staging protocol. Of these, 38 patients underwent definitive surgical management of their organ-confined disease. The final pathologic stage and tumor volumes were then obtained post operatively. Comparative analysis of final pathologic tumor volumes was performed to determine the sensitivity of DWI for the detection of prostate cancer.
RESULTS:
The average pathological prostate size was 48.9g. Of the 38 patients final staging revealed 26 as T2 and 12 as T3 tumors. Pathological tumor volumes ranged from 0.5% to 40% of the gland. DWI identified tumor volumes ranged from 0 to 55%. Comparison with pre-operative DWI revealed that imaging was able to correctly identify 71% of tumors and was unable to visualize tumor volumes of less than 3% prostate volume. In all patients with T3 disease, evidence of tumor burden was visualized on DWI. Similarly, all patients in whom DWI was negative for disease had T2 pathology.
CONCLUSIONS:
MRI with DWI is an effective means of identifying prostate cancer on pre-operative imaging. The limits of detection suggest that in patients with pathologic tumor volumes less than 3% of the prostate, MRI with DWI is unable to identify disease. Therefore, presence of a positive biopsy with negative MRI may suggest the presence of clinically insignificant amounts of prostate cancer. In addition, this technique may have a role in following patients on active surveillance.


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