ESTRO 38 Abstract book

S563 ESTRO 38

propose different approaches that would enable the beam size reduction required to obtain proton minibeams.

PO-1017 Towards magnetically focused proton minibeams: investigating the limits of a clinical PBS nozzle T. Schneider 1 , L. De Marzi 2 , A. Patriarca 2 , Y. Prezado 1 1 Imagerie et Modélisation en Neurobiologie et Cancérologie IMNC, CNRS - Université Paris 11 - Université Paris 7, Orsay, France ; 2 Institut Curie - PSL Research University, Centre de Protonthérapie d'Orsay - Radiation Oncology Departmentn, Paris, France Purpose or Objective Proton minibeam radiation therapy [1] is a novel and promising technique which already showed a remarkable widening of the therapeutic window for radioresistant tumours [2,3]. So far, this technique has been implemented at a clinical centre [4,5] by means of a multi- slit collimator. The goal of this work was to evaluate the feasibility of generating proton minibeams at a clinical beamline without the need of an external collimator. This would maximise the dose rate, reduce the neutron production and pave the way for 3D intensity-modulated treatment planning. The biological data obtained in recent experiments indicates that the tolerances of normal tissue can be increased when irradiated with narrower beams. In particular, in order to maximize the normal tissue sparing, beam widths narrower than 3 mm are preferred [2, 6]. However, in the current clinical configuration, the beam spot size at isocentre is 18 mm at 100 MeV and 10 mm at 200 MeV (values referring to the full width at half maximum [FWHM] of the lateral beam profile). Material and Methods The Monte Carlo simulation toolkit TOPAS v.3.1.p02 [7] was used to model a complete pencil beam scanning nozzle (IBA Proteus PLUS) including the quadrupole and dipole magnets. The model was benchmarked against experimental data and the impact of different modifications of the beamline on the spot size and beam divergence was evaluated. Among others, the modifications included changing the magnetic field of the focusing elements, adding extra focusing magnets and reducing the distance between certain nozzle components. Results The limits of the current nozzle have been established: the beam widths cannot be made smaller than 12.3 mm at 100 MeV or 6.4 mm at 200 MeV by changing only the magnetic fields. The addition of extra quadrupole magnets at the exit of the nozzle allows to obtain widths as small as 1.8 mm at 100 MeV and 1.4 mm at 200 MeV. Furthermore, the reduction of nozzle dimensions, keeping only essential components allows to reach 1.1 mm at 100 MeV and 0.6 mm at 200 MeV. Conclusion This is the first study evaluating a collimator-free generation of proton minibeams at a clinical centre. Our first results indicate that the implementation at current clinical beamlines will be challenging. However, we

References [1] https://doi.org/10.1118/1.4791648

[2] https://doi.org/10.1038/s41598-017-14786-y [3] https://doi.org/10.1016/S0167-8140(18)30879-X

[4] https://doi.org/10.1118/1.4935868 [5] https://doi.org/10.1002/mp.13209

[6] Proton minibeam radiation therapy leads to a superior tumor control than standard proton therapy in RG2 glioma- bearing rats. Submitted to Int. J. Radiat. Oncol. Biol. Phys. [7] https://doi.org/10.1118/1.4758060 PO-1018 Current status of pediatric image-guided radiation therapy in Europe: An international survey C. Windmeijer 1 , A. Bel 1 , R. De Jong 1 , B. Balgobind 1 , G. Collaboration 2 , C. Rasch 1 , I. Van Dijk 1 1 Amsterdam UMC- location AMC, Radiation Oncology, Amsterdam, The Netherlands ; 2 Participating institutions, Radiation Oncology, Various Europe, The Netherlands Purpose or Objective Image-guided radiotherapy (IGRT) enables high precision tumor treatment while sparing healthy tissues. Particularly in pediatric radiotherapy the value of IGRT is widely acknowledged, but there is no consensus on the ‘best practice’. With this survey we aim to evaluate clinical pediatric IGRT patterns in European radiotherapy institutes. Material and Methods An eight-domain survey based on seven treatment sites was sent to members of the Pediatric Radiation Oncology Society and/or our IGRT project-based consortium in 70 European institutes. The domains include items on radiotherapy preparation, planning and delivery. Responses were collected from June-September 2018. Results In total, 42/70 institutes (60%) responded; 33/42 (79%) treat children, one of which focuses exclusively on total body irradiation. The number of children treated annually varies between institutes and per site from 1 to 130 (Figure 1). Photon/electron therapy is used in 26/33 (79%) centers, and 3/33 (9%) use photon therapy only. Proton therapy (PT) is available in 5/33 (15%) institutes, whereas 7/28 (25%) of photon centers refer to proton centers; 2/33 (6%) use both photon and proton therapy. To immobilize patients, facial masks are used in 100% of brain, craniospinal axis (CSA) and head-and-neck (H&N) radiotherapy (all devices in Table 1). Most institutes (89% (thorax), 93% (abdomen), 96% (extremities) and 100% for other sites) use 3DCT scans to define the treatment target. Also MRI (range, 79% for thorax to 97% for brain), PET (range, 21% for CSA to 79% for H&N), and, for thorax and abdomen, 4DCT scans (by respectively 43% and 31% of institutes) are used. IMRT/VMAT is the most common treatment technique (range, 71% for CSA to 87% for brain), followed by 3DCRT (range, 36% for H&N to 69% for