Room: Exhibit Hall
Purpose: By using the dosimetry-based gap width, this study aims to develop a simple and reliable method for determining the DLG.
Methods: A Varian TrueBeam linac with 6 MV, 10 MV, 6 MV FFF and 10 MV FFF photon beams and equipped with the 120 Millennium MLC and the Eclipseâ„¢ treatment planning system (TPS) was used in this study. Integral doses of sliding fields and static MLC fields with different gap widths were measured with an ion chamber and GAFCHROMIC EBT3 films, respectively. DLG was derived from a linear extrapolation to zero dose. The average MLC leaf transmission to the gap reading for each gap (RgT) were calculated with geometric and physical gap widths, respectively. The physical gap width was determined according to the dosimetric profile measured by the EBT3 film. Additionally, the optimal DLG value was identified by plan dose measurements, as the value that produced the closest agreement between the planned and measured doses. DLGs derived from geometric and dosimetric gap width method, and from the film measurements method, and from the optimal process, were obtained and compared.
Results: DLG derived from geometric gap width showed a significantly lower value (difference about 0.5 mm) than that from physical gap width and from film measurements and from plan optimal value. The method in deriving DLGs by correcting the geometric gap widths to the physical gap widths was showed good agreements to the plan optimal values (within 0.2 mm).
Conclusion: The DLGs derived from the method of physical gap widths were consistent to the values derived from film measurements and from the plan optimal process. A simple and reliable method to determine DLG for rounded leaf-end MLC systems was established. This method provides a reliable assessment of DLG value required during TPS commissioning.
Dosimetry, MLC, Treatment Planning
TH- External beam- photons: Standard field computational dosimetry