Setup uncertainties for inter-fractional head and neck cancer in radiotherapy

PURPOSE The aim of this study is to determine the inter-fractional motion of cervical spine in radiotherapy (RT). MATERIALS AND METHODS Eleven localized head and neck cancer patients who were treated from April 2014 to September 2015 were evaluated. Every patient underwent 3 times of computed tomography (CT) simulation with equivalent setting. Left-right (LR, x) and antero-posterior (AP, z) directional shift of cervical spine were evaluated using 33 number of CT image. In regard to random error, geometric changes were evaluated by 22 data set (compared the first obtained CT to second or third CT) by one-sample T test. Systemic error was evaluated by each patients' data set (11 pairs) by paired T test. RESULTS The mean random error of LR and AP translational shift of cervical spine were -0.39 ± 3.24 mm and -0.57 ± 0.99 mm respectively. The mean random error of translational change of AP direction showed statistical significance (p = 0.014). The mean random error of x and z rotational shift were -0.07 ± 0.29° and -0.05 ± 0.35°, respectively. The mean systemic error of translational shift of LR and AP direction were -0.64 ± 2.57 mm and -0.33 ± 1.22 mm, respectively. The mean systemic error of rotational shift of x and z were 0.01 ± 0.18° and -0.27 ± 0.33°, respectively. The mean systemic error of rotational changes of z direction showed statistical difference (p = 0.022). CONCLUSIONS We have to be aware of the inter-fractional motion of cervical spine in head and neck RT and give enough margins in RT planning.