Purpose: To assess the accuracy of ultrasound-based repositioning (BAT) before prostate radiation with fiducial-based three-dimensional matching with cone-beam computed tomography (CBCT).
Patients and methods: Fifty-four positionings in 8 patients with 125I seeds/intraprostatic calcifications as fiducials were evaluated. Patients were initially positioned according to skin marks and after this according to bony structures based on CBCT. Prostate position correction was then performed with BAT. Residual error after repositioning based on skin marks, bony anatomy, and BAT was estimated by a second CBCT based on user-independent automatic fiducial registration.
Results: Overall mean value (MV+/-SD) residual error after BAT based on fiducial registration by CBCT was 0.7+/-1.7 mm in x (group systematic error [M]=0.5 mm; SD of systematic error [Sigma]=0.8 mm; SD of random error [sigma]=1.4 mm), 0.9+/-3.3 mm in y (M=0.5 mm, Sigma=2.2 mm, sigma=2.8 mm), and -1.7+/-3.4 mm in z (M=-1.7 mm, Sigma=2.3 mm, sigma=3.0 mm) directions, whereas residual error relative to positioning based on skin marks was 2.1+/-4.6 mm in x (M=2.6 mm, Sigma=3.3 mm, sigma=3.9 mm), -4.8+/-8.5 mm in y (M=-4.4 mm, Sigma=3.7 mm, sigma=6.7 mm), and -5.2+/-3.6 mm in z (M=-4.8 mm, Sigma=1.7 mm, sigma=3.5 mm) directions and relative to positioning based on bony anatomy was 0+/-1.8 mm in x (M=0.2 mm, Sigma=0.9 mm, sigma=1.1 mm), -3.5+/-6.8 mm in y (M=-3.0 mm, Sigma=1.8 mm, sigma=3.7 mm), and -1.9+/-5.2 mm in z (M=-2.0 mm, Sigma=1.3 mm, sigma=4.0 mm) directions.
Conclusions: BAT improved the daily repositioning accuracy over skin marks or even bony anatomy. The results obtained with BAT are within the precision of extracranial stereotactic procedures and represent values that can be achieved with several users with different education levels. If sonographic visibility is insufficient, CBCT or kV/MV portal imaging with implanted fiducials are recommended.