Purpose: To quantify interobserver variation of prostate and seminal vesicle delineations using CT only versus CT + MRI in consensus reading with a radiologist.
Material and methods: The prostate and seminal vesicles of 13 patients treated with intensity-modulated radiotherapy for prostatic adenocarcinoma were retrospectively delineated by three radiation oncologists on CT only and on CT + MRI in consensus reading with a radiologist. The volumes and margin positions were calculated and intermodality and interobserver variations were assessed for the clinical target volume (CTV), seminal vesicles, prostate and three prostatic subdivisions (apical, middle and basal third).
Results: Using CT + MRI as compared to CT alone, the mean CTV, prostate and seminal vesicle volumes significantly decreased by 6.54%, 5.21% and 10.47%, respectively. More importantly, their standard deviations significantly decreased by 63.06%, 62.65% and 44.83%, respectively. The highest level of variation was found at the prostatic apex, followed by the prostatic base and seminal vesicles.
Conclusion: Addition of MRI to CT in consensus reading with a radiologist results in a moderate decrease of the CTV, but an important decrease of the interobserver delineation variation, especially at the prostatic apex.