Use of pelvic CT scanning to evaluate pubic arch interference of transperineal prostate brachytherapy

To determine the necessity of preoperative evaluation of pubic arch interference in patients with small prostate volumes.

CT scans from 97 consecutive, unselected patients with stage T1 or T2 prostatic carcinoma who had transperineal I-125 or Pd-103 implants at the University of Washington in 1997 were analyzed for pubic arch interference. Transrectal ultrasound (TRUS) was performed with 6.0-MHz transducer with the patient in the lithotomy position and the patient's thighs vertical, similar to that used during the implant procedure. CT scans were obtained with the patient in the supine position, with 0.5-cm images taken at every 0.5 cm. To check for potential arch interference, the largest prostate cross-section was overlaid on the narrowest portion of the pubic arch. The overlap of the pubic arch and the prostate margin is measured at right angles to the inner pubic surface. The prostate volume obtained from the TRUS images was compared with the degree of pubic arch interference in order to determine whether TRUS volume predicted for interference.

There was considerable variability in pubic arch interference between patients. The mm of pubic arch overlap with the prostatic margin varied from -11 mm to 20 mm. Patients with larger prostate volumes generally had more pubic arch interference, but the degree of interference was only loosely related to the prostate volume (r = 0.46).

The degree of pubic arch interference is highly variable from one patient to the next and the TRUS volume cannot reliably predict patients who do or do not need a pelvic CT to detect potential arch interference.