Comparison of dosimetric effects of MLC positional errors on VMAT and IMRT plans for SBRT radiotherapy in non-small cell lung cancer

Abstract

The positional accuracy of multi-leaf collimators (MLC) is important in stereotactic body radiotherapy (SBRT). The aim of this study was to investigate the impact between MLC positional error and dosimetry of volume intensity modulated (VMAT) and general intensity modulated (IMRT) plans for non-small cell lung cancer (NSCLC). Fifteen patients with NSCLC were selected to design the 360 SBRT-VMAT plans and the 360 SBRT-IMRT error plans. The DICOM files for these treatment plans were imported into a proprietary computer program that introduced delivery errors. Random and systematic MLC position (0.1, 0.2, 0.5, 1.0, 1.5, and 2.0 mm) errors were introduced. The systematic errors were shift errors (caused by gravity), opening errors, and closing errors. The CI, GI, d2cm and generalized equivalent uniform dose (gEUD) were calculated for the original plan and all treatment plans, accounting for the errors. Dose sensitivity was calculated using linear regression for MLC position errors. The random MLC errors were relatively insignificant. MLC shift, opening, and closing errors had a significant effect on the dose distribution of the SBRT plan. VMAT was more significant than IMRT. To ensure that the gEUD variation of PTV is controlled within 2%, the shift error, opening error, and closing error of IMRT should be less than 2.4 mm, 1.15 mm, and 0.97 mm, respectively. For VMAT, the shift error, opening error, and closing error should be less than 0.95 mm, 0.32 mm, and 0.38 mm, respectively. The dose sensitivity results obtained in this study can be used as a guide for patient-based quality assurance efforts. The position error of the MLC system had a significant impact on the gEUD of the SBRT technology. The MLC systematic error has a greater dosimetric impact on the VMAT plan than on the IMRT plan for SBRT, which should be carefully monitored.

Assessing quality assurance of multi‐leaf collimator using the structural similarity index

“…Based on previous studies, a 1 mm MLC shift can result in a dose change of −2.1% for the PTV, 0.04% for the cord, −0.7% for the heart, and −0.07% for the lung in a VMAT plan. 37 That means that Gamma test fails to detect around −10% dose change for PTV caused by the MLC shift deviation. On the contrary, the dose change can be controlled to stay under 6%.…”

Evaluating AAPM‐TG‐218 recommendations: Gamma index tolerance and action limits in IMRT and VMAT quality assurance using SunCHECK

“…[6][7][8] With radiation therapy delivery, uncertainty arises from factors such as errors in multi-leaf collimator leaf positioning, instability in gantry rotation, instability of the treatment couch, fluctuations in beam stability, and accelerator utilization time. 9,10 Following dose verification, it is feasible to conduct a comparative analysis of outcomes by overlaying isodose line distributions; however, visual quantification of the disparity is impractical. Hence, as indicated in the American Association of Physicists in Medicine Task Group (AAPM-TG) 218 report, the recommended approach for assessing dose verification outcomes is the gamma passing rate (%GP) technique.…”

“…With radiation therapy delivery, uncertainty arises from factors such as errors in multi‐leaf collimator leaf positioning, instability in gantry rotation, instability of the treatment couch, fluctuations in beam stability, and accelerator utilization time. 9 , 10 …”