ESTRO 2023 - Abstract Book

S1489

Digital Posters

ESTRO 2023

PO-1768 TLD verification of dose delivery in VMAT plan

A. Embriaco 1 , P. Martucci 2 , V. De Coste 1 , M. Pimpinella 1 , S. Russo 3 , C. Fiandra 4 , L. Masi 5 , P. De Felice 1 , M. Stasi 6

1 INMRI, ENEA, Rome, Italy; 2 University Tor Vergata, ENEA-INMRI, Rome, Italy; 3 AUSL Toscana Centro, Health Physics, Florence, Italy; 4 University of Turin, Oncology, Turin, Italy; 5 IFCA, Medical Physics Unit, Florence, Italy; 6 AO Ordine Mauriziano, Health Physics, Turin, Italy Purpose or Objective The Italian Association of Medical Physics (AIFM) in cooperation with the Italian National Institute of Ionizing Radiation Metrology (ENEA-INMRI) offers a certified audit service to radiotherapy centres for reference dosimetry in photon beams. Connected to the service, a technical protocol for verification of dose delivery in VMAT treatments is being developed. Here we report on results obtained using our audit dosimetry system in a set of non-reference irradiation conditions. Materials and Methods The absorbed dose is measured in a cylindrical water phantom with a diameter of 30 cm and a length of 50 cm. The prototype of the phantom has two different inserts for a Farmer ionization chamber and/or TLD dosimeters and for a micro ionization chamber. The measurements were performed with a 6 MV photon beam (Elekta Synergy Linac). Thermoluminescent dosimeters (TLD-100), assembled in a waterproof PMMA holder, are applied to measure the delivered dose. TLD are calibrated at ENEA-INMRI in terms of absorbed dose to water in a reference 60Co γ -beam. The measurement signal is the average value of 10 TLD chips. Different configurations are examined and planned with Monaco TPS on the CT images of the phantom: a) 4 box fields 10 cm x 10 cm at different angles (0°, 90°, 180° and 270°) to delivery an uniform dose in the target volume; b) 360° dynamic conformal arc shaped on target volume; c) VMAT plans for Head and Neck (HN) treatment with different levels of complexity. The target volume in condition a) and b) is a sphere of 6 cm diameter and an homogeneous dose prescription of 2 Gy to the 98% of the target volume is applied. The VMAT plan simulated a Simultaneous Integrated Boost HN treatment for two target volumes with two different dose prescriptions (2-1.5 Gy) to the 95% of each target volume. Several Organs at Risk are considered with proper dose constraints. A ROI simulating the TLD volume is also defined and dose homogeneity within 2 % is requested. Three different VMAT plans are planned and delivered: Results Ionization chamber measurements show that planned and delivered absorbed dose in conditions a) and b) agree within 0.6%. In the same conditions, the agreement between TLD and ionization chamber measurements is around 1.0% that is well within the TLD measurement uncertainty. In VMAT plans, the percentage difference between TLD measured dose and TPS dose on the TLD ROI is up to about 2.7%. Conclusion Our TLD dosimetry system has proven to be suitable for dose measurements in full VMAT conditions. The final cylindrical phantom will also have a microDiamond insert and fiducial markers for CyberKnife plan verifications. A concomitant pilot study involving some Italian RT centers started to evaluate the complexity of VMAT treatment planned by different Linac TPS systems. 1 Centre Interdisciplinaire de Nanoscience de Marseille- CNRS, Department of Physics, Marseille, France; 2 Aix-Marseille University-CNRS, Department of Physics, Marseille, France; 3 Institut Paoli-Calmettes, Department of Radiation Therapy, Marseille, France Purpose or Objective Scintillating dosimetry offers promising techniques in modern small-field radiation therapy (RT) treatment, owing to the miniature dosimeter with a small sensitive volume that could correspond to water equivalency. However, such a compact detector needs to be adapted to the real treatment plan. Hence, the purpose of this research work is to introduce a novel real-time micro-dosimetry technique that could promote accurate dose measurement in external and internal beam RT treatment. Materials and Methods This study is focused on the performance evaluation of a novel micro dosimetry technique for small-field RT treatment. The micro-dosimeter (MSD) consists of an X-ray excited scintillating materials at the extremity of a multimode silica (SiO2) optical fiber (see optical micrograph image attached). System performances were evaluated under LINAC (6 and 15 MV) photon and electron beam, and brachytherapy (BT) source (192Ir). All the measurements were performed using IBA water tank phantoms by following IAEA standards with the TRS-398 protocol and TG43-U1 recommendations for BT used in the patient treatment. Simulation for the specific volume detectors was performed by GATE software (based on Geant4) to 1. 2 arcs with a total number of 316 CP and 944 MU; 2. 2 arcs with a total number of 286 CP and 784 MU; 3. 4 arcs with a total number of 333 CP and 870 MU. TLD measurements are compared with calculated dose on the TLD ROI. PO-1769 High resolution micro-dosimetry in modern radiation therapy treatment S.B.C. Debnath 1 , D. Tonneau 2 , C. Fauquet 2 , A.R. Tallet 3 , M. Ferre 3 , J. Darreon 3