ESTRO 2021 Abstract Book

S126

ESTRO 2021

Conclusion k Q

values of the new PinPoint 3D 31022 chamber for the dosimetry of high energy megavoltage photon beams have been reported. Conforming to the methodology adopted in the TRS 398 update, these results complement the k Q values published recently for the upcoming update.

OC-0197 Dose response of diode-type detectors in magnetic field under small field conditions I. Blum 1 , T. Tekin 1 , B. Delfs 1 , A. Schönfeld 1 , R. Kapsch 2 , B. Poppe 1 , H.K. Looe 1 1 University Clinic of Medical Radiation Physics, Medical Campus Pius Hospital, Carl von Ossietzky University, Oldenburg, Germany; 2 Physikalische-Technische Bundesanstalt, PTB, Braunschweig, Germany Purpose or Objective The aim of the present work is to investigate the dose response of two diode detectors (PTW microDiamond 60019 and PTW microSilicon 60023) in magnetic field under small field conditions experimentally and with Monte Carlo simulations down to the smallest nominal field size of 0.5 cm x 0.5 cm and up to a magnetic field strength of 1.5 T. Materials and Methods Measurements were performed at the National Metrology Institute of Germany (PTB, Braunschweig) with a clinical linear accelerator equipped with an electromagnet using a 6 MV photon beam. Quadratic field sizes with nominal side lengths between 0.5 and 4.0 cm that correspond to the equivalent field sizes between 0.63 and 4.27 cm at the measurement depth of 5 cm were used. The magnetic field was varied between 0.35 T and 1.4 T. 2D dose profiles were measured using calibrated EBT3 radiochromic films. The experimental results of the dose response have been complemented by Monte Carlo simulations up to 1.5 T. Simulations were performed using the EGSnrc code by implementing field size dependent virtual sources derived from the film measurements. All other conditions were the same as during the measurements. Furthermore, detailed simulations were performed to quantify the small field perturbation effects, such as the volume-averaging and density effect, as well as the role of detector’s components on its dose response in magnetic field. Results The dose response normalized to the field-free case of both diode-type detectors decreases with increasing magnetic field. At an equivalent field size of 4.27 cm, a reduction of dose response up to 10% could be observed in 1.5 T magnetic field for both detectors. Under small field conditions, the observed decrement of dose response in magnetic field become less prominent. For example, the dose response decreases by only up to 5% for an equivalent field size of 0.63 cm at 1.5 T. The results from the Monte Carlo simulations show good agreement (within 1%) with the measurements for all investigated field sizes and magnetic field strengths. The small field output correction factors for both detectors in magnetic field are smaller than those in the magnetic field-free case, where correction up to 6% at 1.5 T was required for the smallest field investigated. The volume-averaging correction factors of both detectors are not dependent on the magnetic field. Detailed simulations revealed that the enhanced-density components within the detectors are causing the field size and magnetic field dependent dose response. Conclusion The dose response of the microDiamond and microSilicon under small field conditions has been shown experimentally to depend on the magnetic field strength. The decrease of the dose response with increasing magnetic field is less prominent in smaller field sizes (equivalent field size < 1.99 cm). The enhanced-density detector components beneath the sensitive volume are shown to be responsible for the observed magnetic field dependence. OC-0198 First microdosimetry 2D maps with an array of new 3D-cylindrical microdetectors in proton therapy C. Guardiola 1 , D. Bachiller-Perea 2 , C. Fleta 3 , D. Quirion 4 , L. De Marzi 5 , A. Maia Leite3 6 , F. Gómez 7 1 Université Paris ‒ Saclay, CNRS/IN2P3, Laboratoire de Physique des 2 Infinis Irène Joliot Curie, 91405 Orsay, France; 2 Université Paris ‒ Saclay, CNRS/IN2P3, , Laboratoire de Physique des 2 Infinis Irène Joliot Curie, 91405 Orsay, France; 3 Centro Nacional de Microelectrónica (IMB ‒ CNM, CSIC), Radiation Detector Group, 08193, Bellaterra , Spain; 4 Centro Nacional de Microelectrónica (IMB ‒ CNM, CSIC),, Radiation Detector Group, 08193, Bellaterra , Spain; 5 Institut Curie, PSL Research University , Centre de protonthérapie d’Orsay, 91898 Orsay , France; 6 Institut Curie, PSL Research University, , Centre de protonthérapie d’Orsay, 91898 Orsay , France; 7 Universidad de Santiago de Compostela, Dep. de Física de Partículas, , 15782, Santiago de Compostela, Spain