ESTRO 38 Abstract book

S341 ESTRO 38

reporting, the quantities SRT near maximum and SRT near minimum dose are introduced that take into account that for small volumes a minimum absolute volume is required in the assessment of the quantities. This educational lecture will present a review of ICRU Report 91 (2017) including its rationale, small field dosimetry, volume definition, dose calculation, IGRT and QA as well as the prescription guidelines. The discussion will integrate this with the IAEA TRS-483 recommendations on small field calibration as well as with ongoing work by the AAPM Work Group on SBRT on the extraction of clinical and radiobiological information from the published SBRT outcomes literature. Educational objectives: 1. to understand the implication of small photon radiation beams in calibration, relative dose measurement and treatment planning dose calculations in the context of Report 91 and TRS-483; 2. to learn about the ICRU Report 91 recommendations on volumes definition and dose-volume prescription, recording and reporting. SP-0642 How to select patients for radiotherapy with protons instead of photons J. Balosso 1 , J. Thariat 2 , J.L. Habrand 2 , T. Tessonnier 3 , P. Lesueur 2 , A. Chaikh 4 , D. Stefan 5 , J.M. Fontbonne 4 1 Centre François Baclesse and University Of Grenoble- Alpes, Radiation Oncology / Archade, Caen, France; 2 university Unicaen- Centre François Baclesse And Archade, Radiation Oncology, Caen, France; 3 Centre François Baclesse, Medical Physics, Caen, France; 4 laboratory of Corpuscular Physics- CNRS- and ARCHADE, Ami, Caen, France; 5 Centre François Baclesse, Radiation Oncology, Caen, France Abstract text Epidemiology of eligible cases for protontherapy, as well as selection criteria, are very controversial. Actually, the size of the eligible population is more a question of care offer than a true scientific question, since protons allow almost constantly a better dose distribution than photons for rather the same biological efficiency, thus assuming fewer side effects. On the other hand, western European states as typical “providence states” are committed to provide to every patient the best possible cares. To be sustainable, this extremely costly policy needs a careful selection of the medical procedures that are the most “cost efficient”. Protontherapy is at best ten times more expensive than X- rays, this is the point. Thus, for a given patient and a given tumour, what is the most cost-effective procedure between X-rays and protontherapy? Raw physical dosimetric data cannot fully address this question. A difference of dose may not always translate in better outcomes and cost-effectiveness and, thus, may not be meaningful clinically. Nevertheless, radiation oncologists are aware of a benefit of proton therapy in given situations because of the better dose distribution of protons. These situations are : when dose differences can lead to reduction of toxicities or side effects in a significant health cost sparing way and can improve QOL; when proton therapy results in dramatic reduction of integral dose, which is a critical advantage regarding second cancers for young patients or even of vital importance for genetically radiosensitive patients; when protons allow significant dose escalation for radioresistant tumours; when dose sparing with protons preserves options to irradiate future tumours in case they occur in the same anatomical region (rectal cancer years after cervix Teaching Lecture: How to select patients for radiotherapy with protons instead of photons

SP-0640 Integration of PET imaging in radiation treatment planning U.Nestle’ 1 1 Medical Center- Faculty of Medicine- University of Freiburg, Department of Radiation Oncology, Freiburg im Breisgau, Germany

Abstract not received

Teaching Lecture: Implemention and practice of SRS and SBRT: Consensus guidelines and protocols

SP-0641 Implementation and practice of SRS and SBRT: Consensus guidelines and protocols J. Seuntjens 1 , E. Lartigau 2 1 Mcgill University Health Centre, Cedars Cancer Centre- Medical Physics, Montreal, Canada; 2 centre Oscar Lambret, Radiothérapie, Lille, France Abstract text Stereotactic Radiation Therapy (SRT) is a widely used radiation therapy technique relying on accurate delivery of highly conformal, sharply delineated high doses in few fractions to small target volumes and an accurate avoidance of critical risk organs. SRT is used intracranially (a.k.a. SRS) or extracranially (SBRT or SABR). SRT places major demands on clinicians with respect to peculiarities about the physics and dosimetry of small photon beams and the planning algorithm’s limitations. Image guidance and quality assurance procedures are also more stringent. Finally, the radiation biology of hypofractionated radiation treatment for tumors and normal tissues must ideally be considered. ICRU Report 91 (2017) arose from the lack of guidelines consistent with previous ICRU Reports about prescription, recording and reporting including Report 50 (ICRU, 1993), 62, (ICRU, 1999), and 83, (ICRU, 2010) for these types of clinical treatments. The Report provides a historical background and definitions of SRS and SBRT, emphasizing the characteristics stereotactic localization, hypofractionation, small beams and small targets, inverse optimization, image guidance; and the need for precise volume definition. Clinical prescriptions and dose tolerances for hypofractionation are still the subject of investigation and clinical debate and, aside from stating the limitations of LQ models and alternative models in this context, the Report is not prescriptive on this matter. Three main features that dominate the dosimetry of small beams from accelerators are a lack of charged-particle equilibrium, partial source occlusion and the importance of size and construction details of the detector used. In the Report, IAEA-AAPM TRS-483 (2017) recommendations are retained for reference dosimetry in a machine-specific reference field ( msr ) and for small fields, more than a single detector is recommended to determine relative output factors where the measured data should be corrected with the detector type-specific field output correction factor data. For reasons of dose accuracy required in extracranial situations, ICRU Report 91 mandates the use of advanced type-b model-based dose calculation algorithms. As SRT is characterized by high gradients in the dose distribution and hypofractionation it comes with the need to perform daily correction and verification of target location at a sub-mm precision using IGRT and appropriate QA procedures. ICRU 91 maintains the definition of prescription and risk volumes consistent with previous ICRU documents and the report discusses imaging modalities for GTV definition. The Report maintains prescription of absorbed dose to the isodose surface D V that covers an optimal percentage volume of the PTV while optimally restricting dose to the planning risk volume (PRV). With regards to recording and