During helical tomotherapy, gantry angle accuracy is one of the vital geometric factors that assure accurate dose delivery to the target and organs at risk adjacent to it. The purpose of this study is to investigate the dosimetric impact of gantry angle misalignment on the target volume and critical organs during helical tomotherapy treatment. Five prostate cases were chosen to calculate the effects of gantry angle deviations on both patient-specific delivery quality assurance (DQA) and helical tomotherapy treatment plans. For DQA plans, the cheese phantom was rotated for up to +/- 5° from the preset position to simulate the gantry angle deviations during tomotherapy. Point doses at 5 mm below the isocenter and the dose distribution for each gantry angle were measured and reconstructed, respectively. For helical tomotherapy treatment plans, the same gantry misalignment effect was simulated by adjusting the automatic roll correction for up to +/- 5° using Planned Adaptive software. Variations of dose volume histograms (DVHs) and isodose lines were evaluated for both target and critical organs. There was no significant difference found, however, among the point dose measurements for gantry rotation up to +/- 5° in DQA plans. Shifts of isodose lines could be observed for gantry rotations larger than +/- 27°. Dosimetric discrepancies (less than 2%) were also found among DVHs of the PTV in the cases when gantry angle misalignment was larger than +/- 2°. However, for DVHs of either bladder or rectum under different gantry rotations, no significant differences were detected when gantry angle errors were up to +/- 5°. In summary, point dose measurements alone cannot reveal the dosimetric deviation due to gantry angle misalignment in DQA plans. For a 5° gantry deviation, the dose to PTV increased by 0.5% comparing to the planned dose. The influence on organs at risk, i.e., rectum and bladder, is also negligible. Further studies are needed on the dosimetric impacts of gantry angle deviations for other treatment sites.

Key words: Tomotherapy; Treatment uncertainty; Quality assurance; Adaptive planning; and Gantry angle uncertainty.