ESTRO 2021 Abstract Book

S1368

ESTRO 2021

photon beams. The gamma counting began 20 minutes after the irradiations. A GEANT4 simulation was run for every sample, allowing quantitative results of the measured activities. Irradiations with 15MV led to (n,γ) activation of short-lived isotopes : In the leaf, the measured activity was 1556 Bq just after the irradiation and we observed various gamma lines from 187W, 57Ni and 56Mn. In the new filter, the activity was 1097 Bq and the gamma signature of 56Mn, 56Ni, 57Ni and 59Co was clearly present. The germanium detector allowed to measure gamma lines with relative intensities down to ~0.1%, thank to its very low background. Irradiation at 6MV led only to a small activation of 56Mn. Gammaspectroscopic data was taken several times after the irradiation to monitor the time evolution and the total activity. Conclusion Treating patients with 15MV photon beams activate long-lived radioisotopes in the linac head. With this work, it was possible to identify these isotopes, and highlight the created short-lived radioisotopes. In comparison with portable germanium detectors directly placed under the linac head, our method is able to decorrelate the activation of single linac components, or the activation of recent irradiations. PO-1648 Beam modelling and MLC parameters optimization in a RayStation TPS and TrueBeam linac M. Pinto Monedero 1 , J. Martínez Ortega 2 , P. Sánchez Rubio 2 , A. Montes Uruen 3 , A. López Corella 2 , M. Torres López 1 , J.L. Colado 1 1 Hospital Universitario Puerta de Hierro Majadahonda, Medical Physics Department, Madrid, Spain; 2 Hospital Universitario Puerta de Hierro Majadahonda, Medical Physics Deparment, Madrid, Spain; 3 Hospital Universitario Puerta de Hierro Majadahonda, Medical Physics, Madrid, Spain Purpose or Objective Beam modelling in a TPS system is critical due to its influence in clinical dose calculation. Adjusting MLC parameters is even more important in those treatment units dedicated to high complexity treatments using dIMRT and VMAT techniques, small fields and very often hypofracionation. When modelling TrueBeam 120HD MLC beams in the RayStation TPS very little documentation was found with reference data set nor describing the optimization process of the MLC parameters. The purpose of this work is to describe the beam modeling process and the optimization of MLC parameters in the RayStation TPS and a TrueBeam 120HD MLC linac. Materials and Methods A TrueBeam 120HD MLC linac ( Varian Medical Systems, USA) and the RayStation TPS v8A ( RaySearchLabs , Sweden) were used. Measurements for beam modelling were performed using a water tank phantom and different PTV detectors (PTW, Germany). Beams were modeled following TPS user manual guidelines, comparing open field profiles collimated with jaws. This preliminary model was tested through point dose comparison in a water phantom, in and out the field and in the penumbras. Tolerances from TRS-480 were applied. RayStation uses the following parameters to model MLC: transmission, leaf tip width, tongue and grove (T&G), leaf-tip offset, gain and curvature. A first approach to MLC parameters was performed by comparison of open fields MLC collimated profiles. Two methods described by A.Sanini el al and J.Sáez et al. were applied to do a finer optimization of MLC parameter and results were tested by planning, measuring and comparing clinical plans from TG119. Final selection of MLC parameters obtained from both methods was decided through analysis of the agreement between planned dose distributions calculated by the treatment planning system (TPS) and measured dose distributions of the clinical TG119 plans. Results MLC parameters obtained for the preliminary model, the A.Savini et al, J.Saez el al. model and the final model are presented at table 1. Validation results of the models by comparison of the agreement between planned dose distributions calculated by the treatment planning system (TPS) and measured dose distributions of the clinical TG119 plans are shown at table 2.

Preliminary models showed discrepancies greater than ±2% in high dose regions in some of the clinical plan for