ESTRO 2021 Abstract Book

S476

ESTRO 2021

volumes. Results

A total of 376 patients was treated between January 2011 and May 2020, of which 18% required full replanning. 52% of these cases was attributable to shortcomings of the motion model. The total use of back-up plans decreased from 20% from 2011 to 5-10% beyond 2014. Figure 1 shows that this and protocol adjustments have improved the PotD strategy over time, resulting in a mean decrease of 500 cc in PTV w volume. A stabilization is visible after 2015. Figure 2 displays the boxplots of the PTV BU , PTV w , and PTV fully adapt of patients treated in the second half of 2016 and later. This figure illustrates that the use of the plan library decreased PTV w with 17% compared to PTV BU and that a fully online-adaptive strategy could increase this reduction to 37%.

Conclusion This long-term analysis shows that plan-library-based PotD can consistently reduce PTV volume by 17%. Further reduction is possible, but at the expense of a higher workload.

OC-0614 Patterns Of Practice in Adaptive and Real-Time Particle Therapy, part II: interfractional changes P. Trnkova 1 , Y. Zhang 2 , B. Heijmen 3 , C. Richter 4 , M. Aznar 5 , F. Albertini 2 , A. Bolsi 2 , J. Daartz 6 , A. Knopf 7,8 , J. Bertholet 9 1 Medical University of Vienna, Department of Radiation Oncology, Vienna, Austria; 2 Paul Scherrer Institute, Center for Proton Therapy, Villigen-PSI, Switzerland; 3 Erasmus Medical Center, Department of Radiotherapy, Rotterdam, The Netherlands; 4 Technical University Dresden, OncoRay - National Center for Radiation Research in Oncology, Dresden, Germany; 5 University of Manchester, Division of Cancer Science, Faculty of Biology, Medicine and Health, Manchester, United Kingdom; 6 Massachusetts General Hospital, Department of Radiation Oncology, Boston, USA; 7 University Medical Center Groningen, Department of Radiation Oncology, Groningen, The Netherlands; 8 Center of Integrated Oncology Cologne, University Hospital of Cologne, Department I of Internal Medicine, Cologne, Germany; 9 Inselspital, Bern University Hospital, Division of Medical Radiation Physics, Bern, Switzerland

Purpose or Objective Online and offline treatment plan adaptation (here defined as Adaptive Particle Therapy, APT) and