Endorectal balloon (ERB) in helical tomotherapy (HT) for localized prostate cancer: a case report of dosimetric analysis

Radiation therapy (RT) for localized prostate cancer yields oncological outcomes similar to those following radical prostatectomy, but is associated with more anorectal toxicity. An endorectal balloon (ERB) has been utilized to decrease the incidental dose to the rectal wall. However, few studies analyzed whether the ERB can further spare the rectum in helical tomotherapy (HT), which by itself can be used to treat prostate cancer while minimizing irradiation of surrounding critical tissues. Here, we report a 64-year-old man with pathologically proven prostate adenocarcinoma (stage T2cN0M0). He underwent definitive RT using HT with a hypofractionated scheme of 70 Gy in 28 fractions. Simulation CT was performed twice: with and without ERB application. The ERB was filled with 70 mL of air. Two intensity-modulated RT (IMRT) plans were generated for each CT image set (with and without ERB) and compared about the dose to the anorectum. The rectal volume receiving ≥40 Gy (V40Gy) was reduced from 43.4% to 34.6% with ERB use (20.3% reduction). This reduction rate increased continuously up to V70Gy (48.2% reduction). The anal volume reduction was approximately 50% from V5Gy to V15Gy. The patient tolerated all ERB insertions well and there were no severe acute toxicities. ERB had a further anorectal-sparing effect in this case of prostate cancer treated by highly conformal HT, beyond the generally recommended dose-volume constraints of hypofractionated IMRT.