Dosimetric Discrepancies Due To Positional Errors In Mlc Movement During Stereotactic Lung Vmat

We introduce a method that compares the multi leaf collimator (MLC) leaf positions between planned and delivered treatments for every control point in five lung stereotactic body radiation therapy (SBRT) treatment plans. Dose is recalculated for the new MLC positions to measure impact on treatment delivery. Five plans from patients previously treated with 2-arc volumetric modulated arc therapy (VMAT) SBRT for non-small cell lung carcinoma were used as a basis for this study. Treatment planning and delivery were done in binned dose rate mode. Machine MLC position accuracy is tested weekly using a standard picket fence test, with a 1 mm tolerance. Each plan was delivered three times on a linear accelerator and the MLC positions at each control point were obtained from the machine log file using an in-house program. The difference between planned and treatment MLC positions were compared to examine systematic or random errors. Further, a treatment planning system plan was generated with treatment MLC positions to investigate changes in dose. The original plan and the generated plan were compared using various treatment site specific metrics. A 3D gamma map (2% dose difference, 2 mm distance to agreement) of the planned and treated dose maps was used to further assess discrepancies in dose. The average difference in MLC position for all the control points in each plan was 2.8 ± 2.2 mm, and greater for the first arc in 4 of 5 plans. The leaves with the maximum difference for all plans were constrained to the central, modulating leaves. In arc 1 of each plan, the greatest difference for each measurement occurred systematically within the same 3 leaves for 4 of 5 plans. Averaging over discrepancies in all control points, the MLC with the maximum difference was found to be consistent for the same plan, as well as for all plans delivered. The control point with maximum error for all MLCs was not consistent within a plan. In recalculated plans with the MLC positions from the machine log files, maximum dose to the spinal canal did increase by 26 cGy on average; however it remained under the 1800 cGy limit. PTV dose conformity, homogeneity and gradient did not appear to be significantly compromised. Three-dimensional gamma analysis showed no significant difference between planned and treated dose maps, with gamma > 98% in all cases. Most patient QA methods are resolution-limited and would be unlikely to recognize dosimetric discrepancies due to MLC misplacement. We have found errors as large as 5 mm in MLC position occur during the delivery of lung VMAT SBRT; however, their dosimetric impact was not significant. Since we account solely for the discrepancy in MLC position, further studies are warranted to explore the effect of dose rate and gantry angle change in dose delivered.