Is an Endorectal Balloon Beneficial for Rectal Sparing After Spacer Implantation in Prostate Cancer Patients Treated With Hypofractionated Intensity-modulated Proton Beam Therapy? A Dosimetric and Radiobiological Comparison Study

BackgroundThe aim of this study is to examine the dosimetric influence of endorectal balloons (ERB) in rectal sparing in prostate cancer patients with implanted hydrogel rectum spacers treated with dose escalated/hypofractionated intensity modulated proton beam therapy (IMPT). MethodsTen patients with localised prostate cancer included in the ProRegPros study and treated at our centre were invastigated in this study. All patients underwent a placement of hydrogel rectum spacers before planning. Two planning CTs (with and without 120 cm3 fluid-filled ERB) were acquired for each patient. Dose prescription was set according to the used simultaneous integrated boost strategy with 72 Gray (Gy)/2.4 Gy/5 x weekly to prostate+1 cm of the seminal vesicle, and 60 Gy/2 Gy/5 x weekly to prostate+2 cm of the seminal vesicle. Planning with two lateral-opposed IMPT beams was performed in both CTs. Rectal dosimetry values including dose-volume statistics and normal tissue complication probability (NTCP) in both plans were compared (non-ERB plans vs. ERB plans).ResultsFor ERB plans compared to non-ERB, the reductions were 8.51 ± 5.25 Gy (RBE) (p= 0.000) and 15.76 ± 11.11Gy (P= 0.001) for the mean and the median rectal dose, respectively. No significant reductions in rectal volumes receiving high dose levels were found. The use of ERB resulted in significant reduction in rectal volume receiving 50 Gy (RBE), 40 Gy (RBE), 30 Gy (RBE), 20 Gy (RBE), and 10 Gy (RBE) with P values of 0.034, 0.008, 0.003, 0.001, and 0.001, respectively. For the anterior rectum, no differences between ERB and non-ERB plans were observed. For the posterior rectum, ERB reduced rectal volumes received 30 Gy (RBE), 20 Gy (RBE), and 10 Gy (RBE) with P values of 0.019, 0.003, and 0.001, respectively. No significant reductions in mean or median rectal toxicity (Late rectal bleeding ≥2, necrosis/stenosis, and Late rectal toxicity ≥ 3) were observed when using the ERB according to NTCP models.ConclusionThe ERB reduced rectal volumes exposed to intermediate/low dose levels. However, no significant reduction in rectal volumes receiving high/intermediate doses could be observed. No benefit but also no disadvantage of the ERB for late rectal toxicity was found according to available NTCP models.