ESTRO 36 Abstract Book

S537 ESTRO 36 _______________________________________________________________________________________________

2 University of Naples Federico II, Department of Physics, Napoli, Italy 3 Azienda Ospedaliera Universitaria Federico II, Radiation Oncology Department, Napoli, Italy 4 National Council of Research CNR, Institute of Biostructures and Bioimaging, Napoli, Italy Purpose or Objective The innovative radiotherapy techniques such as Intensity- Modulated Radiation Therapy (IMRT) and Volumetric- Modulated Arc Therapy (VMAT) allow for more conformity of dose to the tumor target and sparing of healthy tissues. However, these techniques require an increase of monitor units (MUs) and therefore an increase of treatment/delivery times for each fraction. In addition, a higher dose outside the field caused by photons scattering in the flattening filter (FF) is expected. Flattening Filter- Free (FFF) photon beams can deliver higher dose rates and reduce the treatment time by about a factor 4 compared to conventional photon beams. Additional benefits also include reduced head scatter, a lower peripheral dose and neutron contamination. Purpose of the present study is to compare the radiobiological effects of FFF versus FF photon beams in mammary epithelial tumor cells (MCF7). Material and Methods MCF7 cells were irradiated with conventional and FFF 6MV photon beams using a TrueBeamSTx (Varian Medical Systems). Different dose rate values were considered (600 MU/min and 1400 MU/min). The cells were exposed to 0.25, 0.5, 1.0, 2.0 and 4.0 Gy doses. The number of monitor units required to deliver the desired doses to the cells was calculated using Pinnacle 3 (Philips) Treatment Planning System (TPS). Irradiations were performed with the flasks placed on 5 cm of equivalent water phantom (RW3) slabs and gantry angle at 180° to deliver homogeneous dose to the cell layer. A check of the actual dose delivered to cells was done exposing 9 thermo- luminescent dosimeters (LiF:Mg,Ti TLD-100) placed on the bottom of one of the irradiated flask. Clonogenic cell survival of MCF7 cells was determined. Cell survival data were fitted to linear-quadratic model. Results In the investigated dose range (0-4Gy), no statistically significant differences on breast cancer cell survival curves was observed a) with or without flattening filter (600 MU/min vs. 600 FFF MU/min) and b) at different dose rates (600 FFF MU/min vs 1400 FFF MU/min). Cell survival curves are reported in figure 1.

dose was 50 Gy in 25 fractions to the PTV. Monte Carlo based dose calculation engine was the preferred choice as it is more accurate at low dose levels which is more relevant for estimating SCR. Appropriate model parameters were taken from the literature for the mechanistic model to calculate excess absolute risk (EAR), lifetime attributable risk (LAR), integral dose and relative risk (RR) for both lungs, contralateral breast, heart and spinal cord. Results The mean MU in IMRT and VMAT plans were 751.1±133.3 and 1004.8±180 respectively for IMRT and VMAT. The mean EAR values per 10,000 person years (PY) estimated for IMRT and VMAT treatments including gender-specific correction with and without age correction factor are shown in figure 3. The mean EAR values with one standard deviation without age correction were 42.4±11.3, 10.6±6.0, 12.3±6.7, 1.9±0.7 and 0.6±0.3 for left lung, right lung, contralateral breast, heart and spinal cord respectively for the IMRT plans. These values were 51.9±19.7, 28.7±11.4, 31.9±13.4, 2.3±0.8 and 1.5±0.8 for the VMAT plans. However the values were reduced with age correction, especially for the contralateral breast. The values obtained with age correction were 44.6±11.9, 11.2±6.4, 5.4±4.0, 1.4±0.5 and 0.3±0.2 for left lung, right lung, contralateral breast, heart and spinal cord respectively for the IMRT treatments and 54.6±20.6, 30.2±12.0, 13.8±8.6, 1.6±0.6 and 0.9±0.5 for the VMAT treatments.

Conclusion Results showed VMAT plans had a higher risk of developing second malignancy in lung, contralateral breast, heart and cord compared to IMRT plans. However, the increase in risk was found to be marginal. The increase in risk was greater in both IMRT and VMAT for left lung and contralateral breast compared to other organs included in the study. Incorporating the age correction factor decreased the risk of contralateral breast SCR. No strong correlation was found between EAR and MU. PO-0972 Breast cancer cell survival using flattening filter-free beam compared to a standard flattened beam M. Boccia 1 , L. Manti 2 , S. Clemente 3 , C. Oliviero 3 , F. Perozziello 2 , R. Liuzzi 4 , M. Conson 1 , L. Cella 4 , R. Pacelli 1 1 Federico II University School of Medicine, Department of Advanced Biomedical Sciences, Napoli, Italy