Location of the prostatic apex and its relationship to the ischial tuberosities

Although modern computerized tomography scans have revolutionized the three-dimensional treatment planning for external beam radiation therapy for prostate cancer, the prostate apex is often difficult to precisely define. Some institutions routinely use the ischial tuberosities to define the lower border of external beam fields for prostate cancer, while others recommend a retrograde urethrogram. This study was undertaken to estimate the accuracy of using the bottom of the ischial tuberosities to define the lower border of the external beam fields for Stages T1, T2, and T3 prostate cancer.

The anatomic location of the apex of the prostate was determined in 153 implant patients either by direct surgical exposure of the prostate (133 patients) or by using transrectal ultrasound (20 patients). The prostate apex position relative to the ischial tuberosities was determined and plotted on a schematic of the bony pelvic structures drawn to scale.

There was excellent agreement in the estimate of the location of the prostate apex between the two methods (surgery vs. ultrasound) used. The prostate apex was located above the ischial tuberosities in 152 of the 153 patients studied (99.3%). Seven of the 153 patients (4.6%) had a prostate apex which was less than 1.5 cm above the ischial tuberosities and 3 of the 153 patients (2%) had an apex-tuberosity distance of less than 1 cm.

This study indicates that locating the inferior border of the external beam fields at the ischial tuberosity adequately treats at least 95.4% of all prostate patients with a margin of 1.5 centimeters or more below the prostate apex. In addition, the external beam policy of locating the inferior border at the ischial tuberosities has produced: (a) excellent 10-year clinical local control rates of 88% for Stage T1 and T2 patients and 82% for Stage T3 patients, and (b) 5-year and 10-year biochemical (normal prostate specific antigen) and clinical disease free survival rates for T1 and T2 patients which are similar to surgery.