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

Purpose: 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 purpos e of this study was to evaluate the feasibility and accuracy of daily ultra sound-guided localization utilizing daily CT as a standard. The relatively simple computer-assisted ultrasound-based system is designed to be an effic ient means of achieving daily accuracy. Methods and Materials: After five weeks of conformal external beam radiatio n therapy, 23 patients underwent a second CT simulation. Prostate-only fiel ds 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 w as performed. Ten of the above patients also underwent prostate localizatio n with a newly developed ultrasound-based system (BAT(TM)) that is designed to facilitate patient positioning at the treatment machine. The portable s ystem, which electronically imports the CT simulation target contours and i socenter, is situated adjacent to the treatment couch. Transverse and sagit tal suprapubic ultrasound images are captured, and the system overlays the corresponding CT contours relative to the machine isocenter. The CT contour s are maneuvered in three dimensions by a touch screen menu to match the ul trasound images. The system then displays the 3-D couch shifts required to produce field alignment. Results: The BAT(TM) ultrasound system produced good quality images with mi nimal operator training required. The localization process was completed in less than 5 min. The absolute magnitude difference between CT and ultrasou nd was small (AIP 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). Analys is confirmed a significant correlation of isocenter shifts (A/P 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. Conclusions: Daily CT localization is a precise method to improve daily tar get localization in prostate carcinoma. However, it requires significant hu man and technical resources that limit its widespread applicability. Conver sely, localization with the BAT(TM) ultrasound system is simple and expedit ious by virtue of its ability to image the prostate at the treatment machin e in the treatment position. Our initial evaluation revealed ultrasound tar geting to be functionally equivalent to CT. This ultrasound technology is p romising and warrants further investigation in more patients and at other a natomical sites. (C) 1999 Elsevier Science Inc.