Minimal benefit of an endorectal balloon for prostate immobilization as verified by daily localization

We wanted to investigate whether using an endorectal balloon (ERB) in lieu of image guidance is reasonable. We compared daily prostate motion in 2 cohorts of patients with fiducial markers implanted in the prostate, one group with the ERB and the other without. Twenty-nine patients were treated using intensity-modulated radiation therapy: 14 with an ERB, and 15 without. All had fiducial markers placed in the prostate. We reviewed the daily displacements necessary to place the isocenter on the prostate as determined by portal imaging. In addition, we used the data to determine whether there is a change in prostate motion over the treatment course. The average prostate displacement for patients treated without an ERB was slightly greater than the average displacement for patients treated with the ERB. However, the difference observed with the ERB was not statistically significant (p > 0.05). The margins necessary to encompass the prostate 95% of the time for the patients treated without an ERB in the lateral, cranio/caudal, and anterior/posterior dimensions would be 4.8, 12.1, and 15.2 mm, respectively. When using the ERB, the margins necessary would be 4.1, 10.4, and 11 mm, respectively. Prostate motion in the anterior-posterior direction actually increased over the course of treatment in patients without an ERB. This increase was prevented by use of the ERB. Day-to-day variability of the position of the prostate is reduced in all dimensions with the water-filled ERB, but not significantly statistically. Use of the water-filled ERB did not obviate performing some form of image guidance daily.