Purpose: To analyse changes of cell inactivation and proliferation under therapeutic irradiation conditions along the proton spread out Bragg peak (SOBP) with particular emphasis on its distal declining edge. Materials and methods: HTB140 cells were irradiated at four positions: plateau, middle, distal end and distal declining edge of the 62 MeV proton SOBP. Doses ranged from 2–16 Gy. They were normalised in the middle of SOBP and delivered following the axial physical dose profile. Survival, proliferation and cell cycle were assessed seven days after irradiation. Results: Moving from proximal to distal irradiation position surviving fractions at 2 Gy (SF2) decreased from 0.88–0.59. Increased radiosensitivity of the cells was noticed for the doses below 4 Gy, resulting in two gradients of cell inactivation, stronger for lower and weaker for higher doses. Relative biological effectiveness (RBE) increased from 1.68–2.84 at the distal end of SOBP. A further rise of RBE reaching 7.14 was at its distal declining edge. Following the axial physical dose profile of SOBP the strongest inactivation was attained at its distal end and was comparable to that at its declining edge. Conclusions: Survival data confirmed very high radioresistance of HTB140 cells. An effect similar to low-dose hyper radiosensitivity (HRS) was observed for order of magnitude larger doses. Better response of cells to protons than to γ-rays was illustrated by rather high RBE. Strong killing ability at the SOBP distal declining edge was the consequence of increasing proton linear energy transfer.

Purpose: To measure the ability of protons and γ-rays to effect cell viability and cell survival of human HTB140 melanoma cells. Materials and methods: Exponentially growing HTB140 cells were irradiated close to the Bragg peak maximum of the 62 MeV protons or with ⁶⁰Co γ-rays with single doses, ranging from 8 – 24 Gy. Cell viability using the 3-(4,5-dimethylthiazol-2yl)-2,5-diphenyltetrazolium bromide (MTT) assay was evaluated at 6 h, 24 h, 48 h or 7 days after irradiation and clonogenic survival was assessed at 7 days after irradiation. Cell cycle phase redistribution and the level of apoptosis were evaluated at 6 h and 48 h after irradiation. Results: The study of cell viability as a function of time (cell survival progression) and cell survival, using a clonal assay, demonstrated the considerably stronger inactivation effect of protons compared to γ-rays with a relative biological effectiveness (RBE) of ∼1.64. Cell cycle phase distribution and apoptosis levels with time enabled us to investigate the development and the character of the damage induced by irradiation. Due to the high radio-resistance of HTB140 cells, cell cycle phase redistribution exhibited only a modest cell accumulation in G2/M phase. Protons but not γ-rays induced apoptosis. Conclusions: It appears that protons reduce the number of HTB140 cells by apoptosis as well as by severe DNA damage, while γ-rays eliminate viable cells primarily by the production of irreparable DNA damage. Protons have an increased RBE relative to γ-rays.