ESTRO 38 Abstract book

S176 ESTRO 38

width. Secondary standard dosimeters such as ionization chambers require calibration in conventional x-ray sources with substantially different field sizes, dose-rates, polarizations and spectra to synchrotron beams. These differences add significant uncertainties to measurements of the dose in the synchrotron beam. Calorimeters and free air chambers can determine dosimetric quantities from first principles. Calorimetry has been successfully performed directly on Synchrotron beams as the high dose rates offers the advantage of increased temperature rise compared to conventional clinical linac calorimetry. Free air chambers (FAC) can also be used for absolute dosimetry, however they suffer from strong recombination and electron loss effects in Synchrotron beams. FAC are also useful tools for continuously monitoring in Synchrotron beams. The determination of the peak and valley dose of the microbeams is challenging. Understanding the valley dose is particularly important as this is a key parameter to spare the normal tissue, limiting the clinical dose delivered. Film and microdiamond detectors are commercially available and with careful use and analysis have had success in microbeam dosimetry. Other promising detectors are high spatial resolution fibre optic dosimeters (FOD) and silicon detector arrays. SP-0353 Compact microbeam sources and microbeam treatment planning S. Bartzsch 1,2 1 Klinikum Rechts Der Isar- Technical University Munich, Department of Radiation Oncology, Munich, Germany; 2 Helmholtz-Zentrum Munich, Institute Of Innovative Radiotherapy, Munich, Germany Abstract text Microbeam radiation therapy (MRT) is an approach in radiation oncology that modulates radiation doses on a micrometre scale. Homogeneous radiation fields are collimated into 25-100 micrometre wide beamlets with a few hundred micrometre spacing between each other. Abundant preclinical research demonstrated that MRT has the potential to change radiotherapy treatment paradigms for certain tumour types. However, the technical demands of MRT on dosimetry, dose calculation, treatment planning and on the radiation source are high. So far dose calculations in MRT have largely been based on Monte Carlo simulations. However, the small dimensions of the treatment fields, subsequent small voxel sizes and substantial dose differences between high and low dose regions entail long simulation times and often poor statistics. An alternative to Monte Carlo simulations are kernel based dose calculation approaches. Although these approaches are much faster they often lack accuracy at material boundaries due to the utilization of low energy photons in MRT. Hybrid dose calculation approaches use elements of both approaches and provide a very accurate and efficient way to calculate peak doses, valley doses and dose profiles. Even complicated cross-firing geometries can easily be calculated. Currently we implement a hybrid dose calculation engine in the popular treatment planning system Eclipse® (Varian), enabling treatment planning for first clinical trials in MRT. The meaning of peak and valley doses in MRT on tissue reactions remains controversial. In order to plan and carry out first clinical trials in MRT, estimates on the biological effectiveness of microbeams based on the physical dose distribution are required. Recently, it has been suggested to use the concept of equivalent uniform dose (EUD). This concept can be calculated for arbitrary and complex beam geometries and our own in-vitro data demonstrates its applicability. The major obstacle for a clinical application of MRT is the availability of adequate radiation sources. MRT requires high dose rates of more than 100 Gy/s, small beam divergence and photon energies of around 100 to 300

structure (e.g. central nervous system (CNS)) and pediatric cancer. One possible way to overcome this limitation is to employ new modes of radiation dose deposition that activate biological processes different from those acting in standard radiotherapy. An example is the spatial fractionation of the dose. This lecture will give a general overview about this strategy. A particular focus will be put on minibeam radiation therapy (MBRT) and its advantages. MBRT, originated at synchrotrons, can now explored outside large facilities thanks to its successful transfer into cost-effective equipment [1]. This allows a widespread implementation, the realisation of comprehensive and systematic biological studies and an easy transfer to potential clinical trials. In the recent years, the exploration of the possible synergies between the advantages of MBRT and the benefits of charged particles for therapy has started, with techniques like proton and heavy ions MBRT [2-7]. In particular, proton MBRT [2] has been implemented at a clinical center (Orsay proton therapy center) and it has already shown an effectiveness of tumor control equivalent or superior than that of standard PT without the important side effects observed in the latter, thus opening the possibility for more aggressive irradiation schemes [3-5]. Concerning heavy ions MBRT, the dosimetric data obtained supports the exploration of this radiotherapy approach [6,7]. Among the different ions species evaluated, Ne stands as the one leading to the best balance between high peak- to-valley dose ratio and peak-to-valley-LET ratio in normal tissues and high LET values in the target region [6]. The biological mechanisms in MBRT, which are not completely known, seem to contradict the classic RT paradigms. Its exploration offers a whole new horizon of both scientific research and potential future clinical practice. The spatial fractionation of the dose could especially benefit paediatric oncology (central nervous system), whose treatments are limited to the high risk of complications in the development of the infants. [1] Y. Prezado et al, Sci. Reports 7, article number 17295 (2017). [2] Y. Prezado and G. Fois, Med. Phys. 40, 031712, 1–8 (2013). [3] Y. Prezado et al., Scie. Reports 7, article number 14403 (2017). [4] Y. Prezado et al. Scientific Reports 8, article number 16479 (2018). [5] Y. Prezado et al, IJROBP (2018). [6] W. Gonzalez and Y. Prezado, Medical Physics 45, 2620- 2627 (2018). [7] I.Martinez-Rovira et al. Med. Phys. 44, 4223–4229 (2017). SP-0352 Dosimetry measurement in microbeam therapy J. Lye 1 , P. Harty 2 , D. Butler 2 1 Australian Radiation Protection And Nuclear Safety Agency, Australian Clinical Dosimetry Service, Melbourne- Victoria, Australia; 2 Australian Radiation Protection And Nuclear Safety Agency, Psdl, Melbourne- Victoria, Australia Abstract text Microbeam radiation therapy (MRT) with Synchrotron beams continues to evolve from promising research towards clinical or veterinary radiotherapy. The change from research to clinical applications requires many developments including dedicated treatment planning, image guidance, patient quality systems and high quality dosimetry. The focus of this presentation is on the challenge of absolute and relative dosimetry as performed at the Australian Synchrotron. The Synchrotron dosimetry chain consists firstly of absolute dosimetry in a “broadbeam”- in this context field sizes of a few centimetres. The next stage is relative dosimetry in the microbeams approximately 10-100 microns in