ESTRO 2020 Abstract book

S291 ESTRO 2020

control compared with conventional RT at similar broad beam (BB) and MRT valley doses. Material and Methods Male Fisher rats (n=160) bearing 9L gliosarcoma were irradiated with multiport MRT protocols (1 to 5 incidences, 10 Gy valley dose) and animal survival, tumor volume and histologic samples were compared with standard BB irradiations (10 Gy). To study effects of multidirectional MRT on healthy brain tissues, normal male Fisher rats (n=34) were irradiated with 2 and 5 MRT incidences (10 Gy valley dose) and standard BB (10 Gy). Throughout one year post exposure, radiation toxicity was evaluated through behavioral tests and histologic analysis. Results Compared with conventional broad beam (BB) irradiations, microbeam radiation therapy (MRT) sparsely modified normal rat behavior and damaged tissues in the beam- crossing area while unidirectionally irradiated brain areas, outside the target, appeared intact. Ten Gy valley (diffused dose in-between microbeams, 50µm wide, 400µm spacing, 726Gy), delivered by 1 MRT port, improved tumor control and median survival time (MST) of 9L-bearing rats compared with 10Gy BB. The more ports being added, the greater were the effects on tumor growth and survival, while histopathological features of cell death, vascular damage and inflammatory response also increased in tumors. At identical cumulated valley dose, each supplementary MRT port extended MST of 9L rats and an exponential correlation between the number of MRT ports and MST has been found (r²=0.9925). A 10Gy 5 port MRT led to the same MST as a unique BB 25Gy fraction, via the accumulation of peak doses (5x137Gy) in the target, while the reduction of each dose delivered per incidence induced less normal tissue damage. Conclusion Multiport MRT reached unexpected biologic equivalent doses (x2.5), an effect which is not achievable with any other radiotherapy method by basic ballistic effect. The remarkable normal tissue sparing and the outstanding therapeutic index make multiport MRT as a promising innovative method that deserves to be transferred in clinical environment, particularly for dose escalation schedules and local radiation-boost delivery protocols. PH-0482 Inter-observer variation of burned-in fiducial marker positions for MR-only prostate radiotherapy F. Beeksma 1 , J. Visser 1 , M. Boon 1 , K. Goudschaal 1 , M. Bijveld 2 , K. Hinnen 1 , Z. Van Kesteren 1 1 Amsterdam UMC - location AMC, Department of Radiotherapy, Amsterdam, The Netherlands ; 2 Catharina Hospital, Department of Radiotherapy, Eindhoven, The Netherlands Purpose or Objective In the current workflow for prostate radiotherapy the advantage of using MRI for target delineation is reduced due to the uncertainty in the registration with the planning CT. This systematic error contributes to the required PTV margin. In an MR-only workflow a pseudo-CT for treatment planning is generated from a specific MRI sequence, which eliminates the registration error between MRI and planning CT. However, the fiducial markers that are required for position verification are not visible on the pseudo-CT. We have created a semi-automatic method, for burning in Poster Highlights: Poster highlights 16 RTT: Novel strategies for treatment planning and delivery

Results The γH2A.X staining revealed persistent robust DNA damage in the cellular compartments exclusively within the beam paths after all doses up to 24h pi. Staining for type IV collagen does not reveal any structural damage to the basement membrane and ECM at all time points (Fig.2). Accumulation of macrophages negative for γH2A.X were observed along the beam paths over time suggesting that macrophages migrated from the “valley” regions between the beams.

Conclusion The presented data support our hypothesis: (i) microbeams damage the cellular components within the beam path. (ii) The intact macrophages from the surrounding “valley” regions infiltrate and remove cell debris from the beam path. (iii) In the next step, the unaffected lung cells repopulate the “beam” region using the basal membranes as a scaffold. PH-0481 Synchrotron Microbeam Radiation Therapy increases the therapeutic index for brain tumor treatment. L. Eling 1 , A. Bouchet 1 , A. Ocadiz 1 , J. Adam 1 , H. Elleaume 1 , M. Krisch 2 , J.A. Laissue 3 , J. Balosso 4 , R. Serduc 1 1 Synchrotron Radiation for Biomedicine, INSERM UA7, Grenoble, France ; 2 European Synchrotron Radiation Facility, Instrumentation Services and Development Division, Grenoble, France ; 3 University of Bern, Institut of Pathology, Bern, Switzerland ; 4 CRLCC Francois Baclesse- 14000 Caen- France, Radiotherapy, Caen, France Purpose or Objective In this work, we highlight the potential benefit of delivering very high radiation doses (hGy) to brain tumors through multiple arrays of microplanar synchrotron- generated X-rays. Microbeam Radiation Therapy (MRT) has been developed at the European Synchrotron Radiation Facility in Grenoble, France. The physical characteristics (low energy, quasi-null divergence) of synchrotron- generated X-rays allow the fractionation of the incidence beam into microbeams deposing hundreds of Gy in their paths (peak dose) while the dose diffusing in-between the microbeams (valley dose) consists of only 1-5% of the peak dose. MRT has shown to be remarkably well tolerated by normal brain tissue while immature tumor vessels are preferentially damaged, significantly improving tumor