Author: Michael Ashenafi, Nicholas Becerra Espinosa, Mario Ramos Gallardo, Matthew Pacella, Sean M. Tanny 👨🔬
Affiliation: Department of Radiation Oncology, University of Rochester 🌍
Purpose: A new multileaf collimator (MLC) model has been introduced in Varian Eclipse v18.0, with the treatment planning system (TPS) explicitly modeling the leaf end effects. The vendor recommends a range of ion chambers suitable for commissioning measurements. We investigate the impact of ion chamber choice on the Enhanced Leaf Model (ELM) predictions of Dosimetric Leaf Gap (DLG) and Leaf Transmission (LT) factors.
Methods: A 0.125cc scanning ion chamber was used to measure the output at 10 cm depth for MLC field patterns designed for DLG and LT factor determination with a TrueBeam (Varian Medical System, Palo Alto, CA) LINAC in a 1D water tank at 90 cm SSD. The measurements were repeated using a Farmer-type ion chamber and solid water. The measured output for each ion chamber was entered into the TPS for DLG calculation. The DLG for each energy and ion chamber combination was then used to predict the dose delivered to water at a depth of 10 cm (d10). Measured dose was compared to the TPS calculated d10 using all combinations of ion chambers.
Results: DLG differences between scanning and Farmer chambers for 6X, 6FFF, 10FFF, and 16X were -0.020 mm, 0.005 mm, -0.010 mm, and -0.010mm. LT factors between the scanning and Farmer measurements were 1.311%, 0.859%, 0.760%, and 0.814% different. The TPS predicted vs. measured d10 values were closer in agreement to the Farmer chamber measurements, with maximum observed differences of 5.6%, and 1.4% for the scanning and Farmer measurements, respectively.
Conclusion: DLG values obtained from both ion chambers are within vendor-recommended ranges. Both chambers show minimal difference in agreement for the sliding MLC fields, and some differences in measured LT. Farmer chamber measurements can provide dose values which more closely match the TPS prediction when delivering fields with MLC patterns designed for ELM modeling.