Ultrasound probe pressure as a source of error in prostate localization for external beam radiotherapy.

Abstract

A pelvic phantom was constructed to evaluate the effect of ultrasound probe pressure during performance of bipolar acquisition technique (BAT) for prostate localization for radiotherapy. A pelvic phantom of a gelatin mold with a water-filled balloon representing the bladder and rectum and a central encapsulated clay sphere representing the prostate was constructed. This phantom was then scanned using planning computed tomography (CT). The geometric information of the phantom was outlined in two planes. The phantom was then scanned using the BAT system with mild and moderate ultrasound probe pressure. Differences in prostate depth between the CT and BAT systems were displayed. A difference of 1 cm between the phantom surface and the prostate could be produced with moderate ultrasound probe pressure. The differences were similar between the CT- and BAT-generated contours and were dependent on the ultrasound probe pressure. Care must be taken not to cause any alteration in prostate localization with increasing ultrasound probe pressure when using BAT localization. Increased probe pressure may introduce errors in prostate localization and under dose the target.