ESTRO 2020 Abstract book

S354 ESTRO 2020

All cases with a positive PC had bilateral neck (as opposed to unilateral) RT and had definitive (as opposed to postoperative) RT. 64% of patients with positive PC had a primary oropharynx carcinoma, other tumor sites included nasopharynx, hypopharynx and larynx. 71% were node- positive, often with bulky disease. No cases of laryngeal carcinoma with N0-status but treatment of elective nodal volumes qualified for PrTh. Qualification for PrTh was based on ΔNTCP for Gr 2 xerostomia in 9 patients, for Gr 2 dysphagia in 1, for Gr 3 dysphagia in 2 and for combined Gr 2 xerostomia & dysphagia in 2 Conclusion Using the model-based approach, approximately 25% of HNC patients qualified for protontherapy based on predicted reduction of moderate to severe xerostomia and/or dysphagia 1 Korevaar EW et al. Radiother Oncol 2019, in press OC-0580 Bringing FLASH to the clinic: treatment planning considerations for ultrahigh dose-rate proton beams P. Van Marlen 1 , M. Dahele 1 , M. Folkerts 2 , E. Abel 2 , B. Slotman 1 , W. Verbakel 1 1 Amsterdam UMC, Radiation Oncology, Amsterdam, The Netherlands ; 2 Varian Medical Systems, Varian Medical Systems, Palo Alto, USA Purpose or Objective Pre-clinical research into ultrahigh dose-rate (e.g. ≥40Gy/s) “FLASH”-radiotherapy suggests a decrease in side-effects compared to conventional irradiation, while maintaining tumor control. There are multiple treatment planning considerations for FLASH: dosimetric, temporal and spatial parameters all seem relevant. Until recently, most FLASH experiments used single field electron or photon beams, with a homogeneous dose-rate. A possible solution for clinical FLASH delivery for deeper lying tumors is to use scanning proton beams, which have no field size limitation. In this case, tissue becomes subject to an entire range of spatially dependent dose-rates. We systematically investigated dose-rate distributions and delivery times for proton FLASH-plans using stereotactic lung irradiation as the paradigm. Material and Methods Stereotactic lung radiotherapy FLASH-plans (3x18Gy to 95% of the PTV), using 10 scanning proton transmission beams of 244MeV (Bragg peak behind the body), were made for 7 patients with the intention to achieve OAR dose≤clinical VMAT-plans. Currently, only beams ≥244MeV can be delivered at FLASH dose-rates; transmission beams do not require in-room energy modulation and are not influenced by range uncertainties. Irradiation time per voxel and dose rate distributions were calculated. Beams with spot peak dose rate (SPDR, dose rate in the middle of the spot) of 100, 200 and 360Gy/s were investigated. Evaluated parameters were: quality of the FLASH-plans compared to the VMAT-plans, dose-rate distribution within a beam, overall irradiation time and number of times tissue is irradiated. A FLASH dose-rate threshold of 40Gy/s was used. Results Transmission beam FLASH-plans had ITV and PTV doses comparable to VMAT-plans, but achieved lower doses for most OARs, with no heart and contralateral lung dose. For SPDR=100Gy/s ~40% of dose is delivered at FLASH dose- rates, while for SPDR=360Gy/s this increased to ~75% (Fig 1). For SPDR=360Gy/s, the average dose per beam is 0.95Gy and the beam area receiving ≥90% FLASH

contribution received an average dose of 1.45Gy, while FLASH-contributions ≤10% received a dose of 0.065Gy. 100% FLASH dose-rate cannot be achieved due to small (non-FLASH) contributions from distant spots with lower dose-rates. The beam irradiation time per beam varied between 10-150ms, with almost 95% <100ms. However, not all spots contribute to the dose in a certain beam voxel. 67% of the dose-receiving body volume was irradiated by 1 beam and ~85% by ≤2 beams.

Conclusion The FLASH efficiency of a scanning proton beam increases with SPDR. An SPDR of 100Gy/s is insufficient to deliver FLASH. The methodology proposed in this proof-of- principle study provides a framework for evaluating FLASH characteristics of scanning proton beam plans and can be adapted as FLASH-parameters are better defined. It currently seems logical to optimize plans for the shortest delivery time, maximum amount of high dose-rate coverage, and maximum amount of single beam and continuous irradiation. OC-0581 FLEPAC: A FLASH therapy treatment planning system for electrons, protons, Helium and Carbon ions. F. Van den Heuvel 1 , J. Ruan 1 , F. Fiorini 1 , A. Vella 1 , M. Brooke 1 , A. Kiltie 1 , A. Ryan 1 , B. Vojnovic 1 , M. Hill 1 1 University of Oxford, Dept of Oncology, Oxford, United Kingdom Purpose or Objective To present a treatment planning system (FTPS) which can model flash related effects using an oxygen depletion model and apply them to flash experiments carried out in our department. As well as published data from other groups. The goal of this paper is three fold: 1) Relate the model to micro-dosimetric calculations, 2) Test a treatment planning system by re-constructing experiments and published data, 3) Investigate possible applications of an FTPS. Material and Methods We adapted an open source planning system "matRad" (DKFZ, Germany) to calculate the induction of complex damage (any damage more complex than at least a double strand break) of various charged particles at different oxygenation levels. The model was compared to micro dosimetric code MCDS (University of Washington, USA) for an energy and oxygenation level range. We hypothesize that the flash protective effect is a result of total oxygen depletion within a delivered pulse, resulting in a reduced