Daily setup uncertainty analysis for craniospinal irradiation using helical tomotherapy

Setup deviations of a craniospinal irradiation (CSI) patient cohort previously treated with helical tomotherapy were used to derive the optimal planning target volume (PTV) margin for CSI patients treated in the supine position.

The daily setup shifts of 27 CSI tomotherapy patients as determined using pretreatment megavoltage computed tomography verification for a total of 454 fractions were evaluated. Translational displacements in the mediolateral (ML), craniocaudal (CC), and anteroposterior (AP) directions were used to assess the systematic and random setup errors, and to derive the PTV margin required when daily image guidance is unavailable.

The individual patient mean shifts and the corresponding standard deviations in the ML, CC, and AP directions were -0.5 ± 2.1 mm, 1.0 ± 2.7 mm, and 0.7 ± 1.1 mm, respectively. The random errors in the corresponding directions were 1.9 mm, 1.9 mm, and 2.2 mm. The PTV margins required in the absence of daily imaging were 3.7 mm to the right, 4.7 mm to the left, 4.4 mm inferior, 6.4 mm superior, 1.6 mm posterior, and 3.0 mm anterior.

The current PTV margin of 3.0 mm is sufficient to ensure clinical target volume coverage for CSI tomotherapy patients treated with daily megavoltage computed tomography imaging. When pretreatment verification imaging is not used to lower the setup uncertainty, a larger PTV margin of up to 6.4 mm in the CC direction will be needed.