Figure 1.

Irradiation with high LET protons reduced clonogenic survival more effectively than irradiation with photons or low LET protons. (A) Schematic presentation of a proton curve with spread out Bragg peak (SOBP) (green) built up of four proton beams with varying energies (low to high energy proton beams: dark red to orange). (B) Schema presenting dose deposition in tissue after irradiation with photons (blue) and protons (green). The thunderbolts indicate the region of the respective beams used to irradiate glioblastoma cells (blue: photons/X-rays; green: low LET protons; red: high LET protons). (C, D) Glioblastoma cells (A172, LN229, T98G and U373) were irradiated with respective beams at 0 Gy, 2 Gy, 4 Gy and 6 Gy and incubated to allow formation of colonies. Surviving fractions were calculated by determining the quotient of counted colonies to plated cells and subsequent normalization to the respective quotient for non-irradiated cells. (C) Surviving fraction (SF) of different cell lines was compared after irradiation with respective beams at 2 Gy (left panels), 4 Gy (middle panels) and 6 Gy (right panels). (D) The effect of the different beams (blue: irradiation with photons; green: irradiation with low LET protons; red: irradiation with high LET protons) on survival fraction was compared. N ≥ 3. *p < 0.05; **p < 0.01; ***p < 0.001 (blue: significance between irradiation with photons and high LET protons; green: significance between irradiation with low LET protons and high LET protons).