Initial experience with ultrasound localization for positioning prostate cancer patients for external beam radiation therapy

Abstract

Purpose: 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: a) quality assurance measures required; b) reproducibility of the ultrasound measurement; c) comparison of patient position between ultrasound and CT localization (prior to treatment); d) predictive indicators of patient anatomy not well suited for ultrasound localization; and e) measurement of prostate organ displacement resultant from ultrasound probe pressure. Materials and Methods: a) Quality assurance tests are performed on a daily and monthly, and semiannual basis. b) The reproducibility of the ultrasound positioning process was evaluated for 15 patients by same day repeat positioning by the same ultrasound operator and between different operators. c) Measurement of the difference between patient position from CT based localization (conventional) and ultrasound based positioning was done for 15 patients. d) Patient’s pelvic anatomy was evaluated for 22 patients with pre-treatment CT scans to identify predictors of poor ultrasound image quality. Specifically, the spatial orientation of the prostate relative to the skeletal structure; the depth of the prostate from the anterior surface; the size of the bladder; and the thickness of tissue anterior to the bladder was examined. e) The displacement of the prostate organ as a result of the pressure from the ultrasound probed was measured for 11 patients with duplicate CT scans with and without the probe pressure present. Results: a) Quality assurance tests can detect ultrasound equipment defects that could result in patient alignment errors. b) Self verification tests of ultrasound positioning indicates a precision of less than 3 mm 90 per cent of the time. Intra-operator tests of ultrasound positioning indicates a precision of less than 4 mm 90 per cent of the time. c) Differences in patient’s positioning between conventional and ultrasound localization are on average: lateral, 0.7 mm (sd=2.6 mm); vertical, 1.0 mm (sd=5.4 mm); and longitudinal, -0.1 mm (sd=6.1 mm). However, on a single day, the differences are greater that 10 mm in the longitudinal direction 14% of the time, and 9% of the time in the vertical direction. d) Depth to the isocenter, thickness of overlying tissue, position of the prostate relative to the pubic symphysis, and bladder volume were predictive indicators of poor ultrasound imaging. e) The pressure of the ultrasound probe displaced the prostate in 45% of the patients measured an average of 3.1 mm. The remaining 55% of the patients showed no displacement of the prostate. Conclusion: The ultrasound positioning systems is reproducible and may indicate the need for significant positioning moves. The factors that predict poor image quality are the bladder volume, the depth to the isocenter, the thickness of tissue overlying the bladder, and the 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.