Localization of the prostatic apex for radiotherapy treatment planning using urethroscopy

Purpose: To assess the ability of computed tomography (CT) scans and retrograde urethrograms to accurately define the prostatic apex in the craniocaudad dimension, using urethroscopy as a reference. Methods and Materials: Plain film radiographs of the pelvis were obtained in 15 patients with early-stage adenocarcinoma of the prostate, with the tip of a urethroscope held in place at the external sphincter, which most closely approximates the prostatic apex. The scope was then withdrawn, and a retrograde urethrogram was performed. Immediately afterwards, a CT scan of the pelvis was obtained. Because differential filling of the bladder and rectum affects the position of the prostatic apex, patients voided prior to rather than in between the three consecutive studies. Results: The urethroscopy-defined prostatic apex was located 4 ± 8 mm (mean ± SD) superior to the CT-defined apex, 13 ± 3 mm (mean ± SD) superior to the urethrogram tip and 30 ± 7 mm (mean ± SD) superior to the ischial tuberosities. There was significant interobserver variability in the location of the prostatic apex as determined by CT scans. Placement of the inferior border of the radiation portals at the ischial tuberosities would have resulted in irradiation of ≥20 mm bulbar urethra, as defined by the dye column of the retrograde urethrogram, in 6 out of 15 (40%) of the patients and irradiation of <10 mm bulbar urethra in 2 out of 15 (13%) of the patients. Conclusion: Because the prostate blends inferiorly with the urogenital diaphragm, CT scans do not allow one to precisely localize the prostatic apex. Due to anatomic variability, the ischial tuberosities do not allow one to accurately localize the prostatic apex. Retrograde urethrograms provide helpful supplemental information regarding the position of the prostatic apex for radiotherapy treatment planning