Inferences about prostate intrafraction motion from pre- and posttreatment volumetric imaging.

PURPOSE: To evaluate the ability of rectal filling, bladder filling, and prostate localization from pre- and posttreatment volumetric imaging to predict prostate intrafraction motion.

METHODS AND MATERIALS: Pre- and posttreatment cone beam computed tomography images (CBCTs) and intrafractional kV fluoroscopy were acquired at each fraction for 20 prostate patients in supine position, totaling 374 fractions available for analysis. Rectal and bladder filling status were evaluated for each CBCT, and correlation with prostate intrafraction motion measured from kV fluoroscopy was performed. The accuracy of pre and posttreatment prostate localization to predict intrafraction motion was evaluated.

RESULTS: Rectal filling status was a significant predictor of prostate intrafraction motion (p <0.001), and gas volume was correlated with the maximum vector displacement at MV delivery with a correlation coefficient (cc) of 0.37 and p <0.001. Prostate motion was greater for patients who consistently had gas volume >0.5 cm(3) within the imaged region (cc = 0.52, p = 0.028). A weak relationship was found between bladder filling and posterior prostate drift for fractions with gas volume
CONCLUSIONS: Rectal filling is a significant predictor of prostate intrafraction motion, whereas bladder filling is of limited usefulness. Pre- and posttreatment localization can provide a reasonable estimate of prostate motion during MV delivery when intrafraction localization is not available, with an error of 95% within 3.1 mm.