Impact of the use of an endorectal balloon on rectal dosimetry during stereotactic body radiation therapy for localized prostate cancer.

PURPOSE: Endorectal balloons may be of benefit during stereotactic body radiation therapy of the prostate to limit intrafraction prostatic motion and potentially minimize rectal toxicity. We evaluated the effect of the endorectal balloon (ERB) on rectal dosimetry, specifically the absolute volume of rectum receiving high dose. METHODS AND MATERIALS: Eleven patients with localized prostate cancer underwent stereotactic body radiation therapy planning with computed tomography simulation with and without a RadiaDyne ERB inflated with 60 mL of water. Prescription dose was 3625 cGy in 5 fractions of 725 cGy. The V3600 (volume receiving 3600 cGy), V3440 (volume receiving 3440 cGy), and volume receiving 50% of the prescription dose were calculated for both the rectum and rectal wall. Repeat plans were generated using smaller planning target volume margins (reduced to 1 mm from 3 mm posteriorly) and after virtually replacing the water-filled ERB with air equivalent density. Comparisons were made using the Wilcoxon signed-rank test.
RESULTS: The rectal V3600 and V3440 were significantly lower without ERB than with water-filled ERB using standard 3-mm posterior margin (P = .003 for both V3600/V3440), water-filled ERB using reduced 1-mm posterior margin (P = .016 and .003), or air-filled ERB (P = .003 and .004). Regarding the rectal wall, V3600 and V3440 were also significantly lower without ERB than with any ERB, except when using the water-filled ERB with reduced posterior margin (P = .328). The volumes of rectum and rectal wall receiving lower dose (volume receiving 50% of the prescription dose) were not significantly greater without the ERB.
CONCLUSION: We found an increase in the volume of rectum and rectal wall receiving high dose radiation utilizing an ERB. Consideration in using an ERB should account for potential increased rectal dose and subsequent toxicity.