The technological development of radiotherapy equipment enabled the changing of the dose rate over time during irradiation. Further, techniques, such as volumetric modulated arc therapy (VMAT), are becoming more widespread. This study aimed to assess the changes in radiation isocenter and beam alignment with different dose rates and investigate the dose error during VMAT plan delivery.

The Winston–Lutz (WL) test and three-dimensional water phantom profiles were measured at different dose rates using 4, 6 and 10 MV and flattening filter-free (FFF) at 6 and 10 MV of X-rays. Furthermore, the change in alignment with the dose rate was reproduced through beam parameter adjustments, and the change in dose difference (DD) in the VMAT plan was assessed.

The WL test revealed a 0·1–0·3 mm radiation isocenter displacement with dose rate. Further, the beam profile measurement results demonstrated a 0·3 mm beam centre position displacement in both the lateral and gun-target directions caused by the change in dose rate. VMAT plan delivery with a beam whose centre position changed by 0·3 mm caused a 0·2–3% decrease in the previous DD rate.

The radiation isocenter and beam alignment changed based on the dose rate. Evaluating the change in beam alignment at multiple dose rates is recommended when performing irradiation with different dose rates.