Purpose: Image-guided intensity-modulated radiotherapy can improve protection of organs at risk when large abdominal target volumes are irradiated. We estimated the daily positioning accuracy of ultrasound-based image guidance for abdominal target volumes by a direct comparison of daily imaging obtained with cone beam computed tomography (CBCT).
Methods and materials: Daily positioning (n = 83 positionings) of 15 patients was completed by using ultrasound guidance after an initial CBCT was obtained. Residual error after ultrasound was estimated by comparison with a second CBCT. Ultrasound image quality was visually rated using a scale of 1 to 4.
Results: Of 15 patients, 7 patients had good sonographic imaging quality, 5 patients had satisfactory sonographic quality, and 3 patients were excluded because of unsatisfactory sonographic quality. When image quality was good, residual errors after ultrasound were -0.1 +/- 3.11 mm in the x direction (left-right; group systematic error M = -0.09 mm; standard deviation [SD] of systematic error, Sigma = 1.37 mm; SD of the random error, sigma = 2.99 mm), 0.93 +/- 4.31 mm in the y direction (superior-inferior, M = 1.12 mm; Sigma = 2.96 mm; sigma = 3.39 mm), and 0.71 +/- 3.15 mm in the z direction (anteroposterior; M = 1.01 mm; Sigma = 2.46 mm; sigma = 2.24 mm). For patients with satisfactory image quality, residual error after ultrasound was -0.6 +/- 5.26 mm in the x (M = 0.07 mm; Sigma = 5.67 mm; sigma = 4.86 mm), 1.76 +/- 4.92 mm in the y (M = 3.54 mm; Sigma = 4.1 mm; sigma = 5.29 mm), and 1.19 +/- 4.75 mm in the z (M = 0.82 mm; Sigma = 2.86 mm; sigma = 3.05 mm) directions.
Conclusions: In patients from whom good sonographic image quality could be obtained, ultrasound improved daily positioning accuracy. In the case of satisfactory image quality, ultrasound guidance improved accuracy compared to that of skin marks only minimally. If sonographic image quality was unsatisfactory, daily CBCT scanning improved treatment accuracy distinctly over that of ultrasound. Use of daily ultrasound or CBCT imaging can help to reduce PTV margins and protect organs at risk compared to the use of skin mark-based positioning.