Prostate localization using transabdominal ultrasound imaging.

PURPOSE Adding margin around a target is done in an attempt to ensure complete coverage of the target. The B-mode acquisition and targeting (BAT) system allows ultrasound imaging of the prostate in patients with a full bladder. This provides a setup tool for patients with localized prostate cancer that takes into account real-time prostate position and may make it possible to decrease tumor margins. Prostate localization using the conventional setup verification method and daily isocenter shifts recommended by the ultrasound imaging system (BAT) were compared and analyzed. METHODS AND MATERIALS Daily treatment isocenter shifts for patients with localized adenocarcinoma of the prostate, obtained from two different imaging modalities, electronic portal imaging (EPI) and BAT, were calculated. We studied the difference in patient setup error calculated using BAT contour alignment and measured from EPI; the reproducibility of BAT contour alignment; intrafraction prostate motion; and how the BAT imaging procedure itself affected the prostate position. Prostate motion relative to its position during simulation was calculated by subtracting the EPI-measured isocenter shifts from the corresponding BAT-defined isocenter shifts. BAT reproducibility was measured by taking a verification BAT image after the patient was moved according to the initial BAT-defined isocenter shifts. Intrafraction prostate motion was measured by repeating BAT imaging at the end of a treatment fraction. The BAT imaging effect on prostate position was studied by examining the effect of suprapubic pressure on seed position in patients after a seed implant. RESULTS The mean BAT isocenter shifts for prostate motion were 0.32 +/- 0.46 cm in the lateral, 0.31 +/- 0.73 cm in the superoinferior, and 0.32 +/- 0.56 cm in the AP directions. Isocenter shifts obtained from EPI measurements were significantly smaller, with a mean of 0.05 +/- 0.24 cm in the lateral, 0.01 +/- 0.11 cm in the superoinferior and -0.11 +/- 0.29 cm in the AP directions. This larger shift seen by BAT was due to prostate motion. For BAT reproducibility, the results showed that for BAT verification images, 90% of the lateral shifts were <0.2 cm, 93% of the superoinferior shifts were <0.3 cm, and 83% of the AP shifts were <0.2 cm. The mean isocenter shift (intrafraction localization error) during patient treatment fraction was 0.02 +/- 0.28 cm in the lateral, 0.04 +/- 0.48 cm in the superoinferior, and 0.0 +/- 0.32 cm in the AP direction. The BAT procedure itself induced an average motion of 1 mm in the AP and superoinferior directions. CONCLUSIONS Prostate patient setup verification on the basis of bony anatomy position does not reflect the actual prostate position. BAT ultrasound target alignment provides a real-time prostate localization system that may make it possible to measure prostate position variations and reduce margins.