For all 10 patients, CBCTs of all fractions were available, in total 240 images. Accurate CBCT-based dose calculation is difficult with the Elekta Synergy CBCT system used in this study because the Hounsfield Units (HU) numbers are not accurate. To enable daily dose distribution calculation for all images, CT HU numbers were mapped to CBCT images by registering the planning CT to CBCT images using DIR (Figure 2) [14 (link)]. For each pre-fraction CBCT image, the planning CT with accurate HU numbers was deformed to represent CBCT images using a B-spline deformable image registration based on intensity values (VelocityAI, version 3.1.0/3.2.0, Varian Medical Systems, Inc., Palo Alto, CA). Of the two available planning CT’s (i.e., full and empty bladder), we selected the CT with the closest bladder volume to the daily anatomy of the CBCT for this step. Prior to the deformable image registration, a rigid registration was performed to match the bony anatomy. The deformable match was visually assessed to ensure that the body contours and the soft tissue matched sufficiently. The quality of CT-to-CBCT deformable registration in the pelvic area using the VelocityAI software was investigated previously and DIR results were reported to be accurate for dose calculation [15 (link),16 ].
Each daily selected plan was used to calculate the corresponding daily dose distribution.