The role of the urethrogram during simulation for localized prostate cancer.

Urethrograms on 89 consecutive patients with localized prostate cancer were evaluated retrospectively, and the inferior border of the treatment field based on this study was compared with the inferior border that would have been defined by using the lower border of the ischial tuberosities. An analysis of the relationship between the margin used and the dose at the inferior border of the prostate supported our policy of requiring a 2 cm margin for optimal coverage of the prostate. Inclusion of at least 1 cm of proximal penile urethra was essential to ensure this 2 cm margin. Based on this assumption, twenty-five percent of patients would have had an inadequate margin if the lower border of the ischial tuberosities had been used instead of the urethrogram to define the inferior border of the treatment field. Assuming that a margin of more than 3 cm inferiorly is excessive, 11% of patients would have had excessive urethral irradiation if the bottom of the ischial tuberosities had been used to define the inferior border. Combining these two extremes, more than one in three patients would have had an inappropriate inferior margin if the bottom of the ischial tuberosities had been used to define the inferior border of the treatment field. There was no apparent increase in morbidity as a result of the urethrograms or an increase in treatment related toxicity in association with using the treatment fields defined by urethrography. Computed tomography was complimentary in defining the apex of the prostate. These data support the routine use of the urethrograms during simulation for localized prostate cancer. The use of the lower border of the ischial tuberosities to define the inferior border of the treatment field is associated with an unacceptable risk of either underdosing the apical portion of the prostate or overdosing the urethra.