MRI of the prostate after combined radiotherapy (interstitial with external beam irradiation): Histopathological correlation.

Aim: To identify the MRI changes of the prostate after combined (high-dose rate interstitial with external beam) radiotherapy for, localized prostate cancer and to correlate the findings with histology in order to determine t he Value of MR imaging in the follow-up of these patients. Material and met hods: Twenty-three patients underwent MR imaging at 1.5 T between 6 and 24 months after completion of combined radiotherapy. The prostate was imaged w ith axial and coronal T-2-weighted sequences and axial T-1-weighted sequenc es before and after intravenous administration of Cd-DTPA. Quadrant or sext ant biopsy was performed in all cases and three patients with proven persis tence of the tumor underwent salvage prostatectomy. The MRI findings were c ompared with the biopsy results or the large-area sections. Results: On T-2 -weighted images the fibrotically changed peripheral zone was hypointense w hile persistent tumor tissue showed hyperintensity. Solid tumors were depic ted when they had a diameter of 1 cm or more. Persistent tumors of the diff use multifocal type escaped detection. Contrast-enhanced T-1-weighted imagi ng yielded no additional information. The accuracy in detecting persistent tumor was 74%. Conclusions: Histopathologic changes seen after combined rad iotherapy correlate with the findings on T-2-weighted MR images. MR imaging cannot replace follow-up by routine biopsy. Its only role is assessing loc al operability in cases found to have increasing PSA levers during follow-u p. Further studies are needed to determine the role of MR imaging in this p atient population.