Objective  To discuss the role of the helical tomotherapy(HT) system setup function to correct positional errors and analyze error sources. 

Methods  The mega-voltage computed tomography (MVCT) image information of 36 patients with thoracic tumors were investigated. The scanned images obtained from the initial resetting and the second setup resetting were aligned using kilo-voltage com-puted tomography (KVCT), and localization errors were recorded. The localization errors between the initial and the second fractionated resetting were evaluated, and a model of repeated measurement mixed linear regression was developed to analyze error sources. 

Results  Except for the translation error in the lateral directions, the average error of the setup function after reset was generally smaller than that of the initial reset. Especially in the vertical direction, the error of the setup function after re-set was significantly smaller than that of the first reset (0.17±1.14 vs. 5.04±2.43, P< 0.001). Multi-factor statistical analysis showed that BMI might induce translation and rotation in the vertical direction and Roll angle, the regression coefficient β were 0.191 (P=0.001) and 0.034 (P=0.040), respectively. Moreo-ver, translational errors in the vertical direction might be influenced by fractionated dose and resetting approaches. Gender, age and clinical diagnosis were not found to result in obvious effects in localization errors in this study (P>0.05). 

Conclusion  Setup function can correct localization errors, especially translational errors in the vertical direction. The localization accuracy in the vertical direction may be influenced by BMI, fractionated dose and resetting approaches for patients with thoracic tumors having radiotherapy.

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[Li H. Study on MVCT image registration method and setup errors in Helical Tomotherapy[D]. Guang-zhou: Southern Medical University, 2021]