Evaluation Of Kv Cone Beam Computed Tomography-Based Setup Errors In Stereotactic Body Radiation Therapy For Liver Cancer

Due to low contrast, motion artifacts, out-of-target fiducial placement, cone beam computed tomography (CBCT) guided liver radiation therapy may render extra difficulties in target localization. The goal of this work is to evaluate the setup errors in stereotactic body radiation therapy (SBRT) treatment of liver tumors using kV CBCT and to investigate the feasibility of liver-specific margin. A total of 40 CBCT scans of patients with liver malignancies treated with SBRT were reevaluated offline, by matching them to the planning CT scans with a 6-degree rigid registration method as well as with a free-form deformable registration method. The planning target volume (PTV) contours were transferred to the daily CBCT scans based on the registration results and compared with those determined by pretreatment matching. The shifts for the centers of mass (COMs) of the PTVs were calculated. The distances between the COMs determined by pretreatment matching and the offline 6-degree rigid matching in A-P, R-L, and S-I directions were 1.7±2.3 mm, 1.3±1.2 mm, and 5.3±4.3 mm, respectively. The rotational errors alone determined by offline rigid registration were −0.35°±1.8°, 0.72°±2.66°, and 1.69°±1.93°, for pitch, roll, and yaw, respectively. Analysis of CBCT images showed that 87.5%, 55%, and 10% of treated fractions had setup errors of >2 mm, >5 mm, and >10 mm, respectively, in any one of the axes. The distances between the COMs determined by online matching and the deformable registration in A-P, R-L, and S-I directions were 2.2±2 mm, 2.5±2.2 mm, and 4.6±4.2 mm, respectively. The offline 6-degree rigid registration and the deformable registration showed similar results for kV CBCT-based setup errors in liver SBRT. The uncertainties in pretreatment matching of CBCT to planning CT require extra attention about the safety margin, especially in the S-I direction.