Room: Exhibit Hall | Forum 1
Purpose: The dosimetric leaf gap (DLG) is used to model the rounded leaf ends of a single focused MLC. Values reported in the literature are inconsistent, suggesting that the measured DLG is a combination of leaf design and machine-dependent offset. We did Monte Carlo calculations to evaluate the intrinsic DLG due to the leaf design.
Methods: Monte Carlo simulations of the DLG measurement geometry were done using a Geant4 model of the 6 MV beam of a TrueBeam linear accelerator. The Monte Carlo model was stored as a virtual machine and instantiated using Amazon Web Services. Simulations started with pre-calculated phase spaces at the exit plane of the primary collimator. Calculations were done for 2, 4, 6, 10, 14, 16, and 20 mm sweeping gaps, a static closed MLC field, and a 10 cm x 10 cm field. Dose was scored in 1.25 mm voxels for a water phantom at 90 cm SSD and averaged over a 2 cm long, 6 mm diameter cylindrical volume at 100 mm depth to simulate a thimble type ionization chamber. The simulation results were compared with measurements.
Results: The standard deviation of the average dose in the cylindrical volume was < 1% for all simulations. The Monte Carlo calculated transmission was 0.96%, similar to the measured value of 1.1%. The DLG computed from the Monte Carlo simulations was 0.89 mm, larger than the measured 0.48 mm.
Conclusion: The simulations suggest that the MLC controller places the leaf ends closer together than the nominal position. Experiments are underway to confirm this conclusion. Monte Carlo simulations for different beam energies and a second MLC type are also in progress. These Monte Carlo simulations can be used to define vendor specifications for the DLG and thus better standardize MLC dosimetry between machines.
Funding Support, Disclosures, and Conflict of Interest: Research supported by Varian Medical Systems.