Purpose: This study investigated the dosimetric impact of multi-leaf collimator (MLC) positional errors and determined the desired MLC positional accuracy for flattened (FF) and flattening filter-free (FFF) beams in standard-fractionated and hypofractionated radiotherapy.
Methods: Eighty patients (20 each for treatment site) with prostate cancer, lung cancer, brain metastases, or spinal metastases were evaluated. Standard-fractionated and hypofractionated (2 and 7.25, 2 and 13, 4.2 and 10, and 3 and 16 Gy per fraction for prostate, lung, brain metastases and spinal metastases, respectively) volumetric-modulated arc therapy (VMAT) plans were generated using FF and FFF beams. Systematic open/closed MLC positional errors (±0.25, 0.5, 1.0, and 2.0 mm in opposite directions) were introduced in the VMAT plans. The dosimetric impact for the targets and organs-at-risk (OARs) were evaluated based on the generalized equivalent uniform dose (gEUD).
Results: The gEUD deviations with MLC errors of the target volumes were strongly dependent on treatment site (2.46±0.38, 4.47±0.88, 1.74±0.46 and 6.61±1.25 %/mm for prostate, lung, brain, and spine metastases, respectively). The beam modality and dose per fraction had less impact on the gEUD of the target volumes. Among OARs, the gEUD deviations with MLC errors were strongly dependent on the dose per fraction (average 1.28±0.29 and 1.29±0.30 %/mm in prostate and spinal metastases) except for lung and brain metastases (average 0.06±0.03, and 0.14±0.17 Gy/mm, respectively). Maintaining the target to within 2% corresponded to within 0.63, 0.42, 0.81, and 0.27 mm for prostate, lung, spinal metastases, and brain metastases, respectively.
Conclusion: Since the dose errors due to MLC positional errors for VMAT may depend on the treatment sites, beam modalities (FF and FFF beams), and doses per fraction, these should be considered for quality assurance.