Online ultrasound image guidance for radiotherapy of prostate cancer: impact of image acquisition on prostate displacement.

AIM Numerous studies reported the use of ultrasound image-guidance system to assess and correct patient setup during radiotherapy for prostate cancer. We conducted a study to demonstrate and quantify prostate displacement resulting from pressure of the probe on the abdomen during transabdominal ultrasound image acquisition for prostate localization. MATERIAL AND METHODS Ten healthy volunteers were asked to undergo one imaging procedure. The procedure was performed in a condition that simulates the localization of prostate during online ultrasound guidance. A 3D ultrasound machine was used. The procedure started with the placement of the probe on the abdomen above the pubis symphysis. The probe was tilted in a caudal and posterior direction until the prostate and seminal vesicle were visualized. The probe was then fixed with a rigid arm, which maintained the probe in a static position during image acquisition. The probe was then moved, in a short time, stepwise toward the prostate, acquiring images at each step. The prostate and seminal vesicles were identified and selected in all planes. The first 3D volume was used as reference 1, to which all other scans were matched using a gray value matching algorithm. RESULTS Prostate motion was quantified as a 3D translation relative to the patient coordinate system. The resulting translations represented the amount of prostate movement as a function of probe displacement. Between 7 and 11 images were obtained per volunteer, with a maximal probe displacement ranging between 3 and 6 cm. Prostate displacement was measured in all volunteers for all the probe steps and in all directions. The largest displacements occurred in the posterior direction in all volunteers. The absolute prostate motion was less than 5 mm in 100% of the volunteers after 1 cm of probe displacement, in 80% after 1.5 cm, in 40% after 2 cm, in 10% after 2.5 cm, and 0% after 3 cm. To achieved a good-quality ultrasound images, the probe requires an average displacement of 1.2 cm, and this results in an average prostate displacement of 3.1 mm. No correlations were observed between prostate motion and prostate-probe distance or bladder size. CONCLUSION Probe pressure during ultrasound image acquisition causes prostate displacement, which is correlated to the amount of probe displacement from initial contact. The induced uncertainty associated with this process needs to be carefully evaluated to determine a safe margin to be employed during online ultrasound image-guided radiotherapy of the prostate.