Prostate cancer: comparison of retrograde urethrography and computed tomography in radiotherapy planning

To prospectively compare the role of retrograde urethrography and high-quality computed tomography during the treatment planning of patients with prostate cancer.

Forty consecutive men with localized prostate cancer underwent planning computed tomography prior to stimulation. At the time of simulation we performed retrograde urethrography and compared the location of the inferior border of the urogenital diaphragm to the location of the ischial tuberosities and the prostate and base of the penis as seen on the computed tomography scan.

Interobserver identification of the prostatic apex varied in 70% of the cases. Perhaps due to this variability, attempts to place the inferior border of the treatment field in relation to the prostatic apex resulted in an inadequate margin (< 1 cm) beneath the urogenital diaphragm in 5%. In contrast, placing the inferior border at the ischial tuberosities or the base of the penis as seen on computed tomography insured an adequate margin for all patients. The distance from the urogenital diaphragm to the ischial tuberosities and, thus, the potential margin beneath the urogenital diaphragm was > 2 cm in 77%, while the distance from the urogenital diaphragm to the base of the penis was > or = 2 cm in only 43%.

This demonstrates the difficulty in reliably identifying the prostate on computed tomography. Nevertheless, by identifying the base of the penis, planning computed tomography provides adequate information to cover the target volume, and results in minimal overtreatment of normal structures. Urethrograms are not necessary if the computed tomography is properly used or if the ischial tuberosities are used as a standard inferior border, but they can reduce the length of urethra in the treatment volume which could potentially reduce complications.