Abstract Book

S340

ESTRO 37

vary much as a function of field size, but for small fields these would only be applicable to very small detector sizes; the finite size of ionization chambers in comparison with the size of small fields results in unacceptably large fluence perturbation factors. It has also been recognized that the broad-field photon beams used for reference dosimetry are well representative for clinical fields in conformal radiotherapy, but for intensity modulated radiotherapy and stereotactic treatments, involving the superposition of a large number of small fields delivered from different angles, this is no longer the case. IPEM Report 103 [2] was the first to compile a discussion on all problems related to small field dosimetry and to provide Guidance on this matter. A working group endorsed by IAEA and AAPM published a seminal paper (Alfonso et al. [3]) that formalised the dosimetry of small fields and also introduced two new types of reference fields: (i) a static machine-specific reference (msr) field for those modalities that cannot establish conventional reference conditions and (ii) a plan-class specific reference (pcsr) field closer to the complex patient-specific clinical fields thereby facilitating standardization of composite field dosimetry. The first Code of Practice for the dosimetry of small fields in photon beams has been published by members of the same working group as IAEA TRS-483 [4], which contains six chapters providing an introduction, a brief discussion of the physics of small photon fields, the formalism, a comprehensive overview of suitable dosimeters, practical recommendations for msr dosimetry and practical recommendations for the determination of field output factors in small-field photon beams. Two appendices provide comprehensive compilations of data and their uncertainties from the literature that form the basis of the data used in the Code of Practice. Current efforts include an IAEA Coordinated Research Activity to test IAEA TRS-483 in the clinical environment [5] of which a status will be presented and an update of IAEA TRS-398 which will include, similarly as the Addendum to AAPM TG-51, Monte Carlo calculated overall beam quality correction factors [6]. The IAEA also intends to review the status of pcsr dosimetry and explore the possibility of developing recommendations for composite- field reference dosimetry [6]. [1] M. McEwen et al. Addendum to the AAPM's TG-51 protocol for clinical reference dosimetry of high-energy photon beams. Med. Phys., 41:041501, 2014. [2] M. M. Aspradakis et al. Small field MV photon dosimetry. IPEM Report 103. Institute of Physics and Engineering in Medicine, York, UK, 2010. [3] R. Alfonso et al. A new formalism for reference dosimetry of small and nonstandard fields. Med. Phys., 35:5179–5186, 2008. [4] H. Palmans et al. Dosimetry of Small Static Fields used in External Beam Radiotherapy: An IAEA-AAPM international Code of Practice for reference and relative dose determination. Technical Report Series No. 483. International Atomic Energy Agency, Vienna, 2017. [5] W. Lechner et al. Initial experiences in Testing the IAEA/AAPM Code of Practice on Small Field Dosimetry. International Conference on Advances in Radiation Oncology (ICARO-2), Vienna, Austria, 20–23 June 2017 [6] K. Christaki. Personal communication, 2018.

Teaching Lecture: Treatment planning: where are we and, where are we going

SP-0641 Treatment planning: where are we and, where are we going O. Bohoudi 1 1 VUMC, Radiotherapy, Amsterdam, The Netherlands Abstract text Radiation therapy treatment planning has undergone major revolutions in the past decades. Innovations in the field of computerized planning technology and the introduction of advanced radiation delivery equipment have resulted in decreased usage of 2D and 3DCRT techniques, in favor of techniques such as IMRT, VMAT and tomotherapy. In addition, current developments such as MR-guided radiation therapy allow for adapted treatment planning, whereas the introduction of stereotactic treatment delivery and proton beam therapy pose their own specific challenges. This has (had) an enormous impact upon treatment planning itself and on the requirements of physicists and/or RTT. Novel planning aspects that are evaluated in current practice are model-based planning and scripting. This presentation provides an overview of some of the above developments in treatment planning, and more specifically, the following topics will be discussed: – An overview of current treatment planning applications. – Treatment planning and machine learning. – The changing role of planners. – Aspects such as robust and adaptive planning, model- based planning will be illustrated for radiosurgery for multiple brain metastases and adaptive planning for MR- guided radiation therapy. SP-0642 OLIGOCARE, the EORTC ESTRO initiative for oligometastasis: a pragmatic platform P. Ost 1 1 University Hospital Ghent, Radiation Oncology, Gent, Belgium Abstract text Despite its almost universal use, the level of evidence supporting radical local treatment for oligometastases in general, and stereotactic radiotherapy in particular, is low. Furthermore, oncology has evolved and changed rapidly within the last years due to more effective systemic treatment (e.g. targeted drugs, immunotherapy), better imaging and diagnostics for disease staging and characterization (e.g. PET scanning, circulating tumor DNA) and more frequent use of multimodality treatment. These factors all potentially influence the effectiveness of local treatment in the oligometastatic setting. Consequently, there is an urgent need for better patient selection criteria for or against local treatment in general and SRT in particular in the setting of oligo-metastasis.However, it is considered as highly unlikely or even impossible that these questions will be answered within the traditional framework of prospective randomized trials, randomizing patients between systemic treatment only or addition of a local treatment component. This is mostly because of two reasons: 1. There are no major technical challenges to safely treat oligometastases in routine practice despite the lack of mature randomized data. Consequently, the equipoise to enroll patients in trials randomizing between SOC and Joint Symposium: ESTRO-EORTC: New strategies for a clinical research partnership?