To quantify the advantages of a 6 degrees of freedom (dof) versus the conventional 3- or 4-dof correction modality for stereotactic body radiation therapy (SBRT) treatments. Eighty-five patients were fitted with 5-7 infra-red passive markers for optical localization. Data, acquired during the treatment, were analyzed retrospectively to simulate and evaluate the best approach for correcting patient misalignments. After the implementation of each correction, the new position of the target (tumor?s center of mass) was estimated by means of a dedicated stereotactic algorithm. The Euclidean distance between the corrected and the planned location of target point was calculated and compared to the initial mismatching. Initial and after correction median±quartile displacements affecting external control points were 3.74±2.55 mm (initial), 2.45±0.91 mm (3-dof), 2.37±0.95 mm (4-dof), and 2.03±1.47 mm (6-dof). The benefit of a six-parameter adjustment was particularly evident when evaluating the results relative to the target position before and after the re-alignment. In this context, the Euclidean distance between the planned and the current target point turned to 0.82±1.12mm (median±quartile values) after the roto-translation versus the initial displacement of 2.98±2.32mm. No statistical improvements were found after 3- and 4-dof correction (2.73±1.22 mm and 2.60±1.31 mm, respectively). Angular errors were 0.09±0.93° (mean±std). Pitch rotation in abdomen site showed the most relevant deviation, being -0.46±1.27° with a peak value of 5.46°. Translational misalignments were -0.68±2.60 mm (mean±std) with the maximum value of 12 mm along the cranio-caudal direction. We conclude that positioning system platforms featuring 6-dof are preferred for high precision radiation therapy. Data are in line with previous results relative to other sites and represent a relevant record in the framework of SBRT.

Key words: Lung cancer; Abdominal cancer; Set-up errors; Treatment couch; and SBRT.