The location of the prostatic apex on retrograde urethrography and its relationship to the bottom of the ischial tuberosities.

To determine the proportion of patients undertreated if the inferior border of the prostate field is set at the bottom of the ischial tuberosities, we reviewed the ports of 80 patients with prostate cancer who had retrograde urethrography as part of simulation for radiation therapy. For the 75 evaluable urethrograms, the mean distance from the top of the urethrogram cone to the bottom of ischial tuberosities was 1.38 cm (range, -0.48-2.90 cm). A comparison of the inferior border defined by the bottom of the ischial tuberosities and retrograde urethrography showed that 47 of 75 (62.7%) patients would have been undertreated if a margin of 1.5 cm was employed, and the prostatic apex was thought to be directly above the urethrogram cone. If the apex was thought to be 1 cm above the cone, six of 75 (8.0%) patients would have been undertreated, using a margin of 1.5 cm. Although previously published reports have established that using the bottom of the ischial tuberosities as the inferior border of the prostate field results in 10-45% undertreatment, our findings, when adjusted for the variability of prostatic apex location and margin of normal tissue employed, indicate that only 8% may actually be undertreated.