MRI validation of Post-Prostatectomy Radiotherapy Contouring

Abstract

Introduction: Level One Evidence has established the indications for, and importance of, adjuvant radiotherapy following radical prostatectomy. Several guidelines have addressed delineation of the prostate bed but with variable specification of the inferior border relative to the penile bulb or to the first CT slice distal to visible urine in the bladder neck. This work determines the correlation between the caudal aspect of the anastomosis shown by the tip of the urethrogram cone and MRI anatomy. Materials and Methods : Sixteen patients receiving adjuvant radiotherapy following prostatectomy underwent diagnostic MRI in addition to planning CT with Urethrogram. The CT Reference Slice, tip of urethrogram cone and superior aspect of penile bulb were delineated. Results: MRI clearly demonstrates the penile bulb but not the anastomosis. In these 16 patients, the tip of the urethrogram cone was a median 3.9 mm cranial to the penile bulb (range 0-10.3 mm). Conclusion: We show marked variability in the distance between penile bulb and the tip of the urethrogram cone. In all sixteen patients, placing the inferior border of the CTV 15mm cranial to the penile bulb would have failed to treat the caudal aspect of the anastomosis, a frequent site of local relapse, that cannot be reliably landmarked by any other anatomic structure. Individualizing the treatment volume to patient anatomy is the only way to ensure consistent coverage without treating a larger than necessary volume in many patients. We recommend the use of planning urethrogram to minimize the potential for geographic miss.