Variation of 4 MV X-ray dose rate strongly impacts biological response both in vitro and in vivo

Abstract

Whereas an RBE > 1 is described for very low-energy X-ray beams (in the range of 25-50 kV), there is a consensus that the RBE of X-rays (from 0.1 to 3 MeV) is equal to 1, whatever the energy or dose rate of the beam. Comparisons of X-ray beam dose rates are scarce even though these beams are widely used in medical diagnosis or radiotherapy. By using two dose rates (0.63 and 2.5 Gy.min −1) of high-energy X-rays on normal endothelial cells (HUVECs), we have studied the clonogenic assay, but also viability/ mortality, cell cycle analysis and measured cellular senescence by flow cytometry, and have performed gene analysis on custom arrays. In order to consolidate these data, we performed localized irradiation of exteriorized small intestine at 0.63 and 2.5 Gy.min −1. Interestingly, in vivo validation has shown a significantly higher loss of weight at the higher dose when irradiating to 19 Gy a small fragment of exteriorized small intestine of C57Bl6J mice. Nevertheless, no significant differences were observed in lesioned scores between the two dose rates, while bordering epithelium staining indicated twofold greater severe damage at 2.5 Gy.min −1 compared to 0.63 Gy.min −1 at one week post-irradiation. Taken together, these experiments systematically show that the relative biological effectiveness of photons is different from 1 when varying the dose rate of high-energy X-rays. Moreover, these results strongly suggest that, in support of clonogenic assay, multiparametric analysis should be considered to provide an accurate evaluation of the outcome of irradiated cells. Relative biological effectiveness (RBE) is the ratio of the dose of one kind of ionizing radiation relative to another to produce the same biological effect. Several studies have focused on dose rate effects, but they were mainly performed at low dose rates and by using 137 Cs 1,2 or 60 Co 3,4 sources. A few studies have directly compared the in vitro effects of different dose rates of X-rays on cancer cells, but not on normal human cells 5-9. It is also known that RBE increases as LET increases up to 100 KeV.µm −1 , above which RBE decreases because of cellular overkill 10. Moreover, RBE for protons is also described as endpoint-dependent 11 , while there is a consensus that the RBE of X-rays (photons; energy from 0.1 to 3 MeV) is equal to 1, whatever the energy or dose rate of the beam 12. Importantly, higher RBE is described for very low-energy X-ray beams (in the range of 25-50 kV) 13-16. Nevertheless, modern radiotherapy uses medical devices (mostly 6-10 MV) able to deliver doses up to 20 Gy. min −1 , assuming that the RBE of the X-ray beam remains equal to 1 whatever the energy and/or dose rate. To verify this, and build a proof of concept both in vitro and in vivo, we set as our reference a beam of high energy X-rays (4 MV) at 0.63 Gy.min −1 on a Linear Accelerator (LINAC) Elekta Synergy Platform. Independently of the radiation type and delivery technique, several dosimetric studies of water phantoms or of real treatment planning conditions 17-1...

Biological effectiveness of very high gamma dose rate and its implication for radiological protection

“…This conclusion fits well with recently reported effects of ultra-high dose rate (UHDR) on melanoma cells (Buontempo et al 2018 ). Also, the presented results support the recent multiparametric study, demonstrating a higher biological effectiveness of 4 MV photons delivered at a dose rate of 2.5 Gy min −1 as compared to 0.63 Gy min −1 (Ben Kacem et al 2020 ).…”

Deep models of integrated multiscale molecular data decipher the endothelial cell response to ionizing radiation

“…The endothelial cell model of radiation-induced dysfunction we used has been extensively characterized in our previous work ( Ben Kacem et al., 2020 ; Heinonen et al., 2015 , 2018 ; Jaillet et al., 2017 ; Paget et al., 2019 ). In this model, at a dose of 20 Gy, the cells stopped proliferating as early as one day after irradiation and did not proliferate at all by the third day ( Ben Kacem et al., 2020 ; Paget et al., 2019 ). In addition, we estimated that at day 7 post-20 Gy only 2%–3% of cells had divided, and at most once, during the first 7 days after irradiation ( Paget et al., 2019 ).…”

Variation of 4 MV X-ray dose rate in fractionated irradiation strongly impacts biological endothelial cell response in vitro

“…Nonetheless, a clear distinction must be made with very low-energy X-ray beams (in the range of 25-50 k V) for ave been reported (Slonina et al, 2003;Gomolka et al, 2005). Moreover, we recently demonstrated that when using a multiparametric RBE measurement approach, the RBE of photons deviated from 1 when varying the dose rate of high-energy X-rays (Ben Kacem et al, 2020) as well as varying the X-ray energy between 220 kV and 4 MV (Paget et al, 2019).…”