2.5. Positioning performance

The accuracy of using sCT for position verification was checked for 14 prostate patients by comparing sCT/ CBCT registration results to CT/CBCT registrations, during 3 fractions (the first, mid, and last treatment day). Fusions were estimated within a box including bony pelvic anatomy, excluding femoral heads. The sCT and CT images were registered using auto‐match towards CBCT images with a manual correction as in routine. Positioning accuracy was calculated to six degrees of freedom (3 translational and 3 rotational axes) in left‐right (LR), anterior‐posterior (AP) and superior‐inferior (SI) directions. Each vector was calculated using either the CT or the sCT as reference and mean differences were calculated using the following equation:

where ΔVCT/sCT is the intrinsic offset between CT and sCT.