title: The potential value of the neutral comet assay and γH2AX foci assay in assessing the radiosensitivity of carbon beam in human tumor cell lines.
abstract: BACKGROUND
  Carbon ions ((12)C(6+)) are high linear energy transfer (LET) radiation characterized by higher relative biological effectiveness than low LET radiation. The assessment of tumour radiosensitivity would be particularly useful in optimizing the radiation dose during radiotherapy. The aim of the current study was to evaluate the potential value of the neutral comet assay and γH2AX foci assay in assessing (12)C(6+) radiosensitivity of tumour cells.
  
  
  METHODS
  The doses of (12)C(6+) and X-rays used in the present study were 2 and 4 Gy. The survival fraction, DNA double-strand breaks (DSB) and repair kinetics of DSB were assayed with clonogenic survival, neutral comet assay and γH2AX foci assay in human cervical carcinoma HeLa cells, hepatoma HepG2 cells, and mucoepidermoid carcinoma MEC-1 cells at the time points of 0.5, 4, 16 and 24 h after (12)C(6+) and X-rays irradiation.
  
  
  RESULTS
  The survival fraction for 12C6+ irradiation was much more inhibited than for X-rays (p < 0.05) in all three tumour cell lines tested. Substantial amounts of residual damage, assessed by the neutral comet assay, were present after irradiation (p < 0.05). The highest residual damage was observed at 0.5 or 4 h, both for (12)C(6+) and X-ray irradiation. However, the residual damage in HeLa and MEC-1 cells was higher for (12)C(6+) than X-rays (p < 0.05). The strongest induction of γH2AX foci was observed after 30 min, for all three tumour cell lines (p < 0.01). The franction of γH2AX foci persisted for at least 24 h after (12)C6+ irradiation; in HeLa cells and MEC-1 was higher than after X-ray irradiation (p < 0.05). The correlation coefficients between the clonogenic survival, neutral comet assay and γH2AX foci assay were not statistically significant, except for some tumour cells at individual irradiation doses and types.
  
  
  CONCLUSIONS
  Our study demonstrated that the neutral comet assay and γ-H2AX foci assay could be used to assess the radiosensitivity of (12)C(6+) in human tumour cells.
authors.affiliation: Medical College of Northwest University for Nationalities, Lanzhou 730030, PR China.
authors.forename: Jin
authors.initials: J
authors.lastname: Zhao
authors.forename: Zhong
authors.initials: Z
authors.lastname: Guo
authors.forename: Hong
authors.initials: H
authors.lastname: Zhang
authors.forename: Zhenhua
authors.initials: Z
authors.lastname: Wang
authors.forename: Lei
authors.initials: L
authors.lastname: Song
authors.forename: Jianxiu
authors.initials: J
authors.lastname: Ma
authors.forename: Shuyan
authors.initials: S
authors.lastname: Pei
authors.forename: Chenjing
authors.initials: C
authors.lastname: Wang
keywords: DNA double strand breaks; X-rays; carbon ions; human tumour cells; radiation sensitivity; γH2AX