Purpose: To study prostate motion from 4,154 ultrasound alignment fractions on 130 prostate patients treated with conformal radiotherapy or intensity-modulated radiation therapy at the University of Nebraska Medical Center.
Methods and materials: Each prostate patient was immobilized in a vacuum cradle. Daily treatment was verified by ultrasound scan after laser setup with skin marks and before radiation delivery by the same physician responsible for anatomic delineation during planning. Directional statistics were employed to test the significance of shift directions.
Results: Polar histograms showed the prevalence of prostate motion in superior-posterior directions. The average direction was about 27 degrees from the superior axis. The average changes of prostate position in superior to inferior (SI), anterior-posterior (AP), and left to right (LR) directions and in radial distance were 0.25, -0.13, 0.03, and 0.92, cm respectively. Our data indicated that prostate motion was not patient specific, and its average magnitude remained virtually unchanged over time. Recommended planning target volume (PTV) margins for use without ultrasound localization were 0.90 cm in SI, 1.02 cm in AP, and 0.80 cm in LR directions.
Conclusion: Ultrasound localization revealed a predominance of prostate shift from planning position in the superior-posterior direction, with an average closer to the superior axis. The motion data provides recommended margins for PTV.