Dosimetric impacts of endorectal balloon in CyberKnife stereotactic body radiation therapy (SBRT) for early-stage prostate cancer

Abstract

Purpose: In SBRT for prostate cancer, higher fractional dose to the rectum is a major toxicity concern due to using smaller PTV margin and hypofractionation. We investigate the dosimetric impact on rectum using endorectal balloon (ERB) in prostate SBRT.

Materials and methods: Twenty prostate cancer patients were included in a retrospective study, ten with ERB and 10 without ERB. Optimized SBRT plans were generated on CyberKnife MultiPlan for 5 × 7.25 Gy to PTV under RTOG-0938 protocol for early-stage prostate cancer. For the rectum and the anterior half rectum, mean dose and percentage of volumes receiving 50%, 80%, 90%, and 100% prescription dose were compared.

Results: Using ERB, mean dose to the rectum was 62 cGy (P = 0.001) lower per fraction, and 50 cGy (P = 0.024) lower per fraction for the anterior half rectum. The average V_50% , V_80% , V_90% , and V_100% were lower by 9.9% (P = 0.001), 5.3% (P = 0.0002), 3.4% (P = 0.0002), and 1.2% (P = 0.005) for the rectum, and lower by 10.4% (P = 0.009), 8.3% (P = 0.0004), 5.4% (P = 0.0003), and 2.1% (P = 0.003) for the anterior half rectum.

Conclusions: Significant reductions of dose to the rectum using ERB were observed. This may lead to improvement of the rectal toxicity profiles in prostate SBRT.

Keywords: SBRT; CyberKnife; endorectal balloon; prostate cancer; rectum toxicity.

Figure 1

The endorectal balloon (ERB) used for patients included in this planning study.

Figure 2

Typical dose distributions of CyberKnife prostate SBRT plans for a case from the ERB group (left) and a case from the noERB group (right). The top row for axial view, bottom row for sagittal view. The anterior half of the rectum contours were obtained by bisecting the full rectum contours from the midline slice‐by‐slice.

Figure 3

DVHs for a typical case in the ERB group (a) and in the noERB group (b).

Figure 4

DVHs profiles for the rectum volume (left) and anterior half rectum volume (right) in the intermediate‐to‐high dose region 50%–100% of prescription dose 36.25 Gy.