ABSTRACT
The aim of this paper is to study the correction of prostate motion and position during external beam therapy. The correction was performed using a commercially available ultrasound-based repositioning tool. Electronic portal imaging with the use of fiducial markers was used to assess efficacy and accuracy. Patients undergoing radiation treatment for adenocarcinoma of the prostate were enrolled in a positioning study. Fifteen patients had five to six gold fiducial markers implanted in their prostate. These patients were positioned daily in a standard manner and then were repositioned every other day using an ultrasound-based correction system. Every fraction of a patients' treatment was imaged. This yielded 156 image pairs with and 119 pairs without repositioning available for analysis. This group of patients with markers had the following residual positions measured after the use of ultrasound repositioning. A mean error of -0.4 mm (LL), -2.6 mm (CC), and +2.5 mm (AP) with a standard deviation of 4.3, 5.4, and 5.7 mm. In two directions the improvements of treatment using the ultrasound correction were smaller than the precision of this experiment. They were no larger than 0.81 mm (LAT), and 0.95 mm (CC). In the AP direction a significant improvement was found of 1.6 mm. A highly significant correlation (p < 0.001) was found between the residual errors in the cranio-caudal direction and the shifts performed on the basis of the ultrasound measurements (Spearman ranking R = 0.53). We presented a method to objectively estimate improvements by a correction scheme. This method applied to ultrasound-based adjustment showed significant improvement in one direction and no measurable improvement in two other directions.