Inactivation of human normal and tumour cells irradiated with low energy protons

Purpose: To analyse the cell inactivation frequencies induced by low energy protons in human cells with different sensitivity to photon radiation. Materials and methods: Four human cell lines with various sensitivities to photon irradiation were used: the SCC25 and SQ20B derived from human epithe lium tumours of the tongue and larynx, respectively, and the normal lines M /10, derived from human mammary epithelium, and HF19 derived from a lung fi broblast. The cells were irradiated with gamma-rays and proton beams with l inear energy transfer (LET) from 7 to 33 keV/mu m. Clonogenic survival was assessed. Results: Survival curves are reported for each cell line following irradiat ion with gamma-rays and with various proton LETs. The surviving fraction af ter 2 Gy of gamma-rays was 0.72 for SQ20B cells, and 0.28-0.35 for the othe r cell lines. The maximum LET proton effectiveness was generally greater th an that of gamma-rays. In particular there was a marked increase in beam ef fectiveness with increasing LET for the most resistant cells (SQ20B) whose 2 Gy-survival varied from 0.72 with gamma-radiation down to 0.37 with 30 ke V/mu m protons. The relative biological effectiveness (RBE(2Gy gamma)) with the 30 keV/mu m beam, evaluated as the ratio of 2 Gy to the proton dose pr oducing the same inactivation level as that given by 2Gy of gamma-rays, was 3.2, 1.8, 1.3 and 0.8 for SQ20B, M/10, SCC25, and HF19, respectively. Conclusions: RBE for inactivation with high-LET protons increased with the cellular radioresistance to gamma-rays. The cell line with the greatest res istance to gamma-rays was the most responsive to the highest LET proton bea m. A similar trend has also been found in studies reported in the literatur e with He, C, N ions with LET in the range 20-125 keV/mu m on human tumour cell lines.