Abstract Book

S1276

ESTRO 37

Material and Methods Melanoma (A375) and glioma (LN18) cells in 96-well plates were irradiated to 15, 18 and 24 Gy using 10 MV X- rays with two dose-rate settings (400 monitor units (MU) /min and 2400 MU/min) in a linear accelerator using a custom made perspex phantom. Cell viability (tetra- zolium colorimetric cell viability (MTS) assay, Promega CellTiter 96 ® AQueous One) and double-strand DNA breaks (γ-H2AX, In-Cell Western® assay, LI-COR Biotechnology) were measured and compared. In addition, melanoma cells were irradiated to 18 Gy using 10 MV and 2400 MU/min X-rays either continuously (cont) or with 3 minute breaks (break_3) or 10 minute breaks (break_10) after delivering each 3 Gy. Cell viability and double- strand DNA breaks were measured and compared. Graphpad Prism was used to generate graphs and for data analysis. Results For melanoma cells, mean cell viability with 400 MU/min radiation dose-rate for 15, 18 and 24 Gy were 0.66, 0.65, 0.65, and with 2400 MU/min they were 0.65, 0.63, 0.69. For glioma cells, the corresponding values were 0.24, 0.22, 0.22 and 0.23, 0.22, 0.23. These differences were not statistically significant, p = .92 for melanoma and p = .87 for glioma cells ( Figure 1, A & B ). There were no significant differences in double-strand DNA breaks in melanoma cells as measured by the relative γ-H2AX signal intensities with mean values of 99.7, 120, 162.9 for 15, 18 and 24 Gy for 400 MU/min and 118.7, 102, 165.4 for 2400 MU/min dose-rate irradiations (p = .98; Figure 1, C & D ).

Conclusion No significant differences in biological effects were observed with different radiation dose-rates and intra- fraction breaks for typical radiation dose ranges used for clinical treatments with SRT. Further validation of our results with in-vivo models is required. However, we do not find significant evidence for radiobiological differences, atleast in terms of dose rate and intra- fraction breaks, between platforms such as robotic-, isotope-, and high dose rate linac-based SRT. EP-2312 Biological validation of a high-throughput in- vitro radiobiology platform K. Thippu Jayaprakash 1,2 , D. Mostafa 3 , M. Hussein 3 , A. Nisbet 3,4 , R. Shaffer 2 , M. Ajaz 1,2 1 University of Surrey, Department of Clinical and Experimental Medicine, Guildford, United Kingdom 2 St Luke's Cancer Centre- Royal Surrey County Hospital, Department of Oncology, Guildford, United Kingdom 3 St Luke's Cancer Centre- Royal Surrey County Hospital, Department of Medical Physics, Guildford, United Kingdom 4 University of Surrey, Department of Physics, Guildford, United Kingdom Purpose or Objective Pre-clinical in-vitro radiobiology platforms lack efficiency for conducting radiobiology experiments. We have developed and dosimetrically validated a high-throughput in-vitro radiobiology platform utilising an advanced clinical radiation technique, Intensity Modulated Radiotherapy (IMRT) to deliver multiple radiation doses to tissue culture plates in the same plan 1 . Here we report biological validation of this radiobiology platform. Material and Methods Melanoma (A375) and glioma (LN18) cells in 96- and 6- well tissue culture plates were placed in a custom made perspex tissue culture plate holder, and were irradiated in a linear accelerator with dosimetrically verified 96- and 6-well tissue culture plate IMRT plans to target radiation doses of 0, 2, 4, 8, 16 and 24 Gy. Cells were also irradiated with conventional open-field, homogeneous irradiations to the same nominal doses. Cell viability (tetrazolium colorimetric cell viability (MTS) assay, Promega CellTiter 96 ® Aqueous One), cell proliferation (clonogenic assay) and double-strand DNA breaks (quantitative γ-H2AX analysis, In-Cell Western ® assay, LI-COR Biotechnology) of plate IMRT plans were compared to those delivered by open-field irradiations. GraphPad Prism was used to generate graphs and for statistical analysis.

When 18 Gy was delivered with a continuous irradiation plan (cont), mean cell viability for melanoma cells was 0.46 and with intra-fraction breaks, it was 0.46 for break_3 and 0.49 for break_10, these differences were not statistically significant (p = .93 ; Figure 2, A) . No significant differences in double-strand DNA breaks were observed with relative γ-H2AX signal intensities of 107.2, 142.4 and 130.6 for irradiation plans cont, break_3 and break_10 respectively (p = .12 ; Figure 2, B & C) .