Analyzing the patient isocenter positional accuracy with portal imaging acquired at different gantry angles and its dosimetrical impact on the dose delivery for head and neck cancers.

Objective: This study aimed to analyze the effect of different gantry angles on portal imaging in terms of isocenter shifts and their dosimetric impact on dose delivery.

Methods: Thirty patients with head and neck cancers were prospectively selected for this study. The reference anterior to posterior (AP) digitally reconstructed reference radiograph image was obtained at a gantry angle of 0°. The AP portal images were acquired at angles of 0°, 0.5°, 1°, 1.5°, and 2.0°. The average deviation of the isocenter shift with respect to the zero-gantry angle was calculated. To check the dosimetric effects, the reference fluence was compared with different fluences measured at different isocenter shifts.

Results: The average isocenter shift differences in the lateral direction were 0.7 mm, 1.3 mm, 1.9 mm, and 2.5 mm. The average difference was <±0.1 mm for isocenter shift in the longitudinal direction. The results of the statistical analysis showed that the average isocenter shift and gamma pass rate with respect to the different isocenter position errors were significant in the lateral direction.

Conclusion: The results of this study showed that as the gantry angle increased, the isocenter shifted and the percentage of deviation in the lateral direction increased.