Age Response for CHO Cells Exposed to Negative Pions

The variation in radiosensitivity with position in the cell cycle of cultured Chinese hamster cells (line CHO) for exposures to negative pions at the peak position has been compared to the variation for X rays. The cells were first synchronized by mitotic selection and resynchronized in the region of late $G_{1}/\text{early}$ S phase by treatment with hydroxyurea (10-3 M). A fixed dose of 540 rad of negative pions was used to expose cells at different stages in the cell cycle. The results indicate that the variation (ratio of maximum survival to minimum survival) was approximately ten fold for X rays and five fold for negative pions at comparable survival levels.