ESTRO 36 Abstract Book

S105 ESTRO 36 _______________________________________________________________________________________________

compared? Can the resources of ART be motivated by a clinical gain, or have we lost the clinical perspective during the technological development? These are points that will be further addressed in this talk. SP-0208 Development of procedures for safe clinical application of plan-of-the-day adaptive radiotherapy S. Quint 1 , J. Penninkhof 1 , J. Schiphof-Godart 1 , W. Hilst- van der 1 , B. Heijmen 1 , M. Hoogeman 1 1 Erasmus MC Cancer Institute, Radiation Oncology, Rotterdam, The Netherlands Complex tissue variations during the course of a radiotherapy treatment combined with IMRT or VMAT require adaptive approaches using in-room verification of position and shape of the target volume for optimal dose avoidance in organs at risk. In Erasmus MC we have developed an on-line adaptive approach for cervical and bladder cancer patients. Sofar more than 200 patients have been treated with adaptive therapy [Heijkoop S., IJROBP 2014, Jun;90(3):673-9)]. For each patient, an individualized library of treatment plans is pre-treatment established. Each plan in the library is optimally suited for a patient anatomy that can potentially occur during treatment. During each fraction, the plan that best matches the anatomy of the day is selected, based on an acquired CBCT. In this presentation, we will discuss aspects for safe introduction and application of the novel technology, including formal risk analyses, multi-disciplinary involvement, education, and definition of tasks and responsibilities of technologists, physicists, and physicians. A prerequisite of high precision radiotherapy (RT) is high precision of dose planning and delivery. If all involved uncertainties are not accounted for, this will result in a reduced benefit of highly conformal techniques, such as particle therapy (PT). By definition, a plan is robust when treatment goals are met despite uncertainties in patient and beam models and the plan remains acceptable over a range of likely variation. PTV margins are a well- established strategy to guarantee target coverage in photon RT, but showed to be a suboptimal solution in PT. Deviations in particle range entail significant dose deformations, related to the single beam path and require beam specific margin expansions. Uncertainties, and robustness as a consequence, depend on multiple factors: plan optimization, dose calculation accuracy, immobilization systems, image guidance protocols and delivery techniques. First of all, robust beam selection is essential to reduce heterogeneities across the beam path and avoid regions subject to intra and inter-fraction variations in patient anatomy which could determine unexpected severe dose errors. Set up errors and inherent deviations in CT calibration values can be included in plan evaluation and in the optimization process itself. Several approaches have been proposed for robust plan optimization, showing that the cost of robustness is often a reduction of plan conformality and a consequent increase of OAR doses. Planned dose recalculation based on machine log files allows for evaluation of the impact of dose delivery errors, providing important information on plan sensitivity to beam energy or position deviations. The consistency between planned and delivered doses may Symposium: Robust optimisation in protons and photons SP-0209 What is the actual robustness of the plans we deliver in particle therapy and what measures do we take to obtain it S. Molinelli 1 , M. Ciocca 1 1 Fondazione CNAO, Medical Physics, Pavia, Italy

substantially deteriorate when approximation errors occur in the dose calculation algorithm. This influences particle range and causes improper modeling of the Bragg peak degradation and beam lateral spread in heterogeneous media. When comparing TPSs based on different beam models, substantial dose differences can be found, particularly if passive beam modulators are used. While for protons the well-known distal end RBE enhancement can be easily accounted for with a distal margin extension, a more complex issue concerns carbon ions RBE-weighted doses. The RBE dependence on depth, dose, energy, fractionation and cell type is strictly related to the biological model adopted in the TPS. Changing the model or model parameters, impacts on RBE-weighted dose values corresponding to the same absorbed (and delivered) dose, with a significant influence on clinical outcomes. Most clinical TPS in use do not provide any tool for management of plan robustness. Site specific, manual and cumbersome approaches are often required, based on beam geometry constraints and the use of avoidance structure to force and/or prevent radiation pathways. Recent commercial systems provide robust evaluation and optimization tools based on the inclusion of set up errors and CT-HU variation to account for random and systematic range uncertainties. Few attempts have been made in the direction of delivery pattern optimization, in terms of energy layers rescanning, redistribution and filtration and spot editing. Simultaneous plan optimization on multiple CT scans, representing different anatomical conditions involved in the dose delivery phase (e.g.: 4D CT scans, in case of gated treatments to mitigate plan sensitivity against residual organ motion) is, to our knowledge, still missing. A fast and accurate MC engine should be available for dosimetric accuracy assessment in challenging clinical cases, where the calculation algorithm is known to present significant limitations. For carbon ion therapy, TPSs should provide dose averaged LET and fragment spectra distributions, in addition to a flexible selection of different RBE biological models and model parameters. Common plan evaluation metrics, setting a threshold between plan robustness and conformality, are still not available in clinical routine. Retrospective analysis of delivered plans could help in the definition of reference robustness databases in centers with consolidated clinical results. Experimental systems for in-vivo monitoring of particles range provide a direct measure of the uncertainties involved. A new PET scanner able to operate during the actual treatment of H&N tumors has recently been tested, based on the measurement of the β+ activity induced by the interaction of the therapeutic beam with patient tissues. Optimal PT plans should preserve target dose conformity, healthy tissue sparing and robustness towards uncertainties. IGRT protocols to minimize inter- fraction deviations should be integrated with robust plan geometry, optimization and evaluation. Even in a robust dose distribution, due to the sensitivity of particle range to variations in volume, shape and filling of tissues along the beam path, the implementation of adaptive protocols is mandatory for a correct treatment. SP-0210 Minimax robust optimisation applied to IMPT for oropharyngeal tumours S. Van de Water 1 , M. Hoogeman 2 , B. Heijmen 2 2 Erasmus MC Cancer Institute, Department of Radiation Oncology, Rotterdam, The Netherlands Robust optimization techniques increasingly receive attention, especially in the field of particle therapy, as they are considered more effective and more efficient in dealing with treatment uncertainties compared with the use of conventional safety margins. During robust optimization, treatment uncertainties are explicitly included in the mathematical optimization, thereby ensuring adequate irradiation when errors occur during treatment execution. Different approaches to robust