Purpose: To investigate the relative biological effectiveness (RBE) of various photon beams on the HeLa and PC-3 cell lines.
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. Irradiations were done with a 6 MV TrueBeam Linac, and a Multi-Rad irradiation system producing 225 kVp (13mA, 0.5mm Cu filter, 202.1cGy/min) and 50 kVp (30mA, 0.5mm Al filter, 188.1cGy/min) photon beams. EBT3 Gafchromicᵀᴹ film dosimetry was performed to ensure homogenous 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 Linear-Quadratic model. RBE values were calculated at the clinically relevant 2 Gy fraction and 10% SF endpoint, using 225 kVp x-rays as the reference radiation quality.
Results: The photon qualities used in the study so far had more efficient cell killing with decreasing energies for both HeLa and PC-3. Furthermore, PC-3 cells were more radiosensitive than HeLa cells specifically with lower energy x-rays. The RBE₂ for 6 MV and 50 kVp x-rays were 0.877 and 1.07 respectively for the HeLa cell line, and 0.719 and 1.12 respectively for the PC-3 cell line.
Conclusion: For equal doses from photons with different energies, results indicate noticeable differences in RBE which increases with decreasing photon energy across the two cell lines. Clinically, all photons are considered to have an RBE of 1.0, however, our results show that MV x-rays have lower RBE than 50 kVp x-rays. Further measurements will be performed for investigation and characterization of other radiation qualities.
Funding Support, Disclosures, and Conflict of Interest: The project was partially funded by CRC Program (#950-232684) as well as by MEDTEQ+, the PSOv2b program, which is cofounded with TransMedTech, Mitacs, the MUHC Foundation and Alpha Tau Medical. No conflict of interest except that the peer-reviewed grant is co-funded by the company Alpha Tau Medical.
Linear Quadratic Model, RBE, X Rays