ESTRO 2020 Abstract book

S290 ESTRO 2020

neck cancer squamous cell carcinoma cells and normal keratinocytes (α/β > 10).

PH-0479 The influence of the tissue specific parameter α/β on the RBE of protons E. Mara 1 , S. Khachonkham 2 , M. Clausen 3 , P. Kuess 3 , C. Pessy 3 , W. Dörr 3 , D. Georg 3 , S. Gruber 3 1 University of Applied Science, Biomedical Science, Wiener Neustadt, Austria ; 2 Faculty of Medicine Ramathibodi Hospital- Mahidol University, Department of Diagnostic and Therapeutic Radiology, Bangkok, Thailand ; 3 Department of Radiotherapy, Department of Radiation Oncology, Vienna, Austria Purpose or Objective Particle radiotherapy is used to treat an increasing number of patients worldwide. Clinical proton facilities use a generic RBE of 1.1, i.e. protons are assumed being 10 % more effective than photons. Recent research increasingly challenged the concept of a static RBE in proton therapy. In this study, the RBE of scanned protons was studied in- vitro for head-and-neck squamous cell carcinoma-, prostate carcinoma- and melanoma cells as well as normal keratinocytes. Furthermore, RBE correlations with the LET (linear energy transfer) and the intrinsic radiosensitivity factor α/β were evaluated. Material and Methods Cells were irradiated at different positions within a 8 cm long SOPB (proximal, middle and distal position) using two different target depths (Figure 1). Both, target A, covered by lower energies (66 – 136 MeV) and target B, covered by higher energies (137 – 180 MeV) were irradiated with physical doses of 0.5, 1, 2, 4 and 6 Gy. The treatment planning system (TPS) RayStation (V5.99, Raysearch Laboratories, Sweden) was used. Corresponding dose averaged LET (LET d ) was derived using the Monte Carlo algorithm provided in the TPS. As reference irradiation, 200 kV X-rays were used. A negative control was conduct and handled as the irradiated samples only without the irradiation treatment. The LQ model was applied to the in vitro data.

Conclusion The intrinsic α/β ratio of a tissue appears to be a primary influencing factor for the RBE of protons and its sensitivity towards LET changes. These findings indicate a rational patient stratification according to the individual malignancy type and location. Patients with tumours with comparably lower α/β ratios than the surrounding normal tissues may benefit most from proton therapy. Tumours with high α/β ratios in close proximity to organs at risk with low α/β ratios, may respond less while an increased risk for developing significant late adverse effects can be expected. PH-0480 Early repair mechanisms after Synchrotron Microbeam Radiation Therapy in normal lung tissue V. Trappetti 1 , C. Fernandez-Palomo 1 , P. Pellicioli 2 , V. Djonov 1 1 University of bern, Institute of Anatomy, Bern, Switzerland ; 2 Grenoble Alpes University, European Synchrotron Radiation Facility, Grenoble, France Purpose or Objective Synchrotron Microbeam Radiation Therapy (MRT) is a pioneering radiotherapy technique that spatially fractionates the beam of synchrotron X-rays into an array of quasi-parallel micro-planar beamlets. MRT is effective at delaying and even suppressing the growth of several types of tumors, with minor collateral effects on the normal tissue. One of our studies, conducted in rodents, demonstrated that MRT elicited a minor degree of pulmonary fibrosis compared to synchrotron Broad Beam (BB) radiotherapy even at 600 Gy peak-dose and one year post-irradiation (pi). The aim of this project is to elucidate the early cellular and molecular mechanisms responsible for comprehensive tissue repair and minor degree lung fibrosis after MRT. Material and Methods To identify the underlying mechanisms, we established an ex-vivo mouse model of lung tissue slices (Fig.1). To the latter synchrotron MRT was applied (50 μm wide beams; 400 μm center to center) with peak-doses of 100, 200, 400 and 800 Gy. Immunostaining and confocal microscopy were performed at different time points: 1h, 4h, 12h, 24h and 48h pi.

Results A strong correlation between LET and RBE was found for prostate carcinoma- and melanoma cells (α/β < 4.5)(Figure 2). For both low α/β cell lines RBE values between 1.28 and 1.76 were determined, with highest RBE values at the distal SOBP positions. Cell lines with a high α/β ratio showed only a marginal LET-dependency; however, RBE values between 1 and 1.3 were observed for head-and-