Endorectal Balloon in Stereotactic Body Radiation Therapy (sbrt) for Early-Stage Prostate Cancer: A Planning and Dosimetry Analysis

The use of an endorectal balloon (ERB) has been reported to significantly reduce intra-fractional prostate motion during standard fractionated intensity-modulated radiation therapy (IMRT) for prostate cancer. Recently, hypofractionated IMRT and SBRT strategies are under investigation; however, to date, there have been no dosimetric analyses on the use of ERB with these approaches. This study investigates the dose-volume characteristics for the rectum when using ERB as part of SBRT planning for early-stage prostate cancer. Twelve patients with prostate cancer who had received conventional IMRT at our institutions were included in this study. Six patients (Group-ERB) had been planned and treated with ERB containing 60-100 cc of water, and the other six (Group-noERB) without ERB. For each case, an optimized SBRT plan was generated by using sequential optimization in TPS according to the 5 fraction (5 x 7.25 Gy) dose-specification and dose-volume constraints of the RTOG 0938 randomized phase II study of hypofractionated RT for early-stage prostate cancer. Dosimetric characteristics for the rectum were compared between the two groups, including maximum dose, mean dose for the entire rectum and the anterior half of the rectum, and the percentages of rectal volume and anterior-half rectal volume receiving 50%, 80%, 90% and 100% of the prescription dose (36.25 Gy). While the maximum dose to the rectum was comparable between the two groups (range 37.8-39.9 Gy), the mean dose to the entire rectum was lower for Group-ERB (10.3 Gy, range 8.2-13.5 Gy) than Group-noERB (12.9 Gy, range 11.4-15.1 Gy). The mean dose to the anterior half of the rectum was also lower for Group-ERB (15.0 Gy, range 13.0-19.5 Gy) than Group-noERB (16.2 Gy, range 12.2-19.1 Gy). A consistent dosimetric advantage was also noted in the rectal volume receiving 50%, 80%, 90% and 100% of the prescription dose for Group-ERB compared to Group-noERB. The mean V50%, V80%, V90% and V100% of the entire rectum for Group-ERB was 16.7%, 6.3%, 3.5% and 0.9%, in comparison to 26.3%, 10.9%, 6.3% and 1.4% for Group-noERB. Similarly, the mean V50%, V80%, V90% and V100% of the anterior half of the rectum for Group-ERB was 31.6%, 11.8%, 6.6% and 1.6%, in comparison to 39.7%, 18.4%, 10.6% and 2.6% for Group-noERB. Consistent rectal dose-volume improvements were observed with the use of endorectal balloons in five fraction SBRT treatment planning for early-stage prostate cancer. Confirmation of these dosimetric results with additional cases is warranted before investigating the impact of ERB on minimizing acute and long-term rectal toxicity.