Effects of sagging correction calibration error on radiation therapy equipment using image analysis

Purpose: This study investigates the effect of sagging correction errors on image quality and geometric coordinate accuracy.
Methods: This study utilised the Elekta radiotherapy system, ball bearing (BB), Catphan phantom and MultiMet-WL phantom. Ten distinct flex maps (FMs) were acquired by positioning the BB at the accuracy isocentre and introducing shifts of 0.2, 0.4 and 0.6 mm in the left, table and up directions, respectively. Cone-beam computed tomography images of the Catphan phantom were acquired using 10 FMs. The images were analysed for modulation transfer function (MTF) values and geometric coordinates. Additionally, the Winston–Lutz (W-L) test was conducted under reference couch positions and with a 0.3 mm couch shift.
Results: For the Catphan phantom analysis, the standard deviations of MTF10% across FMs were 0.19. The centre-of-gravity coordinates of the insert exhibited shifts of approximately 0.2, 0.4 and 0.6 mm when comparing reference images to those acquired with the shifted FMs. The results of the W-L test with a 0.3 mm couch shift showed radiation isocentre deviations exceeding 1 mm compared to the reference couch positions.
Conclusions: Minor sagging correction calibration errors did not remarkably impact image quality; however, they altered the geometric coordinates of the image isocentre. These calibration errors decreased the accuracy of off-isocentre positioning.