SU-GG-T-95: The Dosimetric Advantages of Varian HD120 MLC Over Millennium120 MLC for Photon External Beam Treatment

Abstract

Purpose: To compare the dosimetricproperties of the Varian HD120 high definition MLC and Millennium 120 MLC and their performances in IMRTtreatment planning.Method and Materials:Dosimetricproperties of the Varian HD120 high definition MLC (minimum leaf width 2.5 mm) is studied using a Trilogy Tx. The penumbra of the MLC‐formed 10cm × 10cm single field at different depth was determined from the reconstructed 3D dose distribution from geldosimetry. Results are compared with those from EDR2 film measurement at Dmax and 10cm depth. Leakage radiation is measured using a farm ion chamber. The same procedures were performed for a Millennium 120 MLC (minimum leaf width 5 mm) on a 21EX. To compare the dose distributions from the HD120 and the Millennium 120 for the same beam arrangement and optimal fluences, a head‐neck IMRT plan was generated using the Eclipse‐Helios optimization engine. The optimal fluences obtained were applied to both the Trilogy with HD120 and the 21EX with Millennium 120. The resultant dose distributions and the dose‐volume histograms from the two plans were compared. 3D dose distributions from these two plans were also verified using geldosimetry.Results: Smaller penumbra regions were identified for the HD120 using gel and film measurements. The leakage radiation from HD120 is smaller. For the two IMRT plans using the same optimal fluences, the dose distribution with HD120 is more uniform over the PTV. The dose distribution also falls off faster than the one with Millennium 120 in the high dose gradient region, as a result of smaller leaf width and penumbra. The doses received at the critical organs scattering radiation with HD120 are also smaller. Conclusion: HD120 provides better dose conformalities around treatment targets and spares critical organs and normal tissue better than Millennium 120 MLC.