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.