Initial experience with ultrasound localization for positioning prostate cancer patients for external beam radiotherapy.

PURPOSE: Transabdominal ultrasound localization of the prostate gland and its immediate surrounding anatomy has been used to guide the positioning of patients for the treatment of prostate cancer. This process was evaluated in terms of (1) the reproducibility of the ultrasound measurement; (2) a comparison of patient position between ultrasound localization and skin marks determined from a CT treatment planning scan; (3) the predictive indicators of patient anatomy not well suited for ultrasound localization; (4) the measurement of prostate organ displacement resulting from ultrasound probe pressure; and (5) quality assurance measures. METHODS AND MATERIALS: The reproducibility of the ultrasound positioning process was evaluated for same-day repeat positioning by the same ultrasound operator (22 patients) and for measurements made by 2 different operators (38 patients). Differences between conventional patient positioning (CT localization with skin markings) and ultrasound-based positioning were determined for 38 patients. The pelvic anatomy was evaluated for 34 patients with pretreatment CT scans to identify predictors of poor ultrasound image quality. The displacement of the prostate resulting from pressure of the ultrasound probe was measured for 16 patients with duplicate CT scans with and without a simulated probe. Finally, daily, monthly, and semiannual quality assurance tests were evaluated.
RESULTS: Self-verification tests of ultrasound positioning indicated a shift of <3 mm in approximately 95% of cases. Interoperator tests indicated shifts of <3 mm in approximately 80-90% of cases. The mean difference in patient positioning between conventional and ultrasound localization for lateral shifts was 0.3 mm (SD 2.5): vertical, 1.3 mm (SD 4.7 mm) and longitudinal, 1.0 mm (SD 5.1). However, on a single day, the differences were >10 mm in 1.5% of lateral shifts, 7% of longitudinal shifts, and 7% of vertical shifts. The depth to the isocenter, thickness of tissue overlying the bladder, and position of the prostate relative to the pubic symphysis, but not the bladder volume, were significant predictive indicators of poor ultrasound imaging. The pressure of the ultrasound probe displaced the prostate in 7 of the 16 patients by an average distance of 3.1 mm; 9 patients (56%) showed no displacement. Finally, the quality assurance tests detected ultrasound equipment defects.
CONCLUSION: The ultrasound positioning system is reproducible and may indicate the need for significant positioning moves. Factors that predict poor image quality are the depth to the isocenter, thickness of tissue overlying the bladder, and position of the prostate relative to the pubic symphysis. The prostate gland may be displaced a small amount by the pressure of the ultrasound probe. A quality assurance program is necessary to detect ultrasound equipment defects that could result in patient alignment errors.