Room: AAPM ePoster Library
Purpose: Varian’s new Halcyon Linac is unique in that it uses two sets of stacked and staggered MLCs rather than standard MLCs with a collimator. The purpose of this project is to determine the quality assurance (QA) procedure that most effectively predicts a potential issue with MLC positioning error before starting patient’s SBRT treatments.
Methods: To replicate a potential MLC positioning error, two prostate SBRT plans (using 2 full arcs VMAT treatments for a total prescription dose of 36.25 Gy in 5 fractions) were copied and the position of one MLC used to modulate dose in the center of the treatment volume was moved by a predetermined distance for each control point. The MLC offsets being simulated were 1-mm, 2-mm, and 3-mm. QA plans were created and delivered on the Halcyon Linac using either portal dosimetry (PD), ArcCHECK, or PTW Octavius phantoms. The composite distribution was evaluated using the gamma pass-rates criteria of 2%/2mm.
Results: Gamma pass-rates for arc-1 of the first patient’s original clinical plan, 1-mm, 2-mm, and 3-mm offset plans for PD were 96%, 95.7%, 95.3%, and 94.8%, respectively. For arc-2 they were 94.5%, 94.3%, 94.3%, and 94.1%, respectively. For 2nd patient’s plan, the pass rates for arc-1 were 91.2%, 91.4%, 90.9, and 90.8, respectively. Arc-2 pass rates were 83.1%, 82.8%, 83.8% and 83.8%, respectively.
Conclusion: The sole results for PD have exhibited a decrease in passing rate with increasing MLC leaf displacement. Our current clinical practice is to evaluate the composite dose distribution. While evaluating field-by-field comparison for PD exhibited the expected trend of decreasing pass-rates with increasing MLC offsets, evaluation of the composite distribution did not show that trend. Further investigation using a field-by-field comparison is currently underway via all 3 QA devices mentioned above.