Plan generation and statistical analysis
±, CF
This study utilized a single core laboratory for analysis of CT images and generation of the DVH data. All target and organ at risk (OAR) volumes were defined and contoured by the same physician. The Core‐Lab physician delineated the target volumes and critical structures on both the pre‐ and post‐implant CT datasets according to predefined guidelines to assure consistency of structure delineation between the two datasets. For each patient, a pre‐ and post‐implant treatment plans were generated using identical dose specifications. Plans were generated containing two fields assigned to the Varian TrueBeam linear accelerator and with 6MV photon energy. The isocenters for both fields are placed within the Planning Target Volume (PTV). Both fields have full arc properties. The chosen gantry rotations are as follows: 179° to 181° (counter‐clockwise) for Field 1, and 181° to 179° degrees (clockwise) for Field 2. The collimator angles used for the fields are 45° for Field 1, and 315° for Field 2 and procedures, assuming same classification of clinical stage for all patients.Treatment plans were optimized using the RapidPlan (knowledge‐based treatment planning system; Varian, Palo Alto, California, USA) to ensure consistency, standardized treatment planning process, and minimize intra‐observer variability. The dose was calculated Varian's AAA algorithm version 15.606. Two treatment plans were used for each patient: VMAT treatment plans were generated for CF of 81 Gy in 45 fractions. The dose constraints to the rectum were generated using RapidPlan's dose volume histogram (DVH) estimation model, which is based upon each patient's individual anatomy. Treatment plans were generated for UH of 36.25 Gy in five fractions. Once again, the rectal dose constraints were generated by RapidPlan's DVH estimation model. Once the plan is normalized such that 98% of the PTV receives 100% of the prescribed dose, the dose statistics were evaluated. For VMAT CF plans rectal V50, V60, V70, and V80 were calculated. For UH plans, a corresponding rectal V22.6, V27.1, V31.37, and V36.25 were established representing 62.5%, 75%, 87.5%, and 100% of the 36.25 Gy prescribed dose.Patient demographic and baseline characteristics between balloon spacer and SpaceOAR are summarized as mean ± standard deviation (SD) for categorical variables and compared using Wilcoxon sum rank test for not normally distribution.The dosimetric variables considered were rectal volume, maximum, and mean dose to the rectum), and volumes (percentage and absolute) of rectum receiving V50, V60, V70, and V80 Gy CF and V22.6 , V27.1 , V31.7, and V36.25 Gy for UH. Wilcoxon sum rank test was used to compare continuous outcomes that are not normally distributed. All reported 