Real-time Study of Prostate Intrafraction Motion During External Beam Radiotherapy with Daily Endorectal Balloon

Purpose: To prospectively investigate intrafraction prostate motion during radiofrequency-guided prostate radiotherapy with implanted electromagnetic transponders when daily endorectal balloon (ERB) is used.

Methods And Materials: Intrafraction prostate motion from 24 patients in 787 treatment sessions was evaluated based on three-dimensional (3D), lateral, cranial-caudal (CC), and anterior-posterior (AP) displacements. The mean percentage of time with 3D, lateral, CC, and AP prostate displacements>2, 3, 4, 5, 6, 7, 8, 9, and 10 mm in 1 minute intervals was calculated for up to 6 minutes of treatment time. Correlation between the mean percentage time with 3D prostate displacement>3 mm vs. treatment week was investigated.

Results: The percentage of time with 3D prostate movement>2, 3, and 4 mm increased with elapsed treatment time (p<0.05). Prostate movement>5 mm was independent of elapsed treatment time (p=0.11). The overall mean time with prostate excursions>3 mm was 5%. Directional analysis showed negligible lateral prostate motion; AP and CC motion were comparable. The fraction of time with 3D prostate movement>3 mm did not depend on treatment week of (p>0.05) over a 4-minute mean treatment time.

Conclusions: Daily endorectal balloon consistently stabilizes the prostate, preventing clinically significant displacement (>5 mm). A 3-mm internal margin may sufficiently account for 95% of intrafraction prostate movement for up to 6 minutes of treatment time. Directional analysis suggests that the lateral internal margin could be further reduced to 2 mm.