ESTRO 2023 - Abstract Book

S2001

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ESTRO 2023

Materials and Methods Cells were cultured under recommended growth conditions to 80% confluency, synchronized in G0/G1 cell cycle phase by serum starving 18 hours prior to irradiation, then trypsinized and plated for clonogenic assays post-irradiation. Irradiation with low LET photons for HeLa and PC3 include 6 MV TB Linac x-rays, and 225 and 50 kVp x-rays, and for HCT116 include 180 and 50 kVp x-rays. All 3 cell lines were also irradiated with a 192-Ir HDR brachytherapy source. EBT3 Gafchromic film dosimetry was performed to ensure homogeneous dose distribution and accurate dose delivery of 2, 4, 6, and 8 Gy (0 Gy control). Post-incubation, colonies formed were fixed/stained, and colonies with ≥ 50 cells were counted to calculate survival fractions (SF). Resulting SFs were plotted into survival curves and fitted to the LQ model. RBE values were calculated at the clinically relevant 2 Gy fraction and 10% SF endpoint, using 225 kVp x-rays (or 180 kVp for HCT116) as the reference radiation quality.

Results The cell killing probability of the investigated photon qualities increased with decreasing photon energies for HeLa, PC-3, and HCT116 cell lines as shown in Figure 1. In all cases, 50 kVp had an RBE >1 whereas 6 MV and 192-Ir irradiations had RBE values <1 as presented in Table 1, with 225 kV (for HeLa and PC-3) and 180 kV (HCT116) x-rays as reference for RBE calculations. HCT116 appears to be the most radiosensitive of the 3 lines tested and HeLa being most radioresistant.