A comparison of daily CT localization to a daily ultrasound-based system in prostate cancer

Daily CT localization has been demonstrated to be a precise method of correcting radiation field placement by reducing setup and organ motion variations to facilitate dose escalation in prostate carcinoma. The purpose of this study was to evaluate the feasibility and accuracy of daily ultrasound-guided localization utilizing daily CT as a standard. The relatively simple computer-assisted ultrasound-based system is designed to be an efficient means of achieving daily accuracy.

After five weeks of conformal external beam radiation therapy, 23 patients underwent a second CT simulation. Prostate-only fields based on this scan were created with no PTV margin. On each of the final conedown treatment days, a repeat CT simulation and isocenter comparison was performed. Ten of the above patients also underwent prostate localization with a newly developed ultrasound-based system (BAT) that is designed to facilitate patient positioning at the treatment machine. The portable system, which electronically imports the CT simulation target contours and isocenter, is situated adjacent to the treatment couch. Transverse and sagittal suprapubic ultrasound images are captured, and the system overlays the corresponding CT contours relative to the machine isocenter. The CT contours are maneuvered in three dimensions by a touch screen menu to match the ultrasound images. The system then displays the 3-D couch shifts required to produce field alignment.

The BAT ultrasound system produced good quality images with minimal operator training required. The localization process was completed in less than 5 min. The absolute magnitude difference between CT and ultrasound was small (A/P range 0 to 5.9 mm, mean 3 mm +/- 1.8; Lat. range 0 to 7.9 mm, mean 2.4 mm +/- 1.8; S/I range 0 to 9 mm, mean 4.6 mm +/- 2.8). Analysis confirmed a significant correlation of isocenter shifts (A/P r = 0.66, p < 0.0001; Lat. r = 0.58, p < 0.003; S/I r = 0.78, p < 0.0001) in all dimensions, and linear regression confirmed the equivalence of the two modalities.

Daily CT localization is a precise method to improve daily target localization in prostate carcinoma. However, it requires significant human and technical resources that limit its widespread applicability. Conversely, localization with the BAT ultrasound system is simple and expeditious by virtue of its ability to image the prostate at the treatment machine in the treatment position. Our initial evaluation revealed ultrasound targeting to be functionally equivalent to CT. This ultrasound technology is promising and warrants further investigation in more patients and at other anatomical sites.