ESTRO 36 Abstract Book

S532 ESTRO 36 _______________________________________________________________________________________________

at different depths on their way through the irradiated volume. Material and Methods A total of six different minibeam sizes were applied to the ear of Balb/c mice using 20 MeV protons. The average dose of 60 Gy was distributed in 4x4 minibeams with beam sizes of σ = 0.09, 0.2, 0.31, 0.45, 0.56 and 0.9 mm and a beam- to-beam distance of 1.8 mm. Inflammatory response, i.e. ear swelling and skin reactions, were observed for 90 days after irradiation. Results The results show a link between the applied beam sizes and the dimension of acute side effects after irradiation. The largest beam sizes lead to significant inflammatory reactions such as ear swelling, erythema and desquamation within 3-4 weeks after irradiation. The maximum skin reactions were reduced with decreasing beam sizes until almost no ear swelling or other visible skin reactions to the irradiation could be detected. Conclusion Our results show that the tissue sparing effect of proton minibeams is highest for the smallest beam sizes as occurring in the superficial layers of an irradiated volume. The positive effect decreases with increasing beam size and is therefore smallest for the biggest beam size which is equivalent to a homogeneous dose as desired in the target volume. However, since all minibeams have significantly reduced acute side effects compared to broad beam irradiation, proton minibeam radiotherapy may offer various possibilities for new approaches in clinical proton radiotherapy. Supported by the DFG Cluster of Excellence: Munich- Centre for Advanced Photonics. PO-0963 RBE variations along the Bragg curve of a 200 MeV proton beam C. Vandevoorde 1 , A. Baeyens 2 , A. Vral 2 , J. Slabbert 3 1 iThemba LABS, Radiation Biophysics, Cape Town, South Africa 2 Ghent University, Basic Medical Sciences, Ghent, Belgium 3 iThemba LABS, Medical Directorate, Cape Town, South Africa Purpose or Objective A lack of strong radiobiological datasets has resulted in the clinical adoption of a fixed, generic relative biological effectiveness (RBE) of 1.1 in current proton therapy (PT). However, in the distal area of the spread-out Bragg peak (SOBP), the RBE is certainly higher than 1.1 due to the rapid decrease in proton energy, resulting in an increased linear energy transfer (LET). Therefore, the RBE was quantified at different positions of the depth-dose profile for the 200 MeV clinical proton beam at iThemba LABS. Material and Methods V-79 fibroblasts were irradiated as monolayers at the plateau, proximal, middle and distal positions, as well as in the distal edge (32% of the maximum dose) of a 7 cm SOBP. At the same time, V-79 cells were also irradiated with 60 Co γ-rays as reference radiation. α and β values were determined from the cell survival curves and the 95% confidence ellipses of these covariant parameters were compared in the analysis. Mean inactivation dose (MID) values were calculated and used for the RBE calculations. Results A large overlap in the 95% confidence ellipses was observed for proton plateau and 60 Co γ-rays, so there is no statistical significant difference in radiation quality. The MID decreases with depth from 3.65 Gy at the entrance plateau, to 3.52 Gy, 3.40 Gy and 3.15 Gy for the proximal, middle and distal position along the SOBP respectively. Since the entrance plateau results were not significantly different from 60 Co γ-rays, RBE was calculated based on the plateau MID as a reference. This resulted in RBE values of 1.04, 1.07 and 1.16 for the proximal, middle and distal positions respectively. Furthermore, a clear separation

was observed between the 95% confidence ellipses for the three positions in the SOBP (see Fig 1). An RBE increase up to 1.46 was determined for the distal fall-off position (see Fig 2).

Conclusion The results obtained in the current study with V-79 fibroblast cells confirm the expected increase in RBE along the proton Bragg curve.

Poster: Radiobiology track: Radiobiology of head and neck cancer

PO-0964 Biomarkers in wound drainage fluids affect response to radiations of head and neck cancer cells M. Mangoni 1 , M. Sottili 1 , T. Gualtieri 2 , A. Javarone 2 , M. Loi 1 , I. Meattini 1 , P. Bonomo 1 , I. Desideri 1 , A. Deganello 2 , L. Livi 1 1 University of Florence, Experimental and Clinical Biomedical Sciences, Firenze, Italy 2 University of Florence, Academic Clinic of Otorynolaryngology and Head and Neck Surgery, Firenze, Italy Purpose or Objective In recent years in head and neck oncology many efforts have been made in order to characterize molecular biomarkers with potential prognostic and therapeutic value. The detection of significant features in the early perioperative setting could possibly lead to a refinement of current adjuvant treatments in high-risk patients. The purpose of our study is to report the feasibility and preliminary results of a pilot prospective study on wound drainage fluids (WDF) analysis in head and neck squamous cells carcinoma (HNSCC) and to evaluate effect of WDF microenvironment on HNSCC response to radiation.