External beam SBRT planning dose and dose reproducibility comparison between with and without endorectal balloon in localised prostate cancer

Abstract

Purpose/Objective(s): The main objective of this study is to evaluate the planning target volume (PTV) and organ at risks (OARs) reproducibility from planning to treatment when endorectal balloon (ERB) is applied in localised prostate cancer, through comparing the dose-volume discrepancies in treatment between with and without ERB application. The second objective is to evaluate the dosimetric impact by ERB to PTV and OARs in localised prostatic SBRT planning. Dose to PTV and OARs were compared between with and without ERB application. Materials/Methods: 20 male patients diagnosed with localised prostate cancer (staging: T1-T2b) were retrospectively recruited and 3 patients were excluded as they were extreme cases. 9 patients who underwent SBRT treatment with ERB application, and 8 patients who underwent conventional VMAT treatment without ERB application. A SBRT plan was generated for all patients and the SBRT planning dose was then compared between two groups. For organ reproducibility, prescribed percentage of all SBRT plans were normalised that V36.25 of PTV was 95% for fair comparison. To calculate treatment dose, deformable image registration was used for deforming planning CT image referenced to cone beam computed tomography images (3 images sets with each set was weekly chosen in first three weeks). Re-calculating the dose with the deformed planning CT images represented treatment dose to patients. Dose-volume discrepancies were calculated as the absolute difference between treatment dose and planning dose. Results: Treatment with ERB application showed better rectal sparing and PTV coverage in SBRT planning. V34.4, V32.625, V29 and V18.125 of rectum in ERB group were significantly lower (p < 0.05) with mean = 3.140%, 5.060%, 8.351% and 25.267% versus 6.479%, 9.174%, 14.712% and 39.529%, respectively). V36.25 of PTV in ERB group was significantly higher (mean = 97.095% versus 95.740% respectively. Besides, ERB showed better rectum and prostate reproducibility in prostatic radiation therapy. The mean of all rectal dose discrepancies in all selected treatment days in were lower in ERB group. Among them, V34.4 and V29 discrepancies in first day (mean = 1.026% and 1.567% versus 4.723% and 5.188 respectively), V32.625 and V29 discrepancies in second day (mean = 1.391% and 1.590% versus 4.241% and 4.721% respectively), and V32.625 discrepancy in third day (mean = 1.234% versus 4.141% respectively) of ERB group were significantly lower. For PTV, Dmin and V34.4 discrepancies in second selected day in ERB group was significantly lower (mean = 4.138 Gy and 1.915% versus 9.256 Gy and 3.338% respectively). Conclusions: Treatment with ERB application shows better rectal sparing effect by pushing the posterior rectal wall away from high dose zone. Rectal dose reduction may provide higher possibility to improve PTV coverage. Also, treatment with ERB application improves rectum and prostate reproducibility, through reproducing the same rectal shape and rectal position relative to prostate, and also the same exerting force to prostate.