Prediction of Intrafraction Prostate Motion: Accuracy of Pre- and Post-Treatment Imaging and Intermittent Imaging

Purpose: To evaluate whether pre- and post-treatment imaging (immediately before and after a radiation therapy treatment fraction) and intermittent imaging (at intervals during a treatment fraction) are accurate predictors of prostate motion during the delivery of radiation. Methods and Materials: The Calypso 4D Localization System was used to continuously track the prostate during radiation delivery in 35 prostate cancer patients, for a total of 1,157 fractions (28-45 per patient). Predictions of prostate motion away from isocenter were modeled for a pre- and post-treatment imaging schedule and for multiple intermittent intrafraction imaging schedules and compared with the actual continuous tracking data. The endpoint was drift of the prostate beyond a certain radial displacement for a duration of more than 30 s, 1 min, and 2 min. Results were used to evaluate the sensitivity and specificity of these models as an evaluation of intrafraction prostate motion. Results: The sensitivity of pre- and post-treatment imaging in determining 30 s of intrafraction prostate motion greater than 3, 5, or 7 mm for all fractions was low, with values of 53%, 49%, and 39%, respectively. The specificity of pre- and post-treatment imaging was high for all displacements. The sensitivity of intermittent imaging improved with increasing sampling rate. Conclusions: These results suggest that pre- and post-treatment imaging is not a sensitive method of assessing intrafraction prostate motion, and that intermittent imaging is sufficiently sensitive only at a high sampling rate. These findings support the value of continuous, real-time tracking in prostate cancer radiation therapy.