ESTRO 38 Abstract book

S118 ESTRO 38

the EMPIR 16NRM03 RTNOM project. Examples of k Q studies will be given and special attention will be paid on how to deal with, e.g., ion recombination and beam non- uniformity.

Sciences Giessen, Institute Of Medical Physics And Radiation Protection, Giessen, Germany

Abstract text The data in IAEA TRS-398 for MV beams was prepared in the mid-1990s, and since that date a number of new developments have taken place, such as the publication of ICRU 90 on key data for measurement standards in the dosimetry of ionizing radiation, free flattening filter beams, new detectors and dosimetry for small fields. IAEA decided to update its protocol and by the end of 2015 asked for volunteers to measure (based on primary standard dosimeters) and calculate (with Monte Carlo codes) updated k Q,Q0 values for reference-class ionization chambers (IC). Primary standard dosimeters for absorbed dose to water are water and graphite calorimeters. IAEA requested primary and secondary standard dosimetry laboratories to measure k Q,Q0 values for at least 5 ICs of the same type, ideally from different manufactured batches and to include one chamber with well-known k Q,Q0 values such as NE 2571. For calculations, the Monte Carlo codes have to pass the Fano test like EGSnrc and PENELOPE. One IC with well-known k Q,Q0 values has to be simulated first to demonstrate appropriate skill for these calculations. The complete results are to be delivered by December 2018 and the data analysis is to begin in 2019. Following IAEA demand, EURAMET decided to launch the EMPIR project 16NRM03 RTNORM in mid-2017 to provide k Q,Q0 factors to the IAEA. In the MV photon beams workgroup, DTU (Denmark), LNE-LNHB (France), NPL (United Kingdoms) and VSL (the Netherlands) were to measure k Q,Q0 factors for 6 IC types and ENEA (Italy), IST- ID (Portugal), STUK (Finland) and THM (Germany) were to calculate k Q,Q0 factors for 10 IC types. This presentation will give a description of the RTNORM measurements and calculations. The results will serve as examples to show which kind of data the IAEA received and how to evaluate them in order to select the ones that will be used for the TRS-398 update. SP-0237 Clinical application of kQ factors for reference dosimetry in flattening filter free (FFF) photon beams L. De Prez 1 , C. Andersen 2 , J. De Pooter 1 , H. Palmans 3 1 VSL, Ionizing Radiation Standards, Delft, The Netherlands; 2 Center For Nuclear Technologies DTU Nutech, Dosimetry, Roskilde, Denmark ; 3 National Physical Laboratory, Radiation Dosimetry, Teddington, United Kingdom Abstract text Flattening filter free (FFF) photon beams were introduced several years ago and are now widespread for application in radiotherapy. There are several clinical advantages of FFF beams over conventional beams with flattening filter (WFF), however differences in beam spectra at the point of interest in a phantom potentially affect the ion chamber response. FFF beams are also non-uniform over the length of a typical reference ion chamber and recombination is usually larger because of the higher dose delivered per pulse. Several studies have described FFF beam characteristics and their effect on beam calibration using conventional reference dosimetry, including the use of k Q factors. Some studies predicted significant discrepancies in k Q factors (0.4% up to 1.0%) if TPR 20,10 based codes of practice (CoPs) were to be used. Recently the IAEA published a new code of practice for dosimetry of small and non-reference static fields, including a description on the application of reference dosimetry for FFF compared to conventional WFF beams. This presentation addresses the application of the new code of practice and how to perform reference dosimetry in FFF beams by applying k Q factors in relation to k Q values available literature and performed by the authors within

SP-0238 TRS 483: past, present and future H. Palmans 1,2

1 EBG Medaustron Gmbh, Medical Physics, Wiener Neustadt, Austria ; 2 National Physical Laboratory, Medical Radiation Science, Teddington, United Kingdom Abstract text IAEA TRS-483 [1] is the first Code of Practice for the dosimetry of small high-energy photon fields published at either national or international level. The subsequent publication of a Summary paper in Medical Physics [2] marks the conclusion of a joint effort by the IAEA and the AAPM in publishing, for the first time, a Code of Practice for dosimetry together. In this presentation, the developments in the past decades that have shaped the small field recommendations in IAEA TRS-483 are presented, followed by an overview of the key steps in its application to the determination of small field output factors. A discussion will then be given on experience with the application of the Code of Practice and new developments and findings that have emerged from those. The aspects related to reference dosimetry for radiotherapy machines that cannot generate the conventional 10 cm × 10 cm reference field, and in particular those pertaining to the reference dosimetry of flattening-filter-free beams, are addressed in the previous presentation in this symposium. The first part of this presentation discusses past developments that have been determining IAEA TRS-483 include the following. There has been the realization that for small fields the collimator setting and field size at FWHM are not congruent and an unambiguous definition of field size is needed. The definition of lateral charged particle equilibrium range has enabled a quantitative criterion to distinguish between fields for which small field conditions and exist and broad beams. A paper by Alfonso et al [3] formally defined a small field output factor as a ratio of detector readings multiplied with a small field output correction factor which enabled the consistent determination of such factors using experiment or Monte Carlo simulations and the subsequent compilation of literature data for the Code of Practice. The work of Cranmer-Sargison et al [4] showed that for the representation of small field output factors and correction factors, the equivalent square field size of irregular small fields is better represented by a field with equal area than by a field with equal scatter conditions, as is the case for broad beams. The second part of the presentation outlines the key steps in applying IAEA TRS-483 to the determination of field output factors. The first step is the selection of suitable measurement equipment. The second step consists of the characterisation of the small field profiles and the derivation of the equivalent square field size from those profiles. Then the alignment procedures for real-time and off-line detectors are considered followed by the selection and interpolation of output correction factors and the determination of their uncertainty. The last part of the presentation discusses the experience with applying the code of practice, new information that has become available in the literature and new detectors that have become commercially available. [1] H. Palmans, P. Andreo, M. S. Huq, J. Seuntjens and K. Christaki, “Dosimetry of small static fields used in external beam radiotherapy: An IAEA-AAPM international Code of Practice for reference and relative dose determination,” IAEA Technical Report Series No. 483, (Vienna, Austria: International Atomic Energy Agency), 2017. [2] H. Palmans, P. Andreo, M. S. Huq, J. Seuntjens, K. E. Christaki and A. Meghzifene, “Dosimetry of small static