Comparison of CT-based treatment planning and retrograde urethrography in determining the prostatic apex at simulation

In 20 consecutive patients who underwent treatment planning, localization of the prostatic apex with CT-based techniques at simulation was compared to location of the apex as defined by retrograde urethrography. In addition, the location of the urethrogram-defined prostatic apex was compared with the bottom of the ischial tuberosities, which is often recommended as the inferior margin of the field. In 15% of the patients there was agreement between the CT-defined apex and the urethrogram-defined apex; in 85% there was discordance. In a majority of patients with discordance, the urethrogram apex was located caudad to the CT-defined apex (71%) with a median difference of .65 cm. In 29% of the patients the urethrogram apex was located superior to the CT-defined apex. Overall, 75% of the patients had discordance between the urethrogram apex and the CT apex of 0.5 cm or greater, 30% had an absolute difference of 1.0 cm or greater. Comparing the location of the prostatic apex with the bottom of the ischial tuberosities revealed that in 15% of the patients the apex was 1.0 cm or less from the bottom of the tuberosities and in 45% it was less than 1.5 cm. This would place the apex of the prostate in the penumbra region of the field and risk undertreatment of the prostate if the bottom of the ischial tuberosities was the inferior margin of the field. Measuring the location of the prostatic apex from the top of the symphysis pubis revealed that a distance of 4.9 cm encompassed the apex in all 20 patients. In the absence of apex localization techniques such as urethrography or radiopaque marker seeds, it is reasonable to use this distance as the apical location to reduce the chance of undertreating or missing the prostate. The causes of discordance between CT and urethrography in defining the prostatic apex will be discussed.