Effect of Dose Rate and Detector Speed Variation on Beam Profile Characteristic in Radiotherapy Dosimetry

flatness and symmetry of the beam profile is essential in radiotherapy quality assurance because these parameters directly affect dose accuracy, treatment safety, and clinical outcomes. This study evaluates the impact of dose-rate variations on beam-profile characteristics, as well as the influence of detector scan speed and positioning speed on flatness and symmetry measurements. Beam profiles—graphical representations of relative dose as a function of distance from the central axis—were acquired at an SSD of 100 cm for a 10 × 10 cm2 field using a semiflex ionization chamber. Measurements were performed at dose rates of 50, 100, 200, 300 and 400 MU/min, and at scan and position speeds of 5, 10, 15, 20 and 25 mm/s. Results show notable deviations in flatness and symmetry at 50 and 100 MU/min compared with the standard 400 MU/min, with differences of up to 2.3% and 5.3% in flatness (crossline and inline, respectively) and 3.5% and 2.4% in symmetry (inline and crossline, respectively) at 50 MU/min, while differences at 100 MU/min were approximately 1% for flatness and up to 1.7% for symmetry. Scan and position speeds also affected flatness and symmetry, though with smaller percentage differences. Overall, beam-profile scanning is recommended at dose rates above 100 MU/min, with scan and position speeds below 15 mm/s to improve flatness and symmetry accuracy.