The extent and serial pattern of interfractional variation in patients with whole pelvic irradiation: A study using a kilovoltage orthogonal on-board imager

The purpose of this study is to assess the extent and serial pattern of setup error of conventional fractionated whole pelvic irradiation using a kilovoltage on-board imager. The daily on-board images of 69 patients were matched with the digitally reconstructed radiographs of simulation on the basis of pelvic bony structure. The shifts along x- (lateral), y- (longitudinal), and z- (vertical) axes, and the 3D vector, were measured. The shift between an origin of the first fraction and each fraction (Δshift^1st) and the shift between an isocenter of simulation and each fraction (Δshift^Sim) were calculated. To evaluate serial changes, the shifts of each fraction were classified into four consecutive sessions, and an ANOVA and chi-square test were used. The systematic error of the Δshift^Sim and Δshift^1st were 2.72 and 1.43mm along the x-axis, 2.98 and 1.28 mm along the y-axis, and 4.26 and 2.39 mm along the z-axis, respectively. The Δshift^Sim and Δshift^1st ≥ 5 mm of the 3D vector occurred in 54.3% and 23.1%, respectively. The recommended margins to cover setup error in case of using Δshift^1st were 3.81, 3.54, and 6.01 mm along x-, y-, and z-axes, whereas those using Δshift^Sim were 6.39, 6.95, and 9.95 mm, respectively. With the passage of time, the Δshift^1st ≥ 5 mm of 3D vector and along any axis in supine setup increased from 14.1% for first session to 22.5% for fourth session (p=0.027) and from 10.8% to 18.5% (p = 0.034), respectively. In prone setup, first session was better than others in the Δshift^1st ≥ 5 mm of 3D vector and along any axis. It is expected that the correction using the on-board images on the first fraction improves geometrical uncertainties and reduces the margin for target coverage. Daily continuous OBI follow-up during conventional fractionated pelvic irradiation can increase the reproducibility and be more effective in the late period.