Dosimetric impact of translational and rotational setup errors for spine stereotactic body radiotherapy: A phantom study

Abstract This study aimed to investigate experimentally the effect of translational and rotational setup errors on 3-dimensional dose distributions by using the gamma index and dose volumetric indices for spine stereotactic body radiotherapy. Treatment plans were designed in accordance with the Radiation Therapy Oncology Group (RTOG) 0631 protocol. Measurements were taken using a Delta4 phantom (ScandiDos, Uppsala, Sweden). Setup errors were generated using the HexaMotion 6D moving platform (ScandiDos). Dose distributions in the presence of setup errors were evaluated, according to the γ passing rate with the 3% and 2 mm criteria (γ 3%/2 mm ) and dose volumetric indices (D 90 for the target volume and D 2 for the spinal cord), using the Delta4 device (ScandiDos). The sensitivity coefficient, which represented the correlation between the γ 3%/2 mm passing rate and dose volumetric indices, was determined to assess robustness against setup errors. Rotational setup errors of 2° were equivalent to translational setup errors of 2 mm for the γ 3%/2 mm passing rate, D 90 for the target, and D 2 for the spinal cord. D 90 for the target had low robustness against a translational setup error in the vertical direction and a rotational setup error in the pitch direction. D 2 for the spinal cord was sensitive to a translational setup error in the lateral direction and a rotational setup error in the roll direction. The positioning accuracy of the rotational setup error, corresponding to the tolerance level of image-guided radiotherapy in the RTOG 0631 protocol, was required to be ≤ 2°.