Endorectal MRI after radiation therapy: Questioning the sextant analysis

Purpose: To evaluate whether the information gained by three coregistration systems (sextant, hemi-prostate, and whole gland) differs significantly, suggesting that one approach should be routinely favored over the others. Despite its known limitations, sextant is the generally accepted standard for magnetic resonance imaging (MRI) and biopsy coregistration; nevertheless, depending on the magnitude of localization errors, other options may be adequate. Materials and Methods: Institutional Review Board approval was obtained and the study was Health Insurance Portability and Accountability Act (HIPAA)-compliant. We identified 70 patients who underwent 1.5 T endorectal MRI of the prostate between 1999 and 2008 after external beam radiotherapy for prostate cancer. A single reader reviewed all T2-weighted images for the presence or absence of tumor. The performance of each approach was quantified using receiver operating characteristic (ROC) curve analysis. Transrectal ultrasound-guided sextant biopsies were used as a standard of reference. Results: The areas under the ROC curve indicating accuracy for each MRI approach were 0.63 (sextant), 0.68 (hemi-prostate), and 0.71 (whole gland). There was no statistically significant difference among these approaches. Conclusion: As expected, the point estimate was higher for the whole-gland approach, but not significantly. Reliable assessment of locally recurrent prostate cancer after external beam radiotherapy by endorectal MRI may be made using a sextant, hemi-prostate, or whole gland approach. The option for one or another approach should not be solely based on estimations of imaging accuracy, but on the purpose of the procedure.