ESTRO 2020 Abstract book

S799 ESTRO 2020

C. Anson Marcos 1 , P. Castro Tejero 1 , D. Hernández González 1 , M. Roch González 1 , A. Viñals Muñoz 1 , R. Fayos- Solà Capilla 1 , L. Pérez González 1 1 University Hospital La Princesa, Medical Physics, Madrid, Spain 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 We have demonstrated that the proposed method can be used to determine the actual DLS for several clinically relevant beams qualities and for standard and high definition MLC types. The agreement between the actual DLSs derived using proposed method and those derived conventionally has been found to be clinically acceptable. PO-1413 Ion chamber beam quality correction factors for brachytherapy dosimetry measurements Z. Thrapsanioti 1 , E. Pantelis 2 , P. Karaiskos 1 , C. Hourdakis 3 1 National and Kapodistrian University of Athens, Medical School Laboratory, Athens, Greece ; 2 National and Kapodistrian University of Athens, Medical Physics Laboratory, Athens, Greece ; 3 Greek Atomic Energy Commission, Licensing and Inspections, Athens, Greece Purpose or Objective The need for a quality control protocol in brachytherapy is well recognized. The purpose of this work is to calculate the beam quality correction factors of a series of detectors for experimental dosimetry measurements around ) for the PTW PinPoint -31014, -31016 and the Exradin A16 thimble chambers were calculated as a function of radial distance from an 192 Ir HDR source centered in 20cm radius spherical water phantom. All calculations were performed using the egs_chamber user code of the EGSnrc Monte Carlo code. All detectors were assumed to be calibrated in a reference 60 Co 10x10cm 2 beam. The contribution of volume averaging and the perturbation of the wall, stem and central electrode to the correction factor was studied. Results Beam quality correction factors of 1.03, 1.01 and 1.02 were calculated for the PinPoint 31014, 31016 and Exradin A16 ion chambers respectively at 1cm distance from the source. These values were found to decrease with increasing distance from the source due to the decrease of the contribution of the volume averaging effect to chambers’ signal. The central electrode and wall material of the chambers was found to perturb the signal in the 192 Ir beam energies by 3% and 1%, respectively at 1cm away from the source. These perturbation values were found to increase with increasing distance from the source due to the increased contribution of low energy scattered photons. Conclusion Beam quality correction factors K Q,Qo for three ion chambers were calculated for dosimetry measurements around 192 Ir brachytherapy sources. Besides volume averaging effect, the signal of the chamber was found to depend on the wall, central electrode, material and distance from the source. PO-1414 Determination of dosimetric leaf gap (DLG) for FFF and WFF beams for a high definition MLC brachytherapy sources. Material and Methods Beam quality correction factors (K Q,Qo