Determination of dosimetric leaf gap (DLG) for FFF and WFF beams for a high definition MLC
PO-1414
Abstract
Determination of dosimetric leaf gap (DLG) for FFF and WFF beams for a high definition MLC
Authors: Anson Marcos|, Cristina(1)*[anson.cristina@gmail.com];Castro Tejero|, Pablo(1);Hernández González |, David(1);Roch González|, María(1);Viñals Muñoz|, Alberto(1);Fayos-Solà Capilla|, Roser(1);Pérez González|, Leopoldo(1);
(1)University Hospital La Princesa, Medical Physics, Madrid, Spain;
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Purpose or Objective
The dosimetric leaf gap (DLG) is used to model the effect of rounded leaf-end of the multileaf collimator (MLC) for dose calculations on the treatment planning system (TPS). The objective of this study is to report DLG values obtained for flattening filter free (FFF) and flattening filter (WFF) beams by using a sliding MLC gap plan and patient optimization.
Material and Methods
DLG and MLC transmission were determined for Varian Truebeam with Millennium 120 HD MLC for Eclipse TPS and 6WFF, 6FFF and 10FFF energies. Firstly, the method used was the sliding MLC gap plan provided by Varian with gap widths of 2, 4, 6, 10, 14, 16, and 20 mm using a Farmer ionization chamber (PTW 30013) in a water phantom (PTW MP3 Phantom Tank). Source to surface distance (SSD) was kept 90 cm and the chamber at 10 cm depth. DLGs were derived by fitting a linear function of gap size against the corrected gap chamber reading and extrapolating to zero [1]. Measurements were done both with the chamber perpendicular and parallel to leaf movement in order to determine the influence of this set-up in the obtained DLG value. Besides, for 6WFF energy the chamber was positioned perpendicular to the leaf movement and 5.5 cm off-axis to obtain information of the off-axis dosimetric area when treating large fields which involves not only the 2.5 mm leaf width in the center but the 5 mm ones off-center.
Secondly, once the obtained DLG value was introduced in the TPS beam data, 10 treated patient plans were recalculated and measured with SRS MapCheck QA phantom for comparing the dose distributions using gamma test. In order to improve the agreement planned-delivered doses, the DLG value was tuned with the aim of finding the optimal DLG which maximizes the gamma passing rate.
DLG values derived from the different set-ups in the sliding MLC gap method and the optimization process were obtained and compared.
Results
DLG and transmission ratio values obtained are shown on table 1. Transmission values have a low variation with the direction of the chamber respect to the leaf movement, as well as DLG values, which showed variations below 1 mm. Instead, when positioning the chamber off-axis, a reduction of 0.154 mm was found for 6WFF. The measured DLG value using sliding window method for the FFF beams resulted in an optimal gamma passing rate in QA patient plans. However, for 6WFF this value had to be increased in 0.5 mm in order to obtain a good agreement between plan and delivered doses.
Conclusion
DLG and transmission values assessment should be verified during TPS commissioning. The sliding MLC gap method using ionization chamber seems to have good agreement with optimization method for FFF beams. Nevertheless, the user should verify the optimal DLG value which provides confidence when implementing the workflow for quality assurance patient plans.
[1] Eclipse Inverse Planning Administration and Physics. Varian Medical Systems