Constraint Analysis and Bladder Wall Low-Dose-Constraint Reduction in Five-Fraction Urethra-Sparing Prostate SBRT

Aim: To compare and analyze dose constraints and target coverage results and to reduce Bladder Wall (Bwall) V18.12 for prostate Stereotactic-Body Radiation Therapy (SBRT) when Seminal Vesicles (SSVV) are included or not. Several indicators based on intersection volumes are obtained to predict constraint fulfillment. Background: Due to prostate’s low alpha-beta ratio and the possibility of increasing the therapeutic ratio several moderate and extreme hypofractionation schemes have been proposed. The scheme selected was a fivefraction urethra-sparing prostate SBRT. Materials and Methods: 150 patients divided into two groups according to the inclusion of SSVV in PTV or not were analyzed. Histograms, average values, standard deviations and degrees of fulfillment were obtained for each constraint or goal and group. A possible reduction of the Bwall V18.12 was addressed by re-optimizing fifty randomly chosen patients. Predictors of constraint fulfilling were obtained by using the intersections of Bwall and Rectum Wall (Rwall) with the PTV. Results: Significant differences in Rwall V32.62 and V29 were obtained when evaluating the influence of SSVV inclusion. A reduction of 12% in the Bwall V18.12 constraint was achieved without compromising coverage and OARs doses. No dependence on the inclusion of SSVV was found. Conclusions: Statistically significant differences have been found in Rwall intermediate-dose constraint when SSVV was included. A reduction of 12% in the Bwall V18.12 constraint has been achieved without compromising the PTV coverage and the rest of OARs constraints. Constraint fulfillment predictors could be useful to evaluate the feasibility of prostate SBRT prior to the planning process for every single patient.