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 epithelium tumours of the tongue and larynx, respectively, and the normal lines M/10, derived from human mammary epithelium, and HF19 derived from a lung fibroblast. The cells were irradiated with γ-rays and proton beams with linear energy transfer (LET) from 7 to 33keV/μm. Clonogenic survival was assessed. Results: Survival curves are reported for each cell line following irradiation with γ-rays and with various proton LETs. The surviving fraction after 2 Gy of γ-rays was 0.72 for SQ20B cells, and 0.28–0.35 for the other cell lines. The maximum LET proton effectiveness was generally greater than that of γ-rays. In particular there was a marked increase in beam effectiveness with increasing LET for the most resistant cells (SQ20B) whose 2 Gy-survival varied from 0.72 with γ-radiation down to 0.37 with 30keV/μm protons. The relative biological effectiveness (RBE(2Gy γ)) with the 30 keV/μm beam, evaluated as the ratio of 2Gy to the proton dose producing the same inactivation level as that given by 2 Gy of γ-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 γ-rays. The cell line with the greatest resistance to γ-rays was the most responsive to the highest LET proton beam. A similar trend has also been found in studies reported in the literature with He, C, N ions with LET in the range 20–125keV/μm on human tumour cell lines. [ABSTRACT FROM AUTHOR]