ESTRO 2020 Abstract Book

S394 ESTRO 2020

Conclusion For the first time, a correlation of patient outcome in respect to plan robustness has been performed for skull base tumors. Our results show that, for this cohort of patients, LF does not correlate with lack of plan robustness, indicating that our planning approach is robust. On the other hand, the analysis of dosimetric parameters seems to indicate that local failures are associated with a reduction of target coverage in the nominal plan, indicating that the dosimetric quality of the nominal plan is of most importance for obtaining tumor control. (1) Albertini et al, PMB, 2011 (2) Lowe et al, PMB, 2016 OC-0703 Patterns of practice for adaptive and real- time radiation therapy: part I intra-fraction motion G. Distefano 1 , J. Bertholet 2 , P. Poulsen 3 , T. Roggen 4 , C. Garibaldi 5 , N. Tilly 6 , J. Booth 7 , U. Oelfke 8 , B. Heijmen 9 , M. Aznar 10 1 St. Luke's Cancer Centre Royal Surrey County Hospital, Radiotherapy Physics, Guildford, United Kingdom ; 2 The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Joint Department of Physics-, London, United Kingdom ; 3 Aarhus University Hospital, Department of Oncology and Danish Center for Particle Therapy, Aarhus, Denmark ; 4 Varian Medical Systems Imaging Laboratory GmbH, Applied Research, Dättwil AG, Switzerland ; 5 European Institute of Oncology IRCCS, IEO- Unit of Radiation Research, Milan, Italy ; 6 Elekta Instruments AB and Uppsala University, Medical Radiation Physics- Department of Immunology- Genetics and Pathology, Uppsala, Sweden ; 7 Royal North Shore Hospital and University of Sydney, Northern Sydney Cancer Centre and Institute of Medical Physics, St.Leonards and Camperdown, Australia ; 8 The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Joint Department of Physics, London, United Kingdom ; 9 Erasmus MC Cancer Institute, Department of Radiation Oncology, Rotterdam, Netherlands Antilles ; 10 School of Medical Sciences- Faculty of Biology- Medicine and Health The University of Manchester, Division of Cancer Sciences, Manchester, United Kingdom Purpose or Objective The patterns of practice for adaptive and real-time radiation therapy (POP-ART RT) study aims to determine to which extent and how these methods are used in clinical practice and to understand the barriers to implementation. Here we report on part I: real-time respiratory motion management (RRMM). Material and Methods An institution-specific questionnaire developed during the 2 nd ESTRO physics workshop was distributed worldwide. The focus was both on current practice and wishes for implementation. Therefore, centres not (yet) doing RRMM were encouraged to participate. RRMM was defined as the use of gating in free-breathing (FB) or breath-hold (BH), or tracking if the beam is continuously realigned with the target in real-time (via robotic or gimbal guidance, MLC or couch tracking). Respondents were asked if they used RRMM for selected tumor sites, the percentage of patients treated with Proffered Papers: Proffered papers 37: 4D and Adaptive RT

Chordoma and Chondrosarcoma patients treated at PSI with pencil beam scanned (PBS) proton therapy between 2003-2017. Material and Methods Between 2003 and 2017, 222 patients with skull-base chondrosarcomas and Chordomas were treated with PBS PT at PSI, to a median total dose of 70CGE and 74CGE respectively. All plans were optimized on a 5mm isotropical expansion of the CTV using the in-house developed treatment planning system. Follow-up MRIs have been systematically acquired and local failures (LFs) identified and contoured in 49 (22%) out of 222 patients. For all cases, treatment robustness to both set-up (1.76 mm for bite-block and 2.25 mm for mask considering Confidence Interval of 85%) and range (3%) have been retrospectively computed, and error bar distributions generated using the 'worst case scenario' approach (1,2). Finally, dosimetric and robustness parameters calculated from Dose Volume Histograms and Error Volume Histograms (EVH) were computed for both the full CTV, as well as the union of the recurrence volume and CTV, for the full delivered treatment plan. Results A minor correlation has been found between target under- dosage and recurrence (Figure 1) with the mean dose (Dmean) to the GTV and CTV being lower (95% compared to 98%) for those patients who suffered a local failure, but this dos-difference was not significant. However, no correlations were found between LF and robustness for either the whole CTV or for the overlapping regions of recurrence (Figure 2). Indeed, dose robustness has been found to be generally better (EVH shifted to the bottom left) in the recurrence region compared to the whole CTV (figure 2).